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RECORD: Darwin, C. R. 1846. Draft of last chap. of South America 'Northern Chile'. CUL-DAR1. Edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)

NOTE: See record in the Darwin Online manuscript catalogue, enter its Identifier here. Reproduced with permission of the Syndics of Cambridge University Library & William Huxley Darwin.

Darwin, C. R. 1846. Geological observations on South America. Being the third part of the geology of the voyage of the Beagle, under the command of Capt. Fitzroy, R.N. during the years 1832 to 1836. London: Smith Elder & Co. F273

Text taken from the published chapter with minor revisions to facilitate electronic searching.

[in the hand of an archivist:] 'Northern Chile; Conclusion' - the last chapter of Geological observations on South America, 1846.

Chapter IX

Northern Chile

Conclusion

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Mr Fletcher, your first page, will be. p. 211, &c)

Ch. 8 VIII

Northern Chile - Conclusion

Section from Illapel to Combarbala; Gypseous formation with silicified wood — Panuncillo — Coquimbo; mines of Arqueros; section up valley; fossils — Guasco, fossils of — Copiapo, section up valley; Las Amolanas, Silicified wood, conglomerates, nature of former land, fossils, thickness of strata, great subsidence — Valley of Despoblado, fossils, tufaceous deposit, complicated dislocations of — Relations between ancient orifices of eruption & subsequent axes of injection — Iquique, Peru, fossils of, salt-deposits — Metalliferous veins — Summary on the Porphyritic conglomerate & Gypseous formations — Great subsidence with partial elevations during the Cretaceo-oolitic period — On the elevation & structure of the Cordillera — Recapitulation on the Tertiary series — Relation between movements of subsidence & volcanic action — Pampean formation — Recent elevatory movements — Long-continued volcanic action in the Cordillera — Conclusion. —

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Chapt. 8

Northern Chile

Valparaiso to Coquimbo. — I have already described the general nature of the rocks in the low country north of Valparaiso, consisting of granites, syenites, greenstones, & altered feldspathic clay-slate. Nearer Coquimbo there is much hornblendic rock & various dusky-coloured porphyries. I will describe only one section in this district, namely, from near Illapel in a N.E. line to the mines of Los Hornos, & thence in a north by east direction to Combarbala, at the foot of the main Cordillera.

Near Illapel, after passing for some distance over granite, andesite, & andesitic porphyry, we come to a greenish stratified feldspathic rock, which I believe is altered clayslate, conformably capped by porphyries & porphyritic conglomerate of great thickness, dipping at an average angle of 20° to N.E. by N. The uppermost beds consist of conglomerates & sandstone only a little metamorphosed, & conformably covered by a Gypseous formation of very great thickness,

but much denuded. This gypseous formation, where first met with, lies in a broad valley or basin, a little southward of the mines of Los Hornos: the lower half alone contains gypsum, not in great masses as in the Cordillera, but in innumerable thin layers, seldom more than an inch or two in thickness. The gypsum is either opaque or transparent, & is associated with carbonate of lime. The layers alternate with numerous varying ones of a calcareous clay-shale (with strong aluminous odour, adhering to the tongue, easily fusible into a pale green glass), more or less indurated, either earthy & cream-coloured, or greenish & hard. The more indurated varieties have a compact, homogeneous almost crystalline fracture, & contain granules of crystallized oxide of iron. Some of the varieties almost resemble hone-stones. There is also a little black, hardly fusible, siliceo-calcareous clayslate, like some of the varieties alternating with gypsum on the Peuquenes range.

The upper half of this gypseous formation is mainly formed of the same calcareous clay-shale rock, but without any gypsum, & varying extremely in nature: it passes from a soft, coarse, earthy, ferruginous state, including particles of quartz, into compact clay-stones with crystallized oxide of iron,—into porcellanic

layers, alternating with seams of calcareous matter,—& into green porcelain-jasper excessively hard, but easily fusible. Strata of this nature alternate with much black & brown siliceo-calcareous slate, remarkable from the wonderful number of huge embedded logs of silicified wood. This wood, according to Mr. R. Brown, is (judging from several specimens) all coniferous. Some of the layers of the black siliceous slate contained irregular angular fragments of imperfect pitchstone, which I believe, as in the Uspallata range, has originated in a metamorphic process. There was one bed of a marly tufaceous nature, & of little specific gravity. Veins of agate & calcareous spar are numerous. The whole of this gypseous formation, especially the upper half, has been injected, metamorphosed, & locally contorted by numerous hillocks of intrusive porphyries, crowded together in an extraordinary manner. These hillocks consist of purple claystone & of various other porphyries, & of much white feldspathic greenstone passing into andesite; this latter variety included in one case crystals of orthitic & albitic feldspar touching each other, & others of hornblende, chlorite, & epidote.

The strata surrounding these intrusive hillocks at the mines of Los Hornos, are intersected by many veins of copper-pyrites, associated with much micaceous iron-ore, & by some of gold: in the neighbourhood of these veins the rocks are blackened & much altered. The gypsum near the intrusive masses is always opaque. One of these hillocks of porphyry was capped by some stratified porphyritic conglomerate, which must have been brought up from below, through the whole immense thickness of the overlying gypseous formation. The lower beds of the gypseous formation resemble the corresponding & probably contemporaneous strata of the main Cordillera; whilst the upper beds in several respects resemble those of the Uspallata chain, & possibly may be contemporaneous with them; for I have endeavoured to show that the Uspallata beds were accumulated subsequently to the gypseous or neocomian formations of the Cordillera.

This pile of strata dips at an angle of about 20° to N.E. by N., close up to the very foot of the Cuesta de Los Hornos, a crooked range of mountains formed of intrusive rocks of the same nature with the above described hillocks. Only in one or two places, on this south-eastern side of the range, I noticed a narrow fringe of the upper gypseous strata brushed up & inclined south-eastward from it. On its north-eastern flank, & likewise on a few of the summits, the stratified porphyritic conglomerate is inclined N.E.: so that, if we disregard the very narrow anticlinal fringe of gypseous strata at its S.E. foot, this range forms a second uniclinal axis of elevation. Proceeding in a north by east direction to the village of Combarbala, we come to a third escarpment of the porphyritic conglomerate, dipping eastwards, & forming the outer range of the main Cordillera. The lower beds were here more jaspery than usual, & they included some white cherty strata & red sandstones, alternating with purple claystone porphyry. Higher up in the Cordillera there appeared to be a line of andesitic rocks; & beyond them, a fourth escarpment of the porphyritic conglomerate, again dipping eastwards or inwards. The overlying gypseous strata, if they ever existed here, have been entirely removed.

Copper Mines of Panuncillo. — From Combarbala to Coquimbo, I traversed the country in a zig-zag direction, crossing & recrossing the porphyritic conglomerate, & finding in the granitic districts an unusual number of mountain-masses composed of various intrusive, porphyritic rocks, many of them andesitic. One common variety was greenish-black, with large crystals of blackish albite. At Panuncillo a short N.N.W. & S.S.E. ridge, with a nucleus formed of greenstone & of a slate-coloured porphyry including crystals of glassy feldspar, deserves notice, from the very singular nature of the almost vertical strata composing it. These consist chiefly of a finer or coarser granular mixture, not very compact, of white carbonate of lime, of protoxide of iron & of yellowish garnets (ascertained by Prof. Miller), each grain being an almost perfect crystal. Some of the varieties consist exclusively of granules of the calcareous spar; & some contain grains of copper ore, &, I believe, of quartz. These strata alternate with a bluish, compact, fusible feldspathic rock. Much of the above granular mixture has, also, a pseudo-brecciated structure, in which fragments are obscurely arranged in planes parallel to those of the stratification, & are conspicuous on the weathered surfaces. The fragments are angular or rounded, small or large, & consist of bluish or reddish compact feldspathic matter, in which a few acicular crystals of feldspar can sometimes be seen. The fragments often blend at their edges into the surrounding granular mass, & seem due to a kind of concretionary action.

These singular rocks are traversed by many copper veins, & appear to rest conformably on a granular mixture (in parts as fine-grained as a sandstone) of quartz, mica, hornblende, & feldspar; & this on fine-grained common gneiss; & this on a laminated mass, composed of pinkish orthitic feldspar, including a few specks of hornblende; & lastly, this on granite, which, together with andesitic rocks, form the surrounding district.

Coquimbo; Mining district of Arqueros. — At Coquimbo the porphyritic conglomerate formation approaches nearer to the Pacific, than in any other part of Chile visited by me, being separated from the coast by a tract only a few miles broad of the usual plutonic rocks, with the addition of a porphyry having a red euritic base. In proceeding to the mines of Arqueros, the strata of porphyritic conglomerate are at first nearly horizontal, an unusual circumstance, & afterwards they dip gently to S.S.E. After having ascended to a considerable height, we come to an undulatory district in which the famous silver mines are situated; my examination was chiefly confined to those of S. Rosa. Most of the rocks in this district are stratified, dipping in various directions, & many of them are of so singular a nature, that at the risk of being tedious I must briefly describe them. The commonest variety is a dull-red, compact, finely-brecciated stone, containing much iron & innumerable white crystallized particles of carbonate of lime & minute extraneous fragments.

Another variety is almost equally common near S. Rosa; it has a bright green, scanty basis, including distinct crystals & patches of white carbonate of lime, & grains of red, semi-micaceous oxide of iron; in parts the basis becomes dark green, & assumes an obscure crystalline arrangement, & occasionally in parts it becomes soft & slightly translucent like soapstone. These red & green rocks are often quite distinct, & often pass into each other; the passage being sometimes affected by a fine brecciated structure, particles of the red & green matter being mingled together. Some of the varieties appear gradually to become porphyritic with feldspar; & all of them are easily fusible into pale or dark-coloured beads, strongly attracted by the magnet. I should perhaps have mistaken several of these stratified rocks for submarine lavas, like some of those described at the Puente del Inca, had I not examined a few leagues eastward of this point, a fine series of analogous but less metamorphosed, sedimentary beds belonging to the gypseous formation, & probably derived from a volcanic source.

This formation is intersected by numerous metalliferous veins, running, though irregularly,

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N.W. & S.E., & generally at right angles to the many dikes. The veins consist of native silver, of muriate of silver, an amalgam of silver, cobalt, antimony, & arsenic,* generally embedded in sulphate of barytes. I was assured by Mr. Lambert, that native copper without a trace of silver has been found in the same vein with native silver without a trace of copper. At the mines of Aristeas, the silver veins are said to be unproductive as soon as they pass into the green strata, whereas at S. Rosa, only two or three miles distant, the reverse happens; & at the time of my visit, the miners were working through a red stratum, in the hopes of the vein becoming productive in the underlying green sedimentary mass. I have a specimen of one of these green rocks, with the usual granules of white calcareous spar & red oxide of iron, abounding with disseminated particles of glittering native & muriate of silver, yet taken at the distance of one yard from any vein,—a circumstance, as I was assured, of very rare occurrence.

Section eastward, up the valley of Coquimbo. — After passing for a few miles over the coast granitic series, we come to the porphyritic conglomerate, with its usual characters, & with some of the beds distinctly displaying their mechanical origin. The strata, where first met with, are, as before stated, only slightly inclined; but near the Hacienda of Pluclaro, we come to an anticlinal axis, with the beds much dislocated & shifted by a great fault, of which not a trace is externally seen in the outline of the hill. I believe that this anticlinal axis can be traced northwards, into the district of Arqueros, where a conspicuous hill called Cerro Blanco, formed of a harsh, cream-coloured euritic rock, including a few crystals of reddish feldspar, & associated with some purplish claystone porphyry, seems to fall on a line of elevation. In descending from the Arqueros district, I crossed on the northern border of the valley, strata inclined eastward from the Pluclaro axis: on the porphyritic conglomerate there rested a mass, some hundred feet thick, of brown argillaceous limestone, in parts crystalline, & in parts almost composed of Hippurites Chilensis d'Orbig.; above this came a black calcareous shale, & on it a red conglomerate. In the brown limestone, with

the Hippurites, there was an impression of a Pecten & a coral, & great numbers of a large Gryphæa, very like, &, according to Prof. E. Forbes, probably identical with G. orientalis, Forbes M.S.,—a cretaceous species (probably upper green-sand) from Verdachellum, in southern India. These fossils seem to occupy nearly the same position with those at the Puente del Inca,—namely, at the top of the porphyritic conglomerate, & at the base of the gypseous formation.

A little above the Hacienda of Pluclaro, I made a detour on the northern side of the valley, to examine the superincumbent gypseous strata, which I estimated at 6,000 feet in thickness. The uppermost beds of the porphyritic conglomerate, on which the gypseous strata conformably rest, are variously coloured, with one very singular & beautiful stratum composed of purple pebbles of various kinds of porphyry, embedded in white calcareous spar, including cavities lined with bright-green crystallized epidote. The whole pile of strata belonging to both formations is inclined, apparently from the above mentioned axis of Pluclaro, at an angle of between 20° & 30° to the east. I will here give a section of the principal beds met with in crossing the entire thickness of the gypseous strata.

Firstly: above the porphyritic conglomerate formation, there is a fine-grained, red, crystalline sandstone.

Secondly: a thick mass of smooth-grained, calcareo-aluminous, shaly rock, often marked with dendritic manganese, & having, where most compact, the external appearance of honestone. It is easily fusible. I shall for the future, for convenience sake, call this variety pseudo-

hone-stone. Some of the varieties are quite black when freshly broken, but all weather into a yellowish-ash coloured, soft, earthy substance, precisely as is the case with the compact shaly rocks of the Peuquenes range. This stratum is of the same general nature with many of the beds near Los Hornos in the Illapel section. In this second bed, or in the underlying red sandstone, (for the surface was partially concealed by detritus), there was a thick mass of gypsum, having the same mineralogical characters with the great beds described in our sections across the Cordillera.

Thirdly; a thick stratum of fine-grained, red, sedimentary matter, easily fusible into a white glass, like the basis of claystone porphyry; but in parts jaspery, in parts brecciated, & including crystalline specks of carbonate of lime. In some of the jaspery layers, & in some of the black siliceous slaty bands, there were irregular seams of imperfect pitchstone, undoubtedly of metamorphic origin, & other seams of brown, crystalline limestone. Here, also, were masses, externally resembling ill preserved silicified wood.

Fourthly & fifthly: calcareous pseudo-honestone; & a thick stratum concealed by detritus.

Sixthly: a thinly stratified mass of bright green, compact, smooth-grained calcareo-argillaceous stone, easily fusible, & emitting a strong aluminous odour: the whole has a highly angulo-concretionary structure; & it resembles, to a certain extent, some of the upper tufaceo-infusorial deposits of the Patagonian tertiary formation. It is in its nature allied to our pseudo-honestone, & it includes well characterized layers of that variety; & other layers of a pale green, harder, &

brecciated variety; & others of red sedimentary matter, like that of bed Three. Some pebbles of porphyries are embedded in the upper part.

Seventhly: red sedimentary matter or sandstone like that of bed One, several hundred feet in thickness, & including jaspery layers, often having a finely brecciated structure.

Eighthly: white, much indurated, almost crystalline tuff, several hundred feet in thickness, including rounded grains of quartz & particles of green matter like that of bed Six. Parts pass into a very pale green, semi-porcellanic stone.

Ninthly: red or brown coarse conglomerate, 300 to 400 feet thick, formed chiefly of pebbles of porphyries, with volcanic particles, in an arenaceous, non-calcareous, fusible basis: the upper two feet are arenaceous without any pebbles.

Tenthly: the last & uppermost stratum here exhibited, is a compact, slate-coloured porphyry, with numerous elongated crystals of glassy feldspar, from 150 to 200 feet in thickness: it lies strictly conformably on the underlying conglomerate, & is undoubtedly a submarine lava.

This great pile of strata has been broken up in several places by intrusive hillocks of purple claystone porphyry, & by dikes of porphyritic greenstone: it is said that a few poor metalliferous veins have been discovered here. From the fusible nature & general appearance of the finer-grained strata, they probably owe their origin (like the allied beds of the Uspallata range, & of the upper Patagonian tertiary formations) to gentle volcanic eruptions, & to the abrasion of volcanic rocks.

Comparing these beds with those in the mining district of Arqueros, we see at both places, rocks easily fusible, of the same peculiar bright green & red colours, containing calcareous matter, often having a finely brecciated structure, often passing into each other & often alternating together: hence I cannot doubt that the only difference between them, lies in the Arqueros beds having been more metamorphosed (in conformity with their more dislocated & injected condition), & consequently in the calcareous matter, oxide of iron & green colouring matter, having been segregated under a more crystalline form.

The strata are inclined, as before stated from 20° to 30° eastward, towards an irregular north & south chain of andesitic porphyry & of porphyritic greenstone, where they are abruptly cut off. In the valley of Coquimbo, near to the H. of Gualliguaca, similar plutonic rocks are met with, apparently a southern prolongation of the above chain; & eastward of it we have an escarpment of the porphyritic conglomerate, with the strata inclined at a small angle eastward, which makes the third escarpment, including that nearest the coast. Proceeding up the valley we come to another north & south line of granite, andesite, & blackish porphyry, which seem to lie in an irregular trough of the porphyritic conglomerate. Again, on the

south side of the R. Claro, there are some irregular granitic hills, which have thrown off the strata of porphyritic conglomerate to the N.W. by W.; but the stratification here has been much disturbed. I did not proceed any further up the valley, & this point is about two-thirds of the distance between the Pacific & the main Cordillera.

I will describe only one other section, namely, on the north side of the R. Claro, which is interesting from containing fossils: the strata are much dislocated by faults & dikes, & are inclined to the north, towards a mountain of andesite & porphyry, into which they appear to become almost blended. As the beds approach this mountain, their inclination increases up to an angle of 70°, & in the upper part, the rocks become highly metamorphosed. The lowest bed visible in this section, is a purplish

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hard sandstone. Secondly, a bed 200 or 300 feet thick, of a white siliceous sandstone, with a calcareous cement, containing seams of slaty sandstone, & of hard yellowish-brown (dolomitic ?) limestone; numerous, well-rounded, little pebbles of quartz are included in the sandstone. Thirdly, a dark coloured limestone with some quartz pebbles, from fifty to sixty feet in thickness, containing numerous silicified shells, presently to be enumerated. Fourthly, very compact, calcareous, jaspery sandstone, passing into (fifthly) a great bed, several hundred feet thick, of conglomerate, composed of pebbles of white, red, & purple porphyries, of sandstone & quartz, cemented by calcareous matter. I observed that some of the finer parts of this conglomerate were much indurated within a foot of a dike eight feet in width, & were rendered of a paler colour with the calcareous matter segregated into white crystallized particles; some parts were stained green from the colouring matter of the dike. Sixthly, a thick mass, obscurely stratified, of a red sedimentary stone or sand-stone, full of crystalline calcareous matter, imperfect crystals of oxide of iron, & I believe of feldspar, & therefore closely resembling some of the highly metamorphosed beds at Arqueros: this bed was capped by, & appeared to pass in its upper part into, rocks similarly coloured, containing calcareous matter, & abounding with minute crystals, mostly elongated & glassy, of reddish albite. Seventhly, a conformable stratum of fine reddish porphyry with large crystals of (albitic ?) feldspar; probably a submarine lava. Eighthly, another conformable bed of green porphyry, with specks of green earth & cream coloured crystals of feldspar. I believe that there are other superincumbent crystalline strata & submarine lavas, but I had not time to examine them.

The upper beds in this section probably correspond with parts of the great gypseous formation; & the lower beds of red sandstone conglomerate & fossiliferous limestone no doubt are the equivalents of the Hippurite stratum, seen in descending from Arqueros to Pluclaro, which there lies conformably upon the porphyritic conglomerate formation. The fossils found in the third bed, consist of—

Pecten Dufreynoyi, d'Orbig. Voyage, Part. Pal.

This species, which occurs here in vast numbers, according to M. d'Orbigny, resembles certain cretaceous forms.

Ostrea hemispherica, d'Orbig. Voyage, &c.

Also resembles, according to the same author, cretaceous forms.

Terebratula ænigma, d'Orbig. Voyage, &c. (Pl. XXII. fig. 10-12,) & Pl. V. fig. 10, 11, 12 of this work.

Is allied, according to M. d'Orbigny, to T. concinna from the Forest Marble. A series of this species, collected in several localities hereafter to be referred to, has been laid before Prof. E. Forbes; & he informs me that many of the specimens are almost undistinguishable from our oolitic T. tetraedra, & that the varieties amongst them are such as are found in that variable species. Generally speaking, the American specimens of T. ænigma may be distinguished from the British T. tetraedra, by the surface having the ribs sharp & well defined to the beak, whilst in the British species they become obsolete & smoothed down; but this difference is not constant. Prof. Forbes adds, that, possibly, internal characters may exist, which would distinguish the American species from its European allies.

Spirifer linguiferoides, E. Forbes, Pl. V. fig. 17, 18.

Professor Forbes states (see Appendix) that this species is very near to S. linguifera of Phillips (a carboniferous limestone fossil), but probably distinct. M. d'Orbigny considers it as perhaps indicating the Jurassic period.

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15 2v

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Coquimbo to Guasco. — The rocks near the coast, & some way inland, do not differ from those described northwards of Valparaiso: we have much greenstone, syenite, feldspathic & jaspery slate, & grauwackes having a basis like that of claystone; there are some large tracts of granite, in which the constituent minerals are sometimes arranged in folia, thus composing an imperfect gneiss. There are two large districts of mica-schist, passing into glossy clayslate, & resembling the great formation in the Chonos Archipelago. In the valley of Guasco, an escarpment of porphyritic conglomerate is first seen high up the valley, about two leagues eastward of the town of Ballenar. I heard of a great gypseous formation in the Cordillera; & a collection of shells made there was given me. These shells are all in the same condition, & appear to have come from the same bed: they consist of—

Turritella Andii, d'Orbig. Voyage, Part. Pal.

Pecten Dufreynoyi, do.

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Valley of Copiapo. — The journey from Guasco to Copiapo, owing to the utterly desert nature of the country, was necessarily so hurried, that I do not consider my notes worth giving. In the valley of Copiapo, some of the sections are very interesting. From the sea to the town of Copiapo, a distance estimated at thirty miles, the mountains are composed of greenstone, granite, andesite, & blackish porphyry, together with some dusky-green feldspathic rocks, which I believe to be altered clayslate: these mountains are crossed by many brown-coloured dikes, running north & south. Above the town, the main valley runs in a south-east & even more southerly course towards the Cordillera; where it is divided into three great ravines, by the northern one of which, called Jolquera, I penetrated for a short distance. The coloured section, fig. 3 in Plate I., gives an eye-sketch of the structure & composition of the mountains on both sides of this valley: a straight east & west line from the town to the Cordillera is perhaps not more than thirty miles, but along the valley the distance is much greater. Wherever the valley trended very southerly, I have endeavoured

to contract the section into its true proportion. This valley, I may add, rises much more gently than any other valley which I saw in Chile.

To commence with our section, for a short distance above the town we have hills of the granitic series, together with some of that rock [A], which I suspect to be

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altered clayslate, but which Prof. G. Rose, judging from specimens collected by Meyen at P. Negro, states is serpentine passing into greenstone. We then come suddenly to the great Gypseous formation [B], without having passed over, differently from in all the sections hitherto described, any of the porphyritic conglomerate. The strata are at first either horizontal or gently inclined westward; then highly inclined in various directions, & contorted by underlying

* Meyen's Reise, &c. Th. 1. s. 394.

masses of intrusive rocks; & lastly, they have a regular eastward dip, & form a tolerably well pronounced north & south line of hills. This formation consists of thin strata, with innumerable alternations, of black, calcareous slate-rock, of calcareo-aluminous stones like those at Coquimbo, which I have called pseudo-honestones, of green jaspery layers, & of pale-purplish, calcareous, soft rottenstone, including seams & veins of gypsum. These strata are conformably overlaid by a great thickness of thinly stratified, compact limestone, with included crystals of carbonate of lime. At a place called Tierra Amarilla, at the foot of a mountain thus composed, there is a broad vein, or perhaps stratum, of a beautiful & curious crystallized mixture, composed, according to Prof. G. Rose,* of sulphate of iron under two forms, & of the sulphates of copper & alumina: the section is so obscure that I could not make out whether this vein or stratum occurred in the gypseous formation, or more probably in some underlying masses [A], which I believe are altered clayslate.

Second Axis of Elevation. — After the gypseous masses [B], we come to a line of hills of unstratified porphyry [C], which on their eastern side blend into strata of great thickness of porphyritic conglomerate, dipping eastward. This latter formation, however, here has not been nearly so much metamorphosed as in most parts of Central Chile;

* Meyen's Reise, &c. Th. 1. s. 394.

true purple claystone porphyry, repeatedly alternating with thick beds of purplish-red conglomerate with the well-rounded, large pebbles of various porphyries, not blended together.

Third Axis of Elevation. — Near the ravine of Los Hornitos, there is a well marked line of elevation, extending for many miles in a N.N.E. & S.S.W. direction, with the strata dipping in most parts (as in the second axis) only in one direction, namely, eastward at an average angle of between 30° & 40°. Close to the mouth of the valley, however, there is, as represented in the section, a steep & high mountain [D], composed of various green & brown intrusive porphyries

Bed (2), a pale lilac, hard but not heavy stone, slightly laminated, including small extraneous fragments, & imperfect as well as some perfect & glassy crystals of feldspar; from 150 to 200 feet in thickness. When examining it in situ, I thought it was certainly a true porphyry, but my specimens now lead me to suspect that it possibly may be a metamorphosed tuff. From its colour it could be traced for a long distance, overlying in one part, quite conformably to the porphyry of bed 1, & in another not distant part, a very thick mass of conglomerate, composed of pebbles of a porphyry chiefly like that of bed 1: this fact shows how the nature of the bottom formerly varied in short horizontal distances. Bed (3), white, much indurated tuff, containing minute pebbles, broken crystals, & scales of mica, varies much in thickness. This bed is remarkable from containing many globular & pear-shaped, externally rusty balls, from the size of an apple to a man's head, of very tough, slate-coloured porphyry, with imperfect crystals of feldspar: in shape these balls do not resemble pebbles, & I believe that they are subaqueous volcanic bombs; they differ from subaërial bombs only in not being vesicular. Bed (4); a dull purplish-red, hard conglomerate, with crystallized particles & veins of carbonate of lime, from 300 to 400 feet in thickness. The pebbles are of claystone porphyries

of many varieties; they are tolerably well rounded, & vary in size from a large apple to a man's head. This bed includes three layers of coarse, black, calcareous, somewhat slaty rock: the upper part passes into a compact red sandstone.

In a formation so highly variable in mineralogical nature, any division not founded on fossil remains, must be extremely arbitrary: nevertheless the beds below the last conglomerate, may, in accordance with all the sections hitherto described, be considered as belonging to the porphyritic conglomerate, & those above it to the gypseous formation, marked [E] in the section. The part of the valley in which the following beds are seen is near Potrero Seco. Bed (5), compact, fine-grained, pale greenish-grey, non-calcareous, indurated mudstone, easily fusible into a pale green & white glass. Bed (6), purplish, coarse-grained, hard sandstone, with broken crystals of feldspar & crystallized particles of carbonate of lime; it possesses a slightly nodular structure. Bed (7), blackish-grey, much indurated, calcareous mudstone, with extraneous particles of unequal size; the whole being in parts finely brecciated. In this mass there is a stratum, twenty feet in thickness, of impure gypsum. Bed (8), a greenish mudstone with several layers of gypsum. Bed (9), a highly indurated, easily fusible, white tuff, thickly mottled with ferruginous matter, & including some white semi-porcellanic layers, which are interlaced with ferruginous veins. This stone closely resembles some of the commonest varieties in the Uspallata chain. Bed (10), a thick bed of rather bright green, indurated mudstone or tuff, with a concretionary nodular structure so strongly developed that the whole mass consists of balls.

Bed (11) I will not attempt to estimate the thickness of the strata in the gypseous formation hitherto described, but it must certainly be very many hundred feet. is at least 800 feet in thickness: it consists of thin layers of whitish, greenish, or more commonly brown, fine-grained indurated tuffs, which crumble into angular fragments: some of the layers are semi-porcellanic, many of them highly ferruginous, & some are almost composed of carbonate of lime & iron with drusy cavities lined with quartz-crystals. Bed (12), dull purplish or greenish or dark-grey, very compact & much indurated mudstone: estimated at 1,500 feet in thickness: in some parts this rock assumes the character of an imperfect coarse clayslate; but viewed under a lens, the basis always has a mottled appearance, with the edges of the minute component particles blending together. Parts are calcareous, & there are numerous veins of highly crystalline carbonate of lime charged with iron. The mass has a nodular structure, & is divided by only a few planes of stratification: there are, however, two layers, each about eighteen inches thick, of a dark-brown, finer-grained stone, having a conchoidal, semi-porcellanic fracture which can be followed with the eye for some miles across the country.

I believe this last great bed is covered by other nearly similar alternations; but the section is here obscured by a tilt from the next porphyritic chain, presently to be

described. I have given this section in detail, as being illustrative of the general character of the mountains in this neighbourhood; but it must not be supposed that any one stratum long preserves the same character. At a distance of between only two & three miles, the green mudstones & white indurated tuffs are to a great extent replaced by red sandstone & black calcareous shaly rocks, alternating together. The white indurated tuff, bed (11), here contains little or no gypsum, whereas on the northern & opposite side of the valley, it is of much greater thickness & abounds with layers of gypsum, some of them alternating with thin seams of crystalline carbonate of lime. The uppermost, dark-coloured, hard mudstone, (bed 12), is in this neighbourhood the most constant stratum. The whole series differs to a considerable extent, especially in its upper part, from that met with at [B B], in the lower part of the valley; nevertheless I do not doubt that they are equivalents.

Fourth Axis of Elevation, (Valley of Copiapo.) — This axis is formed of a chain of mountains [F], of which the central masses (near La Punta) consist of andesite containing green hornblende & coppery mica, & the outer masses of greenish & black porphyries, together with some fine lilac-coloured claystone porphyry; all these porphyries being injected & broken up by small hummocks of andesite. The central great mass of this latter rock, is covered on the eastern side by a black, fine-grained, highly micaceous slate, which, together with the succeeding mountains of porphyry, are traversed by numerous white dikes, branching from the andesite,

them extending in straight lines, to a distance of at least two miles. The mountains of porphyry eastward of the micaceous schist soon, but gradually, assume (as observed in so many other cases) a stratified structure, & can then be recognised as a part of the porphyritic conglomerate formation. These strata [G] are inclined at a high angle to the S.E., & form a mass from 1,500 to 2,000 feet in thickness. The gypseous masses to the west already described, dip directly towards this axis, with the strata only in a few places (one of which is represented in the section) thrown from it: hence this fourth axis is mainly uniclinal towards the S.E., & just like our third axis, only locally anticlinal.

The above strata of porphyritic conglomerate [G] with their south-eastward dip, come abruptly up against beds of the gypseous formation [H], which are gently, but irregularly, inclined westward: so that there is here a synclinal axis & great fault. Further up the valley, here running nearly north & south, the gypseous formation is prolonged for some distance; but the stratification is unintelligible, the whole being broken up by faults, dikes, & metalliferous veins. The strata consist chiefly of

red calcareous sandstones, with numerous veins, in the place of layers, of gypsum; the sandstone is associated with some black calcareous slate-rock, & with green pseudo-honestones, passing into porcelain-jasper. Still further up the valley, near Las Amolanas [I], the gypseous strata become more regular, dipping at an angle of between 30° & 40° to W.S.W., & conformably overlying, near the mouth of the ravine of Jolquera, strata [K] of porphyritic conglomerate. The whole series has been tilted by a partially concealed axis [L] of granite, andesite, & a granitic mixture of white feldspar, quartz, & oxide of iron.

Fifth Axis of Elevation (Valley of Copiapo, near Los Amolanas). — I will describe in some detail the beds [I] seen here, which as just stated, dip to W.S.W., at an angle of from 30° to 40°. I had not time to examine the underlying porphyritic conglomerate, of which the lowest beds, as seen at the mouth of the Jolquera, are highly compact, with crystals of red oxide of iron; & I am not prepared to say whether they are chiefly of volcanic or metamorphic origin. On these beds there rests a coarse purplish conglomerate, very little metamorphosed, composed of pebbles of porphyry, but remarkable from containing one pebble of granite

no instance has occurred in the sections hitherto described. Above this conglomerate, there is a black siliceous claystone, & above it numerous alterations of dark-purplish & green porphyries, which may be considered as the uppermost limit of the porphyritic conglomerate formation.

Above these porphyries comes a coarse, arenaceous conglomerate, the lower half white & the upper half of a pink colour, composed chiefly of pebbles of various porphyries, but with some of red sandstone & jaspery rocks. In some of the more arenaceous parts of the conglomerate, there was an oblique or current lamination; a circumstance which I did not elsewhere observe. Above this conglomerate, there is a vast thickness of thinly stratified, pale-yellowish, siliceous sandstone, passing into a granular quartz-rock, used for grindstones (hence the name of the place Las Amolanas), & certainly belonging to the gypseous formation, as does probably the immediately underlying conglomerate. In this yellowish sandstone there are layers of white & pale-red siliceous conglomerate; other layers with small, well-rounded pebbles of white quartz, like the bed at the R. Claro at Coquimbo;

others of a greenish, fine-grained, less siliceous stone, somewhat resembling the pseudo-honestones lower down the valley; & lastly, others of a black calcareous shale-rock. In one of the layers of conglomerate, there was embedded a fragment of mica-slate, of which this is the first instance; hence, perhaps, it is from a formation of mica-slate, that the numerous small pebbles of quartz, both here & at Coquimbo, have been derived. Not only does the siliceous sandstone include layers of the black, thinly stratified, not fissile, calcareous shale-rock, but in one place the whole mass, especially the upper part, was, in a marvellously short horizontal distance, after frequent alternations replaced by it. When this occurred, a mountain-mass, several thousand feet in thickness was thus composed; the black calcareous shale-rock, however, always included some layers of the pale-yellowish siliceous sandstone, of the red conglomerate, & of the greenish jaspery & pseudo-honestone varieties. It likewise included three or four widely separated layers of a brown limestone, abounding with shells immediately to be described. This pile of strata was in parts traversed by many veins of gypsum. The calcareous shale-rock, though when freshly broken quite black, weathers into an ash-colour: in which respect & in general appearance, it perfectly resembles those great fossiliferous beds of the Peuquenes range, alternating with gypsum & red sandstone, described in the last chapter.

The shells out of the layers of brown limestone, included in the black calcareous shale-rock, which latter, as just stated, replaces the white siliceous sandstone, consist of;—

Pecten Dufreynoyi, d'Orbig. Voyage, Part. Pal.

Turritella Andii, do.

Relations given in the list from Coquimbo.

Astarte Darwinii, E. Forbes, Pl. V. fig. 22, 23.

Gryphæa Darwinii, do. Pl. V. fig. 7.

An intermediate form between G. gigantea & G. incurva.

Gryphæa, nov. spec. ? do. Pl. V. fig. 8 & 9.

Perna Americana, do. Pl. V. fig. 4, 5, 6.

Avicula, nov. spec.

Considered by Mr. G. B. Sowerby as the A. echinata, by M. d'Orbigny as certainly a new & distinct species, having a Jurassic aspect. The specimen has been unfortunately lost.

Terebratula ænigma, d'Orbig. (var. of do. E. Forbes, Pl. V. fig. 13, 14.)

This is the same variety, with that from Guasco, considered by M. D'Orbigny to be a distinct species from his T. ænigma, & related to T. obsoleta.

Plagiostoma & Ammonites, fragments of.

I will reiterate the evidence on the association of these several shells in the several localities.


Coquimbo.
Pecten Dufreynoyi	in the same bed, Rio Claro.
Ostrea hemispherica
Terebratula ænigma
Spirifer linguiferoides

Hippurites Chilensis	same bed, near Arqueros.
Gryphæa orientalis	same bed, near Arqueros.

Terebratula ænigma	in same block of limestone	Collected by M. Domeyko from the same locality, apparently near Arqueros.
———— ignaciana	in same block of limestone
Pecten Dufreynoyi
Ostrea hemispherica
Hippurites Chilensis
Turritella Andii
Nautilus Domeykus

Guasco.
Pecten Dufreynoyi	In a collection from the Cordillera, given me: the specimens all in the same condition.
Turritella Andii
Terebratula ignaciana
———— ænigma, var.
Spirifer Chilensis.

Copiapo.
Pecten Dufreynoyi	Mingled together in alternating beds in the main valley of Copiapo near Las Amolanas, & likewise higher up the valley.
Turritella Andii
Terebratula ænigma, var. as at Guasco
Astarte Darwinii
Gryphæa Darwinii
———— nov. spec. ?
Perna Americana
Avicula, nov. spec.

Terebratula ænigma (true)	Main valley of Copiapo, apparently same formation with that of Amolanas.

Terebratula ænigma (true)	In the same bed, high up the great lateral valley of the Despoblado, in the ravine of Maricongo.
Pecten Dufreynoyi
Gryphæa Darwinii ?

Considering this table, I think it is impossible to doubt that all these fossils belong to the same formation. If, however, the species from Las Amolanas, in the Valley of Copiapo, had, as in the case of those from Guasco, been separately examined, they would probably have been ranked as oolitic; for, although no Spirifers were found here, all the other species, with the exception of the Pecten, Turritella, & Astarte, have a more ancient aspect than cretaceous forms. On the other hand, taking into account the evidence derived from the cretaceous character of these three shells, & of the Hippurites, Gryphæa orientalis, & Ostrea, from Coquimbo, we are driven back to the provisional name already used of cretaceo-oolitic. From geological evidence, I believe this formation to be the equivalent of the neocomian beds of the Cordillera of central Chile.

To return to our section near Las Amolanas:— Above the yellow siliceous sandstone, or the equivalent calcareous slate-rock, with its bands of fossil-shells, according as the one or other prevails, there is a pile of strata, which cannot be less than from 2,000 to 3,000 feet in thickness, in main part composed of a coarse, bright, red conglomerate,

Before making any further remarks, I may state, that higher up this same valley we shall again meet with a similar association of shells; & in the great Despoblado valley, which branches off near the town from that of Copiapo, the Pecten Dufreynoyi, some Gryphites (I believe G. Darwinii), & the true Terebratula ænigma of d'Orbig. were found together in an equivalent formation as will be hereafter seen. A specimen also, I may add, of the true T. ænigma, was given me from the neighbourhood of the famous silver mines of Chanuncillo, a little south of the valley of Copiapo, & these mines, from their position, I have no doubt, lie within the great gypseous formation: the rocks close to one of the silver-veins, judging from fragments shown me, resemble those singular metamorphosed deposits from the mining district of Arqueros, near Coquimbo.

The lower layers of the limestone contained thousands of the Gryphæa; & the upper ones as many of the Turritella, with the Gryphæa (nov. spec.) & Serpulæ adhering to them; in all the layers, the Terebratula & fragments of the Pecten were included. It was evident, from the manner in which the species were grouped together, that they had lived where now embedded.

[Bv]

The Pecten & Turritella

calcareous particles. This whole great pile is remarkable from the thousands of huge, embedded, silicified trunks of trees, one of which was eight feet long, & another eighteen feet in circumference: how marvellous it is, that every vessel in so thick a mass of wood should have been converted into silex! I brought home many specimens, & all of them, according to Mr. R. Brown, present a coniferous structure.

Above this great conglomerate, we have from 200 to 300 feet in thickness of red sandstone; & above this, a stratum of black, calcareous slate-rock, like that which alternates with & replaces the underlying yellowish-white, siliceous sandstone. Close to the junction between this upper black slate-rock & the upper red sandstone, I found the Gryphæa Darwinii, the Turritella Andii, & vast numbers of a bivalve, too imperfect to be recognised. Hence we see that, as far as the evidence of these two shells serves—& the Turritella is an eminently characteristic species—the whole thickness of this vast pile of strata belongs to the same age. Again, above the last-mentioned upper red sandstone, there were several alternations of the black, calcareous slate-rock; but I was unable to ascend to them. All these uppermost strata, like the lower ones, vary extremely in character in short horizontal distances. The gypseous formation, as here seen, has a coarser, more mechanical texture, & contains much more siliceous matter than the corresponding beds

lower down the valley. Its total thickness, together with the upper beds of the porphyritic conglomerate, I estimated at least at 8,000 feet; & only a small portion of the porphyritic conglomerate, which on the eastern flank of the fourth axis of elevation appeared to be from 1,500 to 2,000 feet thick, is here included. As corroborative of the great thickness of the gypseous formation, I may mention that in the Despoblado Valley (which branches from the main valley a little above the town of Copiapo) I found a corresponding pile of red & white sandstones, & of dark, calcareous, semi-jaspery mudstones, rising from a nearly level surface, & thrown into an absolutely vertical position; so that, by pacing, I ascertained their thickness to be nearly 2,700 feet; taking this as a standard of comparison, I estimated the thickness of the strata above the porphyritic conglomerate at 7,000 feet.

The fossils before enumerated, from the limestone-layers in the whitish siliceous sandstone, are now covered, on the least computation, by strata, from 5,000 to 6,000 feet in thickness. Professor E. Forbes thinks that these shells probably lived at a depth of from about thirty to forty fathoms, that is from 180 to 240 feet; anyhow, it is impossible that they could have lived at the depth of from 5,000 to 6,000 feet. Hence in this case, as in that of the Puente del Inca,

we may safely conclude that the bottom of the sea on which the shells lived, subsided, so as to receive the superincumbent submarine strata: & this subsidence must have taken place during the existence of these shells; for, as I have shown, some of them occur high up as well as low down in the series. That the bottom of the sea subsided, is in harmony with the presence of the layers of coarse well-rounded pebbles included throughout this whole pile of strata, as well as of the great upper mass of conglomerate from 2,000 to 3,000 feet thick; for coarse gravel could hardly have been formed or spread out at the profound depths indicated by the thickness of the strata. The subsidence, also, must have been slow to have allowed of this often-recurrent spreading out of the pebbles. Moreover, we shall presently see that the surfaces of some of the streams of porphyritic lava beneath the gypseous formation, are so highly amygdaloidal that it is scarcely possible to believe that they flowed under the vast pressure of a deep ocean. The conclusion of a great subsidence during the existence of these cretaceo-oolitic fossils, may, I believe, be extended to the district of Coquimbo, although, owing to the fossiliferous beds there not being directly covered by the upper gypseous strata, which in the section north of the valley are about 6,000 feet in thickness, I did not there insist on this conclusion.

The pebbles in the above conglomerates, both in the upper & lower beds, are all well rounded, &, though chiefly composed of various porphyries, there are some of red sandstone & of a jaspery stone, both like the rocks intercalated in layers in this same gypseous formation; there was one pebble of mica-slate & some of quartz, together with many particles of quartz. In these respects there is a wide difference between the gypseous conglomerates & those of the porphyritic-conglomerate formation, in which latter, angular & rounded fragments, almost exclusively composed of porphyries, are mingled together, & which, as already often remarked, probably were ejected from craters deep under the sea. From these facts I conclude, that during the formation of the conglomerates, land existed in the neighbourhood, on the shores of which the innumerable pebbles were rounded & thence dispersed, & on which the coniferous forests flourished—for it is improbable that so many thousand logs of wood should have been drifted from any great distance. This land, probably islands, must have been mainly formed of porphyries, with some mica-slate, whence the quartz was derived, & with some red sandstone & jaspery rocks. This latter fact is important, as it shows that in this district, even previously to the deposition of the lower gypseous or cretaceo-oolitic beds, strata of an analogous nature had elsewhere, no doubt in the more central ranges of the Cordillera, been elevated;—thus recalling to our minds

the relations of the Cumbre & Uspallata chains. Having already referred to the great lateral valley of the Despoblado, I may mention that above the 2,700 feet of red & white sandstone & dark mudstone, there is a vast mass of coarse, hard, red conglomerate, some thousand feet in thickness, which contains much silicified wood, & evidently corresponds with the great upper conglomerate at Las Amolanas: here, however, the conglomerate consists almost exclusively of pebbles of granite, & of disintegrated crystals of reddish feldspar & quartz, firmly recemented together. In this case, we may conclude that the land whence the pebbles were derived, & on which the now silicified trees once flourished, was formed of granite.

The mountains near Las Amolanas, composed of the cretaceo-oolitic strata, are interlaced with dikes like a spider's web, to an extent which I have never seen equalled, except in the denuded interior of a volcanic crater: north & south lines, however, predominate. These dikes are composed of green, white, & blackish rocks, all porphyritic with feldspar, & often with large crystals of hornblende. The white varieties approach closely in character to andesite, which composes, as we have seen, the injected axes of so many of the lines of elevation. Some of the green varieties are finely laminated, parallel to the walls of the dikes.

Sixth Axis of Elevation (Valley of Copiapo). — This axis consists of a broad mountainous mass [O] of andesite, composed of albite, brown mica, & chlorite, passing into andesitic granite, with quartz: on its western side it has thrown off, at a considerable angle, a thick mass of stratified porphyries, including much epidote [N N], & remarkable only from being divided into very thin beds, as highly amygdaloidal on their surfaces as sub-aërial lava-streams are often vesicular. This porphyritic formation is conformably covered, as seen some way up the ravine of Jolquera, by a mere remnant of the lower part of the cretaceo-oolitic formation [M M], which in one part encases, as represented in the coloured section, the foot of the andesitic axis [L], of the already described fifth line, & in another part entirely conceals it: in this latter case, the gypseous or cretaceo-oolitic strata falsely appeared to dip under the porphyritic conglomerate of the fifth axis. The lowest bed of the gypseous formation, as seen here [M], is of yellowish siliceous sandstone precisely like that of Amolanas, interlaced in parts with veins of gypsum, & including layers of the black, calcareous, non-fissile slate-rock: the Turritella Andii, Pecten Dufreynoyi, Terebratula ænigma, var., & some Gryphites were embedded in these layers. The sandstone varies in thickness from only twenty to eighty feet; & this variation is caused by the inequalities in the upper surface of an underlying stream of purple claystone porphyry. Hence the above fossils here lie at

the very base of the gypseous or cretaceo-oolitic formation, & hence they were probably once covered up by strata about 7,000 feet in thickness: it is, however, possible, though from the nature of all the other sections in this district not probable, that the porphyritic claystone lava may in this case have invaded a higher level in the series. Above the sandstone there is a considerable mass of much indurated, purplish-black, calcareous claystone, allied in nature to the often-mentioned black calcareous slate-rock.

Eastward of the broad andesitic axis of this sixth line, & penetrated by many dikes from it, there is a great formation [P] of mica-schist, with its usual variations, & passing in one part into a ferruginous quartz-rock. The folia are curved & highly inclined, generally dipping eastward. It is probable that this mica-schist is an old formation, connected with the granitic rocks & metamorphic schists near the coast; & that the one fragment of mica-slate, & the pebbles of quartz low down in the gypseous formation at Las Amolanas, have been derived from it. The mica-schist is succeeded by stratified porphyritic conglomerate [Q] of great thickness, dipping eastward with a high inclination: I have included this latter mountain-mass in the same anticlinal axis with the porphyritic streams [N N]; but I am far from sure that the two masses may not have been independently upheaved.

A text

Proceeding up the ravine, we come to another mass [R] of andesite; & beyond this, we again have a very thick, stratified porphyritic formation [S], dipping at a small angle eastward, & forming the basal part of the main Cordillera. I did not ascend the ravine any higher; but here, near Castaño, I examined several sections, of which I will not give the details, only observing, that the porphyritic beds, or submarine lavas, preponderate greatly in bulk over the alternating sedimentary layers, which have been but little metamorphosed: these latter consist of fine-grained red tuffs & of whitish volcanic gritstones, together with much of a singular, compact rock, having an almost crystalline basis, finely brecciated with red & green fragments, & occasionally including a few large pebbles. The porphyritic lavas are highly amygdaloidal, both on their upper & lower surfaces; they consist chiefly of clay-stone porphyry, but with one common variety, like some of the streams at the Puente del Inca, having a grey mottled basis, abounding with crystals of red hydrous oxide of iron, green ones apparently of epidote, & a few glassy ones of feldspar. This pile of strata differs considerably from the basal strata of the Cordillera in central Chile, & may possibly belong to the upper & gypseous series: I saw, however, in the bed of the valley, one fragment of porphyritic breccia-conglomerate, exactly like those great masses met with in the more southern parts of Chile.

Finally, I must observe, that though I have described between the town of Copiapo & the western flank of the main Cordillera, seven or eight axes of elevation, extending nearly north & south, it must not be supposed that they all run continuously for great distances. As was stated to be the case in our sections across the Cordillera of central Chile, so here most of the lines of elevation, with the exception of the first, third, & fifth, are very short. The stratification is everywhere disturbed & intricate; nowhere have I seen more numerous faults & dikes. The whole district, from the sea to the Cordillera, is more or less metalliferous; & I heard of gold, silver, copper, lead, mercury, & iron veins. The metamorphic action, even in the lower strata, has certainly been far less here than in central Chile.

Valley of the Despoblado. — This great barren valley, which has already been alluded to, enters the main valley of Copiapo a little above the town: it runs at first northerly, then N.E., & more easterly into the Cordillera; I followed its dreary course to the foot of the first main ridge. I will not give a detailed section, because it would be essentially similar to that already given, & because the stratification is exceedingly complicated. After leaving the plutonic hills near the town, I met first, as in the main valley, with the gypseous formation

having the same diversified character as before, & soon afterwards with masses of porphyritic conglomerate, about 1,000 feet in thickness. In the lower part of this formation, there were very thick beds composed of fragments of claystone porphyries, both angular & rounded, with the smaller ones partially blended together & the basis rendered porphyritic; these beds separated distinct streams, from sixty to eighty feet in thickness, of claystone lavas. Near Paipote, also, there was much true porphyritic breccia-conglomerate: nevertheless, few of these masses were metamorphosed to the same degree with the corresponding formation in central Chile. I did not meet in this valley with any true andesite, but only with imperfect andesitic porphyry including large crystals of hornblende: numerous as have been the varieties of intrusive porphyries already mentioned, there were here mountains composed of a new kind, having a compact, smooth, cream-coloured basis, including only a few crystals of feldspar, & mottled with dendritic spots of oxide of iron. There were also some mountains of a porphyry with a brick-red basis, containing irregular, often lens-shaped, patches of compact feldspar, & crystals of feldspar, which latter to my surprise I find to be orthite.

At the foot of the first ridge of the main Cordillera, in the ravine of Maricongo, & at an elevation, which, from the extreme coldness & appearance of the vegetation, I estimated at about 10,000 feet, I found beds of white sandstone & of limestone including the Pecten Dufreynoyi, Terebratula ænigma, & some Gryphites. This ridge, throws the water on the one hand into the Pacific,

as I was informed, into a great gravel-covered, basin-like plain, including a salt-lake, & without any drainage-exit. In crossing the Cordillera by this Pass, it is said that three principal ridges must be traversed, instead of two, or only one as in central Chile.

The crest of this first main ridge & the surrounding mountains, with the exception of a few lofty pinnacles, are capped by a great thickness of a horizontally stratified, tufaceous deposit. The lowest bed is of a pale purple colour, hard, fine-grained, & full of broken crystals of feldspar & scales of mica. The middle bed is coarser & less hard, & hence weathers into very sharp pinnacles; it includes many small fragments of granite, & innumerable ones of all sizes of grey vesicular trachyte, some of which were distinctly rounded. The uppermost bed is about 200 feet in thickness, of a darker colour & apparently hard: but I had not time to ascend to it. These three horizontal beds may be seen for the distance of many leagues, especially westward or in the direction of the Pacific, capping the summits of the mountains, & standing on the opposite sides of the immense valleys at exactly corresponding heights. If united they would form a plain, inclined very slightly towards the Pacific; the beds become thinner in this direction, & the tuff (judging from one point to which I ascended, some way down the valley) finer-grained & of less specific gravity, though still compact

though rendering it probable, would not have convinced me that this mass had been of subaqueous origin, for it is known that volcanic ashes falling on land & moistened by rain often become hard & stratified; but beds thus originating, & owing their consolidation to atmospheric moisture, would have covered almost equally every neighbouring summit, high & low, & would not have left those above a certain exact level absolutely bare; this circumstance seems to me to prove that the volcanic ejections were arrested at their present, widely extended, equable level, & there consolidated by some other means than simple atmospheric moisture; & this no doubt must have been a sheet of water. A lake at this great height, & without a barrier on any one side, is out of the question; consequently we must conclude that the tufaceous matter was anciently deposited beneath the sea. It was certainly deposited before the excavation of the valleys, or at least before their final enlargement;* & I may add, that Mr. Lambert, a gentleman well

Journal, &c. (2d edit.) p. 355

373

This volcanic formation, which I am informed by Mr. Lambert extends far northward, is of interest, as typifying what has taken place on a grander scale on the corresponding western side of the Cordillera of Peru. Under another point of view, however, it possesses a far higher interest, as confirming that conclusion drawn from the structure of the fringes of stratified shingle which are prolonged from the plains at the foot of the Cordillera far up the valleys,—namely, that this great range has been elevated in mass to a height of between 8000 & 9000 feet;* & now, judging from this tufaceous deposit, we may conclude that the horizontal elevation has been in the district of Copiapo about 10,000 feet.

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* I may here mention that on the south side of the main valley of Copiapo, near Potrero Seco, the mountains are capped by a thick mass of horizontally stratified shingle, at a height which I estimated at between 1500 & 2000 feet above the bed of the valley. This shingle, I believe, forms the edge of a wide plain, which stretches southwards between two mountain ranges.

In the valley of the Despoblado, the stratification, as before remarked, has been much disturbed, & in some points to a greater degree than I have anywhere else seen. I will give two cases: a very thick mass of thinly stratified red sandstone, including beds of conglomerate, has been crushed together (as represented in the woodcut) into a yoke or urn-formed trough, so that the strata on both sides have been folded inwards: on the right hand the properly underlying porphyritic claystone conglomerate is seen overlying the sandstone, but it soon becomes vertical, & then is inclined towards the trough, so that the beds radiate like the spokes of a wheel: on the left hand, the inverted porphyritic conglomerate also assumes a dip towards the trough, not gradually as on the right hand, but by means of a vertical fault & synclinal break; & a little still further on towards the left, there is a second great oblique fault, (both shown by the arrow-lines), with the strata dipping to a directly opposite point: In the second case, two low ridges trend together & unite at the head of a little wedge-shaped valley; throughout the right-hand ridge, the strata dip at 45° to the east; in the left-hand ridge, we have the very same strata & at first with exactly the same dip; but in following this ridge

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these mountains are intersected by infinitely numerous dikes, some of which can be seen to rise from hummocks of greenstone, & can be traced for thousands of feet.

up the valley, the strata are seen very regularly to become more & more inclined until they stand vertical, they then gradually fall over, (the basset-edges forming symmetrical serpentine lines along the crest) till at the very head of the valley they are reversed at an angle of 45°: so that at this point the beds have been turned through an angle of 135°; & here there is a kind of anticlinal axis, with the strata on both sides dipping to opposite points at an angle of 45°, but with those on the left hand upside-down.

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On the Eruptive Sources of the Porphyritic Claystone & Greenstone Lavas: — In central Chile, from the extreme metamorphic action, it is in most parts difficult to distinguish between the streams of porphyritic lava & the porphyritic breccia-conglomerate, but here, at Copiapo, they are generally perfectly distinct, & in the Despoblado, I saw for the first time, two great strata of purple claystone porphyry, after having been for a considerable space closely united together, one above the other, become separated by a mass of fragmentary matter, & then both thin out;—the lower one more rapidly than the upper & greater stream. Considering the number & thickness of the streams of porphyritic lava, & the great thickness of the beds of breccia-conglomerate, there can be little doubt that the sources of eruption must originally have been numerous: nevertheless, it is now most difficult even to conjecture the precise point of any one of the ancient submarine craters. I have repeatedly observed mountains of porphyries, more or less distinctly stratified towards their summits or on their flanks, without a trace of stratification in their central & basal parts: in most cases, I believe this is simply due, either to the obliterating effects of metamorphic action, or to such parts having been mainly formed of intrusive porphyries, or to both causes conjoined; in some instances, however, it appeared to me very probable that the great central unstratified masses of porphyry were the now partially denuded nuclei of the old submarine volcanos, & that the stratified parts marked the points whence the streams flowed. In one case alone, & it was in this valley of the Despoblado, I was able actually to trace a thick stratum of purplish porphyry, which for a space of some miles conformably overlay the usual alternating beds of breccia-conglomerates & claystone lavas, until it became united with, & blended into, a mountainous mass of various unstratified porphyries.

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The difficulty of tracing the streams of porphyries to their ancient & doubtless numerous eruptive sources, may be partly explained by the very general disturbance which the Cordillera in most parts has suffered; but I strongly suspect that there is a more specific cause, namely, that the original points of eruption tend to become the points of injection. This in itself does not seem improbable; for where the earth's crust has once yielded, it would be liable to yield again, though the liquefied intrusive matter might not be any longer enabled to reach the submarine surface & flow as lava. I have been led to this conclusion, from having so frequently observed that, where part of an unstratified mountain-mass resembled in mineralogical character the adjoining streams or strata, there were several other kinds of intrusive porphyries & andesitic rocks injected into the same point. As these intrusive mountain-masses form most of the axes-lines in the Cordillera, whether anticlinal, uniclinal, or synclinal, & as the main valleys have generally been hollowed out along these lines, the intrusive masses have generally suffered much denudation. Hence they are apt to stand in some degree isolated, & to be situated at the points where the valleys abruptly bend, or where the main tributaries enter. On this view

in the old points of eruption to become the points of subsequent injection & disturbance, & consequently of denudation, it ceases to be surprising that the streams of lava in the porphyritic claystone conglomerate formation, & in other analogous cases, should most rarely be traceable to their actual sources.

Iquique, Southern Peru. — Differently from what we have seen throughout Chile, the coast here is formed not by the granitic series, but by an escarpment of the porphyritic conglomerate formation, between 2000 & 3000 feet in height.* I had time only for a very short examination; the chief part of the escarpment appears to be composed of various reddish & purple, sometimes laminated, porphyries, resembling those of Chile; & I saw some of the porphyritic breccia-conglomerate; the stratification appeared but little inclined. The uppermost part, judging from the rocks near the famous silver mine of

* The lowest point, where the road crosses the coast-escarpment, is 1,900 feet by the barometer above the level of the sea.

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Huantajaya,* consists of laminated, impure, argillaceous, purplish-grey limestone, associated, I believe, with some purple sandstone. In the limestone, shells are found

This latter species we have seen associated with the fossils of which lists have been given in this chapter, in two places in the valley of Coquimbo, & in the ravine of Maricongo at Copiapo. Considering this fact, & the superposition of these beds on the porphyritic conglomerate formation; &, as we shall immediately see, from their containing much gypsum, & from their otherwise close general resemblance in mineralogical nature with the strata described in the valley of Copiapo, I have little doubt that these fossiliferous beds of Iquique belong to the great cretaceo-oolitic formation of northern Chile. Iquique is situated seven degrees latitude north of Copiapo; & I may here mention, that an Ammonites, nov. spec., & Astarte, nov. spec., were given me from the Cerro Pasco, about ten degrees of latitude north of Iquique, & M. d'Orbigny thinks that they probably indicate a neocomian formation. Again, fifteen degrees of latitude northward, in Colombia, there is a grand fossiliferous deposit, now well known from the labours of Von Buch, Lea, d'Orbigny, & Forbes, which belongs to the earlier stages of the cretaceous system. Hence, bearing in mind the character of the few fossils from Tierra del Fuego, there is some evidence that a great portion of the stratified deposits of the whole vast range of the South American Cordillera, belongs to about the same geological epoch.

Proceeding from the coast escarpment inwards, I crossed, in a space of about thirty miles, an elevated undulatory district, with the beds dipping in various directions. The rocks are of many kinds,—white laminated, sometimes siliceous, sandstone,—purple & red sandstone, sometimes so highly calcareous as to have a crystalline fracture,—argillaceous limestone,—black calcareous slate-rock, like that so often described at Copiapo & other places,—thinly laminated, fine-grained, greenish, indurated, sedimentary, fusible rocks, approaching in character to the so-called pseudo-honestone of Chile, including thin contemporaneous veins of gypsum,—& lastly, much calcareous, laminated porcelain jasper, of a green colour, with red spots, & of extremely easy fusibility: I noticed one conformable stratum of a freckled-brown, feldspathic lava. I may here mention that I heard of great beds of gypsum in the Cordillera. The only novel point in this formation, is the presence of innumerable thin layers of rock-salt, alternating with the laminated & hard, but sometimes earthy, yellowish, or bright red & ferruginous sandstones. The thickest layer of salt was only two inches, & it thinned out at both ends.

* Mr. Bollaert has described (Geolog. Proceedings, vol. ii. p. 598) a singular mass of stratified detritus, gravel, & sand, eighty-one yards in thickness, overlying the limestone, & abounding with loose masses of silver ore. The miners believe that they can attribute these masses to their proper veins.

On one of these saliferous masses I noticed a stratum about twelve feet thick, of dark-brown, hard, brecciated, easily fusible rock, containing grains of quartz & of black oxide of iron, together with numerous imperfect fragments of shells. The problem of the origin of salt is so obscure, that every fact, even geographical position, is worth recording.* With the exception of these saliferous beds, most of the rocks, as already remarked, present a striking general resemblance with the upper parts of the gypseous or cretaceo-oolitic formation of Chile.

Metalliferous Veins.

I have only a few remarks to make on this subject: in nine mining districts, some of them of considerable extent, which I visited in central Chile, I found the principal veins running from between

* It is well known that stratified salt is found in several places on the shores of Peru. The island of San Lorenzo, off Lima, is composed of a pile of thin strata, about 800 feet in thickness, composed of yellowish & purplish, hard, siliceous, or earthy sandstones, alternating with thin layers of shale, which in places passes into a greenish, semi-porcellanic, fusible rock. There are some thin beds of reddish mudstone, & soft ferruginous rotten-stones, with layers of gypsum. In nearly all these varieties, especially in the softer sandstones, there are numerous thin seams of rock-salt: I was informed that one layer has been found two inches in thickness. The manner in which the minutest fissures of the dislocated beds have been penetrated by the salt, apparently by subsequent infiltration, is very curious. On the south side of the island, layers of coal & of impure limestone have been discovered. Hence we here have salt, gypsum, & coal associated together. The strata include veins of quartz, carbonate of lime, [N. & N.W.] to [S. & S.E.]:* in some other places, however, their course appeared quite irregular, as is said to be generally the case in the whole valley of Copiapo: at Tambillos, south of Coquimbo, I saw one large copper vein extending east & west. It is worthy of notice, that the foliation of the gneiss & mica-slate, & the cleavage of the altered clayslate, where such rocks occur, certainly tend to run like the metalliferous veins, though often irregularly, in a direction a little westward of north. At Yaquil, I observed that the principal auriferous veins ran nearly parallel to the grain or imperfect cleavage of the surrounding granitic rocks. With respect to the distribution of the different metals, copper, gold, & iron are generally associated together, & are most frequently found

* These mining districts are Yaquil near Nancagua, where the direction of the chief veins, to which only in all cases I refer, is north & south; in the Uspallata range, the prevailing line is N.N.W. & S.S.E.; in the C. de Prado, it is N.N.W. & S.S.E.; near Illapel, it is N. by W. & S. by E.; at Los Hornos the direction varies from between [N. & N.W.] to [S. & S.E.]; at the C. de los Hornos (further northward), it is N.N.W. & S.S.E.; at Panuncillo, it is N.N.W. & S.S.E.; & lastly at Arqueros, the direction is N.W. & S.E.

‡ Travels in Chile, p. 29.

† The same fact has been observed by Mr. Taylor in Cuba; London, Phil. Journ. vol. xi. p. 21.

‡ Travels in Chile, p. 29.

with many exceptions, as we shall presently see) in the rocks of the lower series, between the Cordillera & the Pacific, namely, in granite, syenite, altered feldspathic clayslate, gneiss, & as near Guasco mica-schist. The copper-ores consist of sulphurets, oxides, & carbonates, sometimes with laminæ of native metal: I was assured that in some cases (as at Panuncillo S.E. of Coquimbo), the upper part of the same vein contains oxides, & the lower part sulphurets of copper.† Gold occurs in its native form; it is believed that, in many cases, the upper part of the vein is the most productive part: this fact, probably, is connected with the abundance of this metal in the stratified detritus of Chile, which must have been chiefly derived from the degradation of the upper portions of the rocks. Very frequently the copper ores, including some gold, are associated with abundant micaceous specular iron. Gold is often

& iron pyrites; they have been dislocated by an injected mass of greenish-brown feldspathic trap.

Not only is salt abundant on the extreme western limits of the district between the Cordillera & the Pacific, but, according to Helms, it is found in the outlying low hills on the eastern flank of the Cordillera. These facts appear to me opposed to the theory, that rock-salt is due to the sinking of water, charged with salt, in mediterranean spaces of the ocean. The general character of the geology of these countries would rather lead to the opinion, that its origin is in some way connected with volcanic heat at the bottom of the sea: see on this subject, Sir R. Murchison's Anniversary address to Geolog. Soc. 1843, p. 65.

found in iron-pyrites: at two gold mines at Yaquil (near Nancagua), I was informed by the proprietor that in one the gold was always associated with copper-pyrites, & in the other with iron-pyrites: in this latter case, it is said that if the vein ceases to contain iron-pyrites, it is yet worth while to continue the search, but if the iron-pyrites, when it reappears, is not auriferous, it is better at once to give up working the vein. Although I believe copper & gold are most frequently found in the lower granitic & metamorphic schistose series, yet these metals

* The same fact has been observed by Mr. Taylor in Cuba; London, Phil. Journ. vol. xi. p. 21.

These superficial beds of well-rounded gravel & sand, containing gold, appeared to me to have been formed under the sea close to the beach, during the slow elevation of the land: Schmidtmeyer‡ remarks that in Chile gold is sought for in shelving banks at the height of some feet on the sides of the streams, & not in their beds, as would have been the case had this metal been deposited by common alluvial action.

occur both in the porphyritic conglomerate formation (as on the flanks of the Bell of Quillota & at Jajuel), & in the superincumbent strata. At Jajuel I was informed that the copper-ore, with some gold, is found only in the greenstones & altered feldspathic clayslate, which alternate with the purple porphyritic conglomerate. Several gold veins & some of copper-ore are worked in several parts of the Uspallata range, both in the metamorphosed strata, which have been shown to have been of probably subsequent origin to the neocomian or gypseous formation of the main Cordillera, & in the intrusive andesitic rocks of that range. At Los Hornos (N.E. of Illapel), likewise, there are numerous veins of copper-pyrites & of gold, both in the strata of the gypseous formation & in the injected hills of andesite & various porphyries.

Silver, in

cold & solitary deserts of the upper Cordillera. There are, however, exceptions to this rule:

Rich argentiferous lead is found in the clayslate of the Uspallata range; & I saw an old silver mine in a hill of syenite at the foot of the Bell of Quillota: I was also assured that silver has been found in the andesitic & porphyritic region between the town of Copiapo & the Pacific. I have stated in a previous part of this chapter, that in two neighbouring mines at Arqueros the veins in one were productive when they traversed the singular green sedimentary beds, & unproductive when crossing the reddish beds; whereas at the other mine exactly the reverse takes place: I have also described the singular & rare case of numerous particles of native silver & of the chloride being disseminated in the green rock at the distance of a yard from the vein. Mercury occurs with silver both at Arqueros & at Copiapo: at the base of C. de los Hornos (S.E. of Coquimbo, a different place from Los Hornos, before mentioned) I saw in a syenitic rock numerous quartzose veins, containing a little cinnabar in nests: there were here other parallel veins of copper & of a ferrugino-auriferous ore. I believe tin has never been found in Chile.

From information given me by Mr. Nixon of Yaquil,* & by others, it appears that in Chile those veins are generally most permanently productive, which consisting of various minerals (sometimes differing but slightly from the surrounding

at Paral (S.E. of Coquimbo) silver is found in the porphyritic conglomerate formation; as I suspect is likewise the case at S. Pedro de Nolasko in the Peuquenes Pass.

include parallel strings rich in metals; such a vein is called a veta real. More commonly the mines are worked only where one, two, or more thin veins or strings running in a different direction, intersect a poor 'veta real:' it is unanimously believed that at such points of intersection (cruceros), the quantity of metal is much greater than that contained in other parts of the intersecting veins. In some cruceros or points of intersection, the metals extend even beyond the walls of the main, broad, stony vein. It is said that the greater the angle of intersection, the greater the produce; & that nearly parallel strings attract each other; in the Uspallata range, I observed that numerous thin auri-ferruginous veins repeatedly ran into knots, & then branched out again. I have already described the remarkable manner, in which rocks of the Uspallata range are indurated & blackened (as if by a blast of gunpowder) to a considerable distance from the metallic veins.

Finally, I may observe, that the presence of metallic veins seems obviously connected with the presence of intrusive rocks, & with the degree of metamorphic action which the different districts of Chile have undergone.† Such metamorphosed areas are generally accompanied by numerous dikes & injected masses of andesite & various porphyries: I have in several places traced the metalliferous veins from the intrusive masses into the encasing

* Sir R. Murchison & his fellow travellers have given some striking facts on this subject in their account of the Ural Mountains. Geolog. Proc. vol. iii. p. 748.

strata. Knowing that the porphyritic conglomerate formation consists of alternate streams of submarine lavas & of the debris of anciently erupted rocks, & that the strata of the upper gypseous formation sometimes include submarine lavas, & are composed of tuffs, mudstones, & mineral substances, probably due to volcanic exhalations,—the richness of these strata is highly remarkable when compared with the erupted beds, often of submarine origin, but not metamorphosed, which compose the numerous islands in the Pacific, Indian, & Atlantic Oceans; for in these islands metals are entirely absent, & their nature even unknown to the aborigines.

Summary on the Geological History of the Chilian Cordillera, & of the southern parts of S. America.

We have seen that the shores of the Pacific, for a space of 1200 miles from Tres Montes to Copiapo, & I believe for a very much greater distance, are composed, with the exception of the tertiary basins, of metamorphic schists, plutonic rocks, & more or less altered clayslate.

On the floor of the ocean thus constituted, vast streams of various purplish claystone & greenstone porphyries were poured forth, together with great alternating piles of angular & rounded fragments of similar rocks ejected from the submarine craters. From the compactness of the streams & fragments, it is probable that, with the exception of some districts in northern Chile, the eruptions took place in profoundly deep water. The orifices of eruption appear to have been studded over a breadth, with some outliers, of from 50 to 100 miles: & closely enough together, both north & south, & east & west, for the ejected matter to form a continuous mass, which in central Chile is more than a mile in thickness. I traced this mound-like mass, for only 450 miles; but judging from what I saw at Iquique, from specimens, & from published accounts, it appears to have a manifold greater length. In the basal parts of the series, & especially towards the flanks of the range, mud, since converted into a feldspathic slaty rock, & sometimes into greenstone, was occasionally deposited between the beds of erupted matter: with this exception the uniformity of the porphyritic rocks is very remarkable.

At the period when the claystone & greenstone porphyries nearly or quite ceased being erupted, that great pile of strata which, from often abounding

I have called the Gypseous formation, was deposited, & feldspathic lavas, together with other singular volcanic rocks, were occasionally poured forth: I am far from pretending that any distinct line of demarkation can be drawn between this formation & the underlying porphyries & porphyritic conglomerate, but in a mass of such great thickness, & between beds of such widely different mineralogical nature, some division was necessary. At about the commencement of the gypseous period, the bottom of the sea here seems first to have been peopled by shells, not many in kind, but abounding in individuals. At the P. del Inca the fossils are embedded near the base of the formation; in the Peuquenes range, at different levels, half-way up, & even higher in the series; hence, in these sections, the whole great pile of strata belongs to the same period: the same remark is applicable to the beds at Copiapo, which attain a thickness of between 7000 & 8000 feet. The fossil shells in the Cordillera of central Chile, in the opinion of all the palæontologists who have examined them, belong to the earlier stages of the cretaceous system; whilst in northern Chile there is a most singular mixture of cretaceous & oolitic forms: from the geological relations, however, of these two districts, I cannot but think that they all belong to nearly the same epoch, which I have provisionally called cretaceo-oolitic.

The strata in this formation, composed of black calcareous shaly-rocks, of red & white, & sometimes siliceous sandstones, of coarse conglomerates, limestones, tuffs, dark mudstones, & those singular fine-grained rocks which I have called pseudo-honestones, vast beds of gypsum, & many other jaspery & scarcely describable varieties, vary & replace each other in short horizontal distances, to an extent, I believe, unequalled even in any tertiary basin. If we picture to ourselves the bottom of the sea, rendered uneven in an extreme degree, with numerous craters, some few occasionally in eruption, but the greater number in the state of solfataras, discharging calcareous, siliceous, ferruginous matters, & gypsum or sulphuric acid to an amount surpassing, perhaps, even the existing sulphureous volcanos of Java,

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Most of these substances are easily fusible, & have apparently been derived either from volcanos still in quiet action, or from the attrition of volcanic products.

From Dr. Gillies's account, this gypseous or cretaceo-oolitic formation extends as far south as the Pass of Planchon, & I followed it northward at intervals for 500 miles: judging from the character of the beds with the Terbratula ænigma, at Iquique, it extends from 400 to 500 miles further: & perhaps even for ten degrees of latitude north of Iquique to the Cerro Pasco, not far from Lima: again, we know that a cretaceous formation, abounding with fossils, is largely developed north of the equator, in Colombia: in Tierra del Fuego, at about this same period, a wide district of clayslate was deposited, which, in its mineralogical characters & external features, might be compared to the Silurian regions of North Wales. The gypseous formation, like that of the porphyritic breccia-conglomerate on which it rests, is of inconsiderable breadth; though of greater breadth in northern than in central Chile.

As the fossil shells in this formation are covered, in the Peuquenes ridge by a great thickness of strata; at the Puente del Inca by at least 5000 feet; at Coquimbo, though the superposition there is less plainly seen, by about 6000 feet; & at Copiapo certainly by 5000 or 6000, & probably by 7000 feet (the same species there recurring in the upper & lower parts of the series), we may feel confident that the bottom of the sea subsided during this cretaceo-oolitic period, so as to allow of the accumulation of the superincumbent submarine strata. This conclusion is confirmed by, or perhaps rather explains, the presence of the many beds at many levels of coarse conglomerate, the well-rounded pebbles in which we cannot believe were transported in very deep water. Even the underlying porphyries at Copiapo, with their highly amygdaloidal surfaces, do not appear to have flowed under great pressure. The great sinking movement thus plainly indicated, must have extended in a north & south line for at least 400 miles, & probably was co-extensive with the gypseous formation.

the extraordinarily numerous silicified trunks of fir-trees at Los Hornos, perhaps at Coquimbo & at two distant points in the valley of Copiapo, indicate that land existed at this period in the neighbourhood. This land, or islands, in the northern part of the district of Copiapo, must have been almost exclusively composed, judging from the nature of the pebbles, of granite: in the southern parts of Copiapo, it must have been mainly formed of claystone porphyries, with some mica-schist, & with much sandstone & jaspery rocks exactly like the rocks in the gypseous formation, & no doubt belonging to its basal

series. In several other places also, during the accumulation of the gypseous formation, its basal parts & the underlying porphyritic conglomerate must likewise have been already partially upheaved & exposed to wear & tear; near the Puente del Inca & at Coquimbo, there must have existed masses of mica-schist or some such rock, whence were derived the many small pebbles of opaque quartz. It follows from these facts, that in some parts of the Cordillera the upper beds of the gypseous formation must lie unconformably on the lower beds; & the whole gypseous formation, in parts, unconformably on the porphyritic conglomerate; although I saw no such cases, yet in many places the gypseous formation is entirely absent; & this, although no doubt generally caused by quite subsequent denudation, may in others be due to the underlying porphyritic conglomerate having been locally upheaved before the deposition of the gypseous strata, & thus having become the source of the pebbles of porphyry embedded in them. In the porphyritic conglomerate formation, in its lower & middle parts, there is very rarely any evidence,

with the exception of the small quartz pebbles at Jajuel near Aconcagua, & of the single pebble of granite at Copiapo, of the existence of neighbouring land: in the upper parts, however, & especially in the district of Copiapo, the number of thoroughly well-rounded pebbles of compact porphyries make me believe, that, as during the prolonged accumulation of the gypseous formation the lower beds had already been locally upheaved & exposed to wear & tear, so it was with the porphyritic conglomerate. Hence in following thus far the geological history of the Cordillera, it may be inferred that the bed of a deep & open, or nearly open, ocean was filled up by porphyritic eruptions, aided probably by some general & some local elevations, to that comparatively shallow level at which the cretaceo-oolitic shells first lived. At this period, the submarine craters yielded at intervals a prodigious supply of gypsum & other mineral exhalations, & occasionally, in certain places, poured forth lavas, chiefly of a feldspathic nature: at this period, islands clothed with fir-trees & composed of porphyries, primary rocks, & the lower gypseous strata had already been locally upheaved, & exposed to the action of the waves;—the general movement, however, at this time having been over a very wide area, one of slow subsidence, prolonged till the bed of the sea sank several thousand feet.

In central Chile, after the deposition of a great thickness of the gypseous strata, & after their upheaval, by which the Cumbre & adjoining ranges were formed, a vast pile of tufaceous matter & submarine lava was accumulated, where the Uspallata chain now stands; also after the deposition & upheaval of the equivalent gypseous strata of the Peuquenes range, the great thick mass of conglomerate in the valley of Tenuyan was accumulated: during the deposition of the Uspallata strata, we know absolutely, from the buried vertical trees, that there was a subsidence of some thousand feet; & we may infer from the nature of the conglomerate in the valley of Tenuyan, that a similar & perhaps contemporaneous movement there took place. We have, then, evidence of a second great period of subsidence; &, as in the case of the subsidence which accompanied the accumulation of the cretaceo-oolitic strata, so this latter subsidence appears to have been complicated by alternate or local elevatory movement—for the vertical trees, buried in the midst of the Uspallata strata, must have grown on dry land, formed by the upheaval of the lower submarine beds. Presently I shall have to recapitulate the facts, showing that at a still later period, namely, at nearly the commencement of the old tertiary deposits of Patagonia & of Chile, the continent stood at nearly its present level, & then, for the third time, slowly subsided

several hundred feet, & was afterwards slowly re-uplifted to its present level.

The highest peaks of the Cordillera appear to consist of active or more commonly dormant volcanos,—such as Tupungato, Maypu, & Aconcagua, which latter stands 23,000 feet above the level of the sea, & many others. The next highest peaks are formed of the gypseous & porphyritic strata, thrown into vertical or highly inclined positions. Besides the elevation thus gained by angular displacements, I infer, without any hesitation,—from the stratified gravel-fringes which gently slope up the valleys of the Cordillera from the gravel-capped plains at their base, which latter are connected with the plains, still covered with recent shells on the coast,—that this great range has been upheaved in mass, by a slow movement, to an amount of at least 8000 feet. In the Despoblado valley, north of Copiapo, the horizontal elevation, judging from the compact, stratified, tufaceous deposit, capping the distant mountains at corresponding heights, was about 10,000 feet. It is very possible, or rather probable, that this elevation in mass may not have been strictly horizontal, but more energetic under the Cordillera, than towards the coast on either side; nevertheless, movements

of this order may be conveniently distinguished from those by which strata have been abruptly broken & upturned. When viewing the Cordillera, before having read Mr. Hopkins's profound Researches on Physical Geology, the conviction was impressed on me, that the angular dislocations, however violent, were quite subordinate in importance to the great upward movement in mass, & that they had been caused by the edges of the wide fissures, which necessarily resulted from the tension of the elevated area, having yielded to the inward rush of fluidified rock, & having thus been upturned.

The ridges formed by the angularly upheaved strata, are seldom of great length: in the central parts of the Cordillera, they are generally parallel to each other, & run in north & south lines; but towards the flanks, they often extend more or less obliquely. The angular displacement has been much more violent in the central than in the exterior main lines; but it has likewise been violent in some of the minor lines on the extreme flanks. The violence has been very unequal on the same short lines; the crust having apparently tended to yield on certain points along the lines of fissures. These points, I have

endeavoured to show, were probably first foci of eruption, & afterwards of injected masses of porphyry & andesite.* The close similarity of the andesitic granites & porphyries, throughout Chile, Tierra del Fuego, & even in Peru, is very remarkable. The prevalence of feldspar cleaving like albite, is common not only to the andesites, but (as I infer from the high authority of Prof. G. Rose, as well as from my own measurements) to the various claystone & greenstone porphyries, & to the trachytic lavas of the Cordillera. The andesitic rocks have in most cases been the last injected ones, & they probably form a continuous dome under this great range: they stand in intimate relationship with the modern lavas; & they seem to have been the immediate agent in metamorphosing the porphyritic conglomerate formation, & often likewise the gypseous strata, to the extraordinary extent to which they have suffered.

With respect to the age at which the several parallel ridges composing the Cordillera were upthrown, I have little evidence. Many of them may have been contemporaneously elevated & injected, in the same

* Mr Murchison, & his companions state (Geolog. Proc. vol. iii. p. 747), that no true granite appears in the higher Ural Mountains; but that syenitic greenstone—a rock closely analogous to our andesites—is far the most abundant of the intrusive rocks.

of several points…

manner* as in volcanic archipelagoes lavas are contemporaneously ejected on the parallel lines of fissure. But the pebbles apparently derived from the wear & tear of the porphyritic conglomerate formation, which are occasionally present in the upper parts of this same formation & are often present in the gypseous formation, together with the pebbles from the basal parts of the latter formation in its upper strata, render it almost certain that portions, we may infer ridges, of these two formations were successively upheaved. In the case of the gigantic Portillo range, we may feel almost certain that a pre-existing granitic line was upraised (not by a single blow, as shown by the highly inclined basaltic streams in the valley on its eastern flank) at a period long subsequent to the upheavement of the parallel Peuquenes range.*B Again, subsequently to the upheavement of the Cumbre chain, that of Uspallata was formed & elevated; & afterwards, I may add, in the plain of Uspallata, beds of sand & gravel were violently

* Volcanic Islands, &c. by the Author, p. 129.

* I have endeavoured to show in my Journal (2d edit. p. 321), that the singular fact of the river, which drains the valley between these two ranges, passing through the Portillo & higher line, is explained by its slow & subsequent elevation. There are many analogous cases in the drainage of rivers: see Edinburgh New Phil. Journal vol. xxviii. p. 33 & 44.

crust has been stretched & yielded many times over the same points. With respect to the age of the axes of elevation between the Pacific & the Cordillera, I know little: but there are some lines which must—namely, those running north & south in Chiloe, those eight or nine east & west, parallel, far-extended, most symmetrical uniclinal lines at P. Rumena, & the short N.W.-S.E. & N.E.-S.W. lines at Concepcion—have been upheaved long after the formation of the Cordillera. Even during the earthquake of 1835, when the linear north & south islet of St. Mary was uplifted several feet above the surrounding area, we perhaps see one feeble step in the formation of a subordinate mountain-axis. In some cases, moreover, for instance, near the baths of Cauquenes, I was forcibly struck with the small size of the breaches cut through the exterior mountain-ranges, compared with the size of the same valleys higher up where entering the Cordillera; & this circumstance appeared to me scarcely explicable, except on the idea of the exterior lines having been subsequently upthrown, & therefore having been exposed to a less amount of denudation. From the manner in which the fringes of gravel are prolonged in unbroken slopes up the valleys of the Cordillera, I infer that most of the greater dislocations took place during the earlier parts of the great elevation in mass: I have, however, elsewhere given a case, & M. de

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Tschudi* has given another, of a ridge thrown up in Peru across the bed of a river, & consequently after the final elevation of the country above the level of the sea.

Ascending to the older tertiary formations, I will not again recapitulate the remarks already given at the end of the Fifth Chapter,—on their great extent, especially along the shores of the Atlantic,—on their antiquity, perhaps corresponding with that of the eocene deposits of Europe,—on the almost entire dissimilarity, though the formations are apparently contemporaneous, of the fossils from the eastern & western coasts, as is likewise the case, even in a still more marked degree, with the shells now living in these opposite though approximate seas,—on the climate of this period not having been more tropical than what might have been expected from the latitudes of the places under which the deposits occur; a circumstance rendered well worthy of notice, from the contrast with what is known to have been the case during the older tertiary periods of Europe, & likewise from the fact of the southern hemisphere

* Reise in Peru, Band 2. s. 8:—Author's Journal, 2d edit. p. 359.

having suffered at a much later period, apparently at the same time with the northern hemisphere, a colder or more equable temperature, as shown by the zones formerly affected by ice-action. Nor will I recapitulate the proofs of the bottom of the sea, both on the eastern & western coast, having subsided 700 or 800 feet during this tertiary period;

the movement having apparently been co-extensive, or nearly co-extensive, with the deposits of this age. Nor will I again give the facts & reasoning on which the proposition was founded, that when the bed of the sea is either stationary or rising, circumstances are far less favourable, than when its level is sinking, to the accumulation of conchiferous deposits of sufficient thickness, extension, & hardness to resist, when upheaved, the ordinary vast amount of denudation. We have seen that the highly remarkable fact of the absence of any extensive formations containing recent shells, either on the eastern or western coasts of the continent,—though these coasts now abound with living mollusca,—though they are, & apparently have always been, as favourable for the deposition of sediment as they were when the tertiary formations were copiously deposited,—& though they have

been upheaved to an amount quite sufficient to bring up strata from the depths the most fertile for animal life,—can be explained in accordance with the above proposition. As a deduction, it was also attempted to be shown, first, that the want of close sequence in the fossils of successive formations, & of successive stages in the same formation, would follow from the improbability of the same area continuing slowly to subside from one whole period to another, or even during a single entire period; & secondly, that certain epochs having been favourable at distant points in the same quarter of the world for the synchronous accumulation of fossiliferous strata, would follow from movements of subsidence having apparently, like those of elevation, contemporaneously affected very large areas.

There is another point which deserves some notice, namely, the analogy between the upper parts of the Patagonian tertiary formation, as well as of the upper possibly contemporaneous beds at Chiloe & Concepcion, with the great gypseous formation of Cordillera; for in both formations, the rocks, in their fusible nature, in their containing gypsum & in many other characters, show a connection, either intimate or remote, with volcanic action; & as the strata in both were accumulated during subsidence, it appears at first natural to connect this sinking movement with a state of high activity in the neighbouring volcanos.

During the cretaceo-oolitic period this certainly appears to have been the case at the Puente del Inca, judging from the number of intercalated lava-streams in the lower 3000 feet of strata; but generally, the volcanic orifices seem at this time to have existed as submarine solfataras, & were certainly quiescent compared with their state during the accumulation of the porphyritic conglomerate formation. During the deposition of the tertiary strata, we know that at S. Cruz, deluges of basaltic lava were poured forth; but as these lie in the upper part of the series, it is possible that the subsidence may at that time have ceased: at Chiloe, I was unable to ascertain to what part of the series the pile of lavas belonged. The Uspallata tuffs & great streams of submarine lavas, were probably intermediate in age between the cretaceo-oolitic & older tertiary formations, & we know from the buried trees that there was a great subsidence during their accumulation; but even in this case, the subsidence may not have been strictly contemporaneous with the great volcanic eruptions, for we must believe in at least one intercalated period of elevation, during which the ground was upraised on which the now buried trees grew. I have been led to make these remarks, & to throw some doubt on the strict contemporaneousness of high volcanic activity & movements of subsidence, from the conviction

impressed on my mind by the study of coral formations,* that these two actions do not generally go on synchronously;—on the contrary, that in volcanic districts, subsidence ceases as soon as the orifices burst forth into renewed action, & only recommences when they again have become dormant.

At a later period, the Pampean mud, of estuary origin, was deposited over a wide area,—in one district conformably on the underlying old tertiary strata, & in another district unconformably on them, after their upheaval & denudation. During & before the accumulation, however, of these old tertiary strata, &, therefore, at a very remote period, sediment, strikingly resembling that of the Pampas, was deposited; showing during how long a time in this case the same agencies were at work in the same area. The deposition of the Pampean estuary mud was accompanied, at least in the southern parts of the Pampas, by an elevatory movement, so that the M. Hermoso beds probably were accumulated after the upheaval of those round the S. Ventana; & those at P. Alta after the upheaval of the Monte Hermoso strata; but there is some reason to suspect that one period

* The Structure, &c. of Coral Reefs, p. 140.

intervened, during which mud was deposited over the coarse sand of the Barrancas de S. Gregorio, & on the higher parts of Banda Oriental. The mammiferous animals characteristic of this formation, many of which differ as much from the present inhabitants of S. America, as do the eocene mammals of Europe from the present ones of that quarter of the globe, certainly co-existed at B. Blanca with twenty species of mollusca, one balanus, & two corals, all now living in the adjoining sea: this is likewise the case in Patagonia with the Macrauchenia, which co-existed with eight shells, still the commonest kinds on that coast. I will not repeat what I have elsewhere said, on the place of habitation, food, wide range, & extinction of the numerous gigantic mammifers, which at this late period inhabited the two Americas.

The nature & grouping of the shells embedded in the old tertiary formations of Patagonia & Chile, show us, that the continent at that period must have stood only a few fathoms below its present level, & that afterwards it subsided over a wide area, 700 or 800 feet. The manner in which it has since been re-brought up to its actual level, was described in detail in the first & second chapters. It was there

that recent shells are found on the shores of the Atlantic, from Tierra del Fuego northward for a space of at least 1180 nautical miles, & at the height of about 100 feet in La Plata, & of 400 feet in Patagonia. The elevatory movements on this side of the continent have been slow; & the coast of Patagonia, up to the height in one part of 950 feet, & in another of 1200 feet, is modelled into eight, great, step-like, gravel-capped plains, extending for hundreds of miles with the same heights; this fact shows that the periods of denudation (which, judging from the amount of matter removed, must have been long-continued) & of elevation were synchronous over surprisingly great lengths of coasts. On the shores of the Pacific, upraised shells of recent species, generally, though not always, in the same proportional numbers as in the adjoining sea, have actually been found over a north & south space of 2075 miles, & there is reason to believe that they occur over a space of 2480 miles. The elevation on this western side of the continent has

not been equable; at Valparaiso, within the period during which upraised shells have remained undecayed on the surface, it has been 1300 feet, whilst at Coquimbo, 200 miles northward, it has been within this same period only 252 feet. At Lima, the land has been uplifted at least eighty-feet since Indian man inhabited that district; but the level within historical times apparently has subsided. At Coquimbo, in a height of 364 feet, the elevation has been interrupted by five periods of comparative rest. At several places the land has been lately, or still is, rising both insensibly & by sudden starts of a few feet during earthquake-shocks; this shows that these two kinds of upward movement are intimately connected together. For a space of 775 miles, upraised recent shells are found on the two opposite sides of the continent; & in the southern half of this space, it may be safely inferred from the slope of the land up to the Cordillera, & from the shells found in the central part of Tierra del Fuego, & high up the river Santa Cruz, that the entire breadth of the continent has been uplifted. From the general occurrence on both coasts of successive lines of escarpments, of sand-dunes & marks of erosion, we must conclude that the elevatory movement has been normally interrupted by periods, when the land either was stationary, or when it rose at so slow a rate as not to resist the average denuding power of the waves, or when it subsided. In the case of the present high sea-cliffs of Patagonia & in other analogous instances, we have seen that the difficulty in understanding how strata can be removed at those depths under the sea, at which the currents & oscillations of the water are depositing a smooth surface of mud, sand, & sifted pebbles, leads to the suspicion that the formation or denudation of such cliffs has been accompanied by a sinking movement.

In South America, everything has taken place on a grand scale, & all geological phenomena are still in active operation. We know how violent at the present day the earthquakes are, we have seen how great an area is now rising, & the plains of tertiary origin are of vast dimensions; an almost straight line can be drawn from Tierra del Fuego for 1600 miles northward, & probably for a much greater distance, which shall intersect no formation older than the Patagonian deposits; so equable has been the upheaval of the beds, that throughout this long line, not a fault in the stratification or abrupt dislocation was anywhere observable. Looking to the basal, metamorphic & plutonic rocks of the continent, the areas formed of them are likewise vast; & their planes of cleavage & foliation strike over surprisingly great spaces in uniform directions. The Cordillera, with its pinnacles here & there rising upwards of 20,000 feet above the level of the sea, ranges in an unbroken line from Tierra del Fuego, apparently to the Arctic circle. This grand range has suffered both the most violent dislocations, & slow, though grand, upward & downward movements in mass; I know not whether the spectacle of its immense valleys, with mountain-masses of once liquefied & intrusive rocks now bared & intersected, or whether the view of those plains, composed of shingle & sediment hence derived, which stretch to the borders of the Atlantic Ocean, is best adapted to excite our astonishment at the amount of wear & tear which these mountains have undergone.

The Cordillera from Tierra del Fuego to

Mexico, is penetrated by volcanic orifices, & those now in action are connected in great trains. The intimate relation between their recent eruptions & the slow elevation of the continent in mass,* appears to me highly important, for no explanation of the one phenomenon can be considered as satisfactory which is not applicable to the other. The permanence of the volcanic action on this chain of mountains is, also, a striking fact; first, we have the deluges of submarine lavas alternating with the porphyritic conglomerate strata, then occasionally feldspathic streams & abundant mineral exhalations during the gypseous or cretaceo-oolitic period: then the eruptions of the Uspallata range, & at an ancient but unknown period, when the sea came up to the eastern foot of the Cordillera, streams of basaltic lava at the foot of the Portillo range; then the old tertiary eruptions; & lastly, there are here & there amongst the mountains much worn & apparently very ancient volcanic formations without any craters; there are, also, craters quite extinct, & others in the condition of solfataras, & others occasionally or habitually in fierce action. Hence it would appear that the Cordillera, has been, probably with some quiescent periods, a source of volcanic matter from an epoch anterior to our cretaceo-oolitic formation to the present day; & now the

* On the Connection of certain Volcanic Phenomena in S. America. Geolog. Transact. vol. v. p. 609.

earthquakes, daily recurrent on some part of the western coast, give little hopes that the subterranean energy is expended.

Recurring to the evidence by which it was shown that some at least of the parallel ridges, which together compose the Cordillera, were successively & slowly upthrown at widely different periods; & that the whole range certainly once, & almost certainly twice, subsided some thousand feet, & being then brought up by a slow movement in mass, again, during the old tertiary formations, subsided several hundred feet, & again was brought up to its present level by a slow & often interrupted movement; we see how opposed is this complicated history of changes slowly effected, to the views of those geologists who believe that this great mountain-chain was formed in late times by a single blow.

look to the analogies drawn from the changes now in progress in the earth's crust, whether to the manner in which volcanic matter is erupted, or to the manner in which the land is historically known to have risen & sunk;

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I have endeavoured elsewhere to show,* that the excessively disturbed condition of the strata in the Cordillera, so far from indicating single periods of extreme violence, presents insuperable difficulties, except on the admission that the masses of once liquefied rocks of the axes were repeatedly injected, with intervals sufficiently long for their successive cooling & consolidation. Finally, if we

* Geolog. Transact. vol. v. p. 626.

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Section of an almost quadrant sloped curved block of

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Citation: John van Wyhe, ed. 2002-. The Complete Work of Charles Darwin Online. (http://darwin-online.org.uk/)