RECORD: Darwin, C. R. Geological diary: Copiapò. (6-7.1835) CUL-DAR36.597-610 Transcribed by Kees Rookmaaker, edited by John van Wyhe. (Darwin Online, http://darwin-online.org.uk/)
REVISION HISTORY: Transcribed by Kees Rookmaaker, edited by John van Wyhe 2.2012. RN1
NOTE: This document, part of the largest scientific document composed by Darwin during the voyage of the Beagle, is written mostly in ink. Marginal notes are here integrated into the text. This section dates from 2 June to 5 July 1845. The period is described in the Beagle Diary, pp. 575-591. Page 605verso includes, what we believe has never before been noted in Darwin historiography, possibly Darwin's oldest surviving use of the word 'evolution' though not in its modern biological sense.
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Reproduced with the permission of the Syndics of Cambridge University Library and William Huxley Darwin.
1835 Guasco — Copiapò 113
We then came to a ridge of pale coloured siliceous & talcaceous slates dipping E 60°-70°. —
To the East of this is a granitic district: the mountains, although in close neighbourhead are composed of 2 different kinds; one being an ordinary sort, the other ferruginous, fine grained with little mica:
each the two are not associated in one mass. — To the Eastward of this there sh is a great escarpement of porphy: breccia. — In our second days journey, there was a long & very regular ridge of laminated rocks dipping Easterly, chiefly consisting of white (alumino siliceo) limestones, more or less compact or crystalline. 2985 2986 — I neither know whether this is In this there were Terebratulae & fragments of shells. — I neither know, whether these belong to the slates of the ridge observed during first day. — nor what connection they have with the porph. breccia — They form a remarkable feature in the form of the country. — I saw very little in whole distance of true. porph. breccia; but rather red sandstones, conglomerates. or true purple porphyries.
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1835 Copiapò 114
to p. 145.
The valley of Copiapò first runs from the sea in
NSW ESE line to the town. — Above this it bends N & S. — Where near the Cordilleras is divided into three ravines of which the most northern one Jolquera penetrates in the most direct line, this however is not more than NW. The others ([Maniflas] & Joluera Pulido) run very southerly. — Hence in the section above the town it will be impossible to contract it into an E & W line, but, by neglecting the S & N bends & taking into account the acute angle formed by the ravine of Jolquera. — The section may be considered as running for about SE by E & NW by W line to the & within the Cordilleras.
From the sea to the Villa, I had little opportunity of examining the geology. — The distance is called 18 leagues, perhaps in a straight line it may be 30 miles. In the whole of this we have unstratified crystalline rocks.
I may perhaps except a little black, obscurely laminated fine grained more or less siliceous feldspathic rock (a); which I believe to be altered clay-slate; outside of this on the coast, we have true granite.
(a) Specimens were brought me from Lavata (about (apart) 100? miles North of Copiapò) by the Schooner of a lead coloured. slaty
si compact siliceo-feldspathic rock. — evidently of the same constitution as these.
1835 Copiapò 115
The more interior hills, which I examined consisted of syenitic greenstone & greendikes — granite & blackish porphyries, that kind which not unfrequently is associated with syenitic greenstone. These hills are remarkable from the vast numbers of brown dykes. — nearly all of which run in a N & S direction. —
Being harder than the granitic rocks: they form the crest or backbone of all the small transverse ridge which enter the main valley. — in the vicinity of the town syenitic greenstone & the blackish porphyries abound. In these hills there are scattered mines of gold, silver & copper. — Of the silver there formerly was a rich ore. —
Above To the East the junction of the Despoblado, we have the escarpement of the gypseous formations:
These strata either dip small angle to the West, are nearly horizontal or irregularly contorted.
This latter case seems owing to upheaved hillocks of greenstones & green feldspathic rocks, which have succeded the syenitic greenstone at Sierra Amarilla a hill is thus constituted (a) 3104 3105; an obscure stratification separates a green variety from a paler kind with little micaceous mineral disseminated. —
(a) I suspect these rocks are similar to the altered slates, so commonly found superimposed on the granitic rocks near the coast. —
1835 Copiapò 116
This hill is traversed by broard vein not very highly inclined (possibly a stratum) of a soft yellow ochreous earth; associated with gypsum & a soluble salt in ferruginous base. 3109 3108 — Sulphate of iron, for which the vein is mined 3106, sulphate of copper & some bright
yellow yellow crystals 3107. — The specimen is poor as an ore: in Atacama sulp of iron is found abundantly. — I have said, where the gypseous formation is first met with the stratification has not much distorted any decided inclination. this is as a n grand is succeeded by extensive, but not high East dip, in places the beds are much disturbed being highly inclined & even vertical. The strata appear to consist of a black calcareous stone, which will be subsequently be mentioned, more or less compact horn stones marked with dendritic manganese; some of which are very hard & green 3102; soft calcareo-aluminous rotten stones, 3103 either whiteish or very pale purple, & including thin layers of gypsum & veins of do. —
These alternate in all manners, the strata being thin & speaking generally of nearly same thickness. — From the various directions in which valleys have been scooped out, the surface of the bare hills present the same lined surface which dead plancks do. — These strata are succeded conformably by a mass
(a) In various parts there are mines of gold, silver & copper in these gypseous strata. — although not perhaps very rich.
1835 Copiapò 117
of thinly stratified compact pale colored conch: fracture siliceous limestone. 3099 3100 3101 — which this is associated some dull coloured limestone, containing broken crystals.
There are highly inclined towards the East & form a rather conspicuous N & S ridge in the country. —
I strongly suspect this is part of the very same line of elevation of the slaty rocks
line of elevation mentioned in my journey from Guasco to this valley. — Perhaps these super gypseous strata. replace. but not in full thickness those rocks, which we shall afterwards see occupy this position. — If this is the case there are the more superior highest ones in this country. (a) —
We then meet a band of unstratified porphyry hills, which on East side blend into a great thickness of dark purple rock of the porph. breccia formation, which has a regular dip to the East. — This does not consist of crystalline rocks & sedimentary ones now blended into one whole; but of distinct strata of purple porphyries separated by numerous & thick ones of a compact purpleish-red conglomerate, the pebbles of porphyry being frequently large & well rounded. — There was one hill of the common purple claystone porphyry, injected amongst the stratified rocks.
(a) If this chain was not elevated anteriorly to the more central ones, — this must be the case, for no degradation would remove so equally superior strata so as to leave in a long distance these calcareous slaty rocks always uppermost. —
1835 Copiapò 118
We have now reached the Hacienda of Potiero Seco (a) — Here in the [Juchada] on South side near the houses (or Los Hornitos) there is a line of disturbance, where I particularly examined the geology. — There is a steep & high hill, composed in its central parts of various porphyries, amongst which masses of "plate porphyry", with green & brown bases are prevalent. — This on the SE is covered by a stratum, with high inclination of some very curious rocks; their character changes incessantly generally being porphyritic with glassy cryst: of feldspar & several other minerals 2997 2998; some of it is highly amygdaloid with carb of lime & yellow mineral. 2996 — I call this a stratum,
but its parallelism with the superior beds is evident on upper surface (Bed: 1), but only in a few places on the lower. — here, especially in higher parts of hills it seems to blend with the unstratified mass: — (Bed 2d) 150-200 ft of a pale lilac hard conch fract base, scarcely crystalline occasionally porphy with glossy scattered cryst of yellow & green feldspar. but has 2999 3000 [receded] —
The rock is sonorous in places slightly laminated. — On a grand scale is strictly conformable to the mass of overlying
(a) omitted. —
I should mention that in the whole of this neighbourhead at
about an elevation of 10-2000 1000 to 1500 ft. there are the remains of a plain of sh stratified shingle. — There are the South side of the valley high up the some extensive cliffs which show the formation is of considerable thickness. — On the North, there are patches at a corresponding height. — The road which I followed from Guasco, crosses part of an this extensive plain & the descent into the valley is over its bro edge. — The higher mountains, which bound the valley rise like islands from it. — The plain slopes to seaward, is clearly of marine origin, & must be one of the highest, which I have seen in Chili. — The greater part of the following section lies beneath its level. — as the strata are then inclined, & the well-rounded gravel rounded horizontal the angular upheaval must have taken place beneath the sea. —
1835 Copiapò 119
strata, which all (neglecting partial disturbances) dip at ∠ 45° to ESE. — On close inspection the upper surface will be found slightly irregular. —
This bed preserving the very same tint of color paralellism, its average thickness &c can be traced for considerable distances. — On North side of valley of Copiapò, it is seen lying on a coarse, but firmly cemented conglomerate, composed of
true similar porphyries to those in (Bed 1).
At the time I thought the fusion of such a stratum
had formed by the same cause which has protruded the hill (A) had perhaps fused these & thus (Bed 1) had originated. — It is more probable that an uneven bottom of the ocean, was formed in one place by bare L porphyry. & in another by pebbles collected from its detritus. — We shall show that the Lilac porphyry is covered by aqueous deposits as it here rests on them. — There can be no doubt but what this has been a subaqueous stream of lava. — To return to our section: upon this lava there is (Bed 3) of very varying thickness, of a white moderately hard, aluminous tableau containing minute pebbles 3012; broken feldspar crystals & scales of mica. —
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1835 Copiapò 120
(Bed 3) In places has brecciated structure: generally very compact especially where of considerable thickness; stained pale purple in patches. — I consider it a fine grained tufa, which perhaps has undergone action of subsequent hear. — It is very remarkable, by containing a good many globular & pear shaped concretions, of the size from apple to man's head 3013
omitted to bring specimen, externally of dark rusty color, very tough, of the identical same brownish lilac porphyry as the underlying bed. —
I say they are concretions from their form, which kind of evidence will be understand, by comparing them to the calcareo hard sandstone concretions in the common sandstone of the tertiary strata of S. America. — I can only explain these by supposing small subaqueous "bombs"; a ball of melted lava shot through water under very high pressure would probably assume either globular or pear-shaped form. — It will at first appear surprising that there is no cellular structure; but neither is there in the lava. —
(bed 4). 2 to 300 ft thick of a conglomerate: the whole is of a dull reddish purple color, the matrix is very compact & hard & contains cryst particles of white carb of lime. 3020 — There are also broard veins of this.
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1835 Copiapò 121
(Bed 4) The pebbles are pretty well rounded: from the size of man's fist to head general size. —
All are of porhyries,
the for the greater proportion of the lilac. lava; proceeding probably from the degradation near within its crater. (a) — There was also one of a white rock, I believe the indurated tufa. 3000 3001 ... 3011 — The other specimens will give a good idea of the remaining 1/3 of pebbles: & will also serve as generally to illustrate all the porph. lavas, which subsequently will be described. —
Within the conglomerate, there are three thin beds of a black, compact non-fissile, coarse grained calcareous clay. slate. 2987 — These are wide apart & the junctions defines. — The conglomerate in its upper parts passes into a compact red sandstone. —
(Bed 5). Light green, fine-grained, compact, nodular structure non calc: sedimentary (altered argillaceous) 2988
(Bed 6) Purplish sandstone, coarser grained, cryst. patch of c. of lime. 2989 — slightly nodular structure. —
(Bed 7). Compact, conch. fract, dirty green, tolerably fine grained calcareous sedimentary rock 2990: sometimes, finely brecciated
(Bed 8) 20 ft: thick, of impure gypsum,
[illeg] in layers some contorted; not very compact, associated with aluminous matter
(a) Till this idea occurred to me, I had often been puzzled to explain presence of rounded pebbles of the subaqueous lavas, which I believed had been produced in deep water. — A crater fill of water, fragments of rocks, (& bombs) & mud would by the constant evolution of gazes during the semi-active phases cause much attrition; which during an eruption would together with angular pieces of lava (or inferior strata) be scattered abroard.
NB. — I believe amongst the 10 specimens one will be found of the Basis or Matrix? —
1835 Copiapò 122
& layers of the green rock as in (Bed 7)
) & 10) Dusky green do stone & again impure gypsum (a)
(Bed 11) A laminated green do stone, with same layers of a siliceo-ferruginous substance, 2991 (particles blended together) also with a quite white, indurated aluminous stone, veined with ferruginous matter, like (1993)
These are covered by thick mass of a brighter green do stone, with remarkable nodular or concretionary structure;
The strata are composed of balls of all sizes
(Bed 12) A great thickness probably at least 800 ft (I really cannot guess how much we have already described certainly many hundred ft.) of much laminated whitish soft aluminous substance; this either pale green or more generally ferruginous & brown. 2992 — is marked with dendritic manganes, traversed by calcareous veins. — Decomposes readily, [easily] into angular bits. — To in patches quite white & more indurated. 2993 — There are many layers of carb of lime & ferruginous matter. (carb: of iron?) 2993
Laminae sometimes contorted. — These are
somewhat similar, only more ferruginous than those at Plueclaro & Los Hornos. —
(Bed 13) Is thick enough to form a mountain. —
I believe the least thickness will be 1500 ft. —
(a) I feel no doubt but what some green strata which alternated with gypsum at Valle del Yeso in Portillo Pass, was of this nature. —
1835 Copiapò 123
consists of sedimentary rocks, which I have called sandstones, but which are too fine grained & compact to deserve that name. — The colors vary, being either dull (1) greenish (2) purplish or (2) brown & often mottled: 3016 either calcareous or not 2995; the particles seem blended together & finer even so as sometimes to approach the constitution of clay-slate. 3017 — The stone is generally very compact with modular structure & not much stratified. — There are numerous veins of carb of lime & brown spar? 3019 — This mass of stone is remarkable by containing 2 exactly parallel strata each about 18 inches thick & 80 ft apart. — They consist of a brown, very compact, so as to possess almost conch: fracture 3018 (& perhaps
brecciated aggregated structure) calcareous stone. — These from weathering project outwards & resemble dykes. — They may be seen for nearly a mile, on each hand, keeping same position & apparent breath in the great amorphous mass of sediments described. — The strata here dip from 30 to 40 to the East; that is a less angle than close to the hill of porphyry. —
Bed this last Bed is covered another of a yellowish color & this again by a purplish. —
This from the curve which the strata take upwards & towards the main range of porphyry (B).
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1835 Copiapò 124
as may be seen in the section & from some faults, is difficult to be distinguished. —
I have described the above section with much particularity, because it is highly illustrative: it must not be supposed that it gives an accurate idea of surrounding district. — There is not one stratum, which
does is not subject to extreme variation. — Even in the same ravine from 1 to 2 miles distant: compare with section, lower in valley
The black compact calcareous slate, forming thick sometimes curved plates, was much more abundant, much of the green sedimentary stone was replaced by red & these three alternated repeatedly; they likewise encroached on & alternated with the the white or ferruginous soft aluminous slaty substances. — These latter appear the proper receptacle for the gypsum, yet here scarcely any was to be found: whilst on North side of the main valley (where
a mine this substance is worked) the formation is much thicker, & so abounds with laminae of gypsum, as to become a gypseous tufa. — Some layers are two & three inches thick & pure; but generally it is very impure & aluminous. 3014 — There also is much laminated with a stalactiform kind of carb of lime. 3015
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1835 Copiapò 125
The hills or parts of mountain, thus constituted are of a uniform & very yellow color. — In this vicinity the upper compact sedimentary rock keeps to one character over the greatest extent. (dikes) (a) —
I look at the inferior subaqueous lavas & purple conglomerate as the upper limit of what corresponds to the porph: breccia further South. — We see above this both the gypseous & supergypseous formations. —
The line of elevation to which all the above easterly dips belong is very extensive. I
can could see it running in a NNE & SSW direction fro many miles in both ways. — The strata, close round Porphyry Hill (A) dip at greater angle than generally & here we also only have a narrow bands of strata with an anticlinal dip. — It is not strictly anticlinal however, the strata rather mantling around it: the dip being W by S. in place of WNW. — The two escarpements converge towards a point, much in same manner as described at Uspallata. — This W edge forms naturally would happen from rounded hill being forced upward through a resisting hard covering. — The Porphyry hill (A) therefore is a point of greatest
(a) All these strata are traversed by dykes. —
1835 Copiapò 126
violence in a long line. —
With this exception, the kind of elevation appears to have been, where by a vertical fault, the continuity of the strata is broken, & a fresh escarpement is found, but with same direction of dip. —
The greater part of these gypseous & super g. strata dip directly towards & abut on a great N & S. band of porphyritic & granitic hills. — In the upper parts of the strata, especially to North of Valley, there is an anticlinal curve, which dips to the West. — This however is subordinate to the general stratification. —
In this group (B) of great irregularly conical mountains the outer ones consist of a pale lilac conch. fracture. base porph. with fine crystals of feldspar. — in short precisely similar to the subaqueous lava. — I cannot doubt, but what the connection of it to one of these great masses in that of a stream & lava to its volcano. (a) —
traverses passes amidst these mountains. — After the lilac porphyry, dark green & blackish porphyries with few crystals of feldspar succeed. — These are traversed by some paps of syenitic greenstone. — Then comes the grand. central mass of this latter rock: Here the feldspar is more or less perfectly foliated
(a) If my view of the Bombs is correct it proves that the eruption took place at no great distance. —
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