RECORD: Darwin, C. R. Geological diary: Valparaiso. (1834-1835) CUL-DAR35.371-376 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)

REVISION HISTORY: Transcribed by Kees Rookmaaker, corrections and editing by John van Wyhe 6.2011. 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.

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Reproduced with the permission of the Syndics of Cambridge University Library and William Huxley Darwin.

See the introduction to the Geological Diary by Gordon Chancellor.


371

Valparaiso 1834 (1)

All used except p. 18-21 on structure of Cauquenes mountains

The prevailing rock in the neighbourhead of the town is gneiss: it abounds with black mica, which thickly coats the layers. — is very subject to decomposition, hence the solid rock is seldom seen, being covered by great mass of reddish earth (a); which is This in many places has so entirely lost its original structure that with the excepting of rocks the presence of some vein it would never be guessed to be in situ.

The formation is however chiefly remarkable by the infinite number of veins which traverse it. These veins are oftentimes parallel to each other, but not generally in same direction with the cleavage: videlicet, gneiss cleaving S by E & N by W, veins running SE & NW. — It is often difficult to know which term, vein or bed, to apply to some of these layers; They are at other times irregular & enlarge into masses, which send off small veins, & then suddenly thins out. — They are clearly contemporaneous veins. —

They are composed of quartz & feldspar, oftentimes each separate, frequently containing mica & becoming coarsely granitic & this structure passes into eurite. — I have seen in the veins, black schist (2156: 2157), also irregular masses of pachnite? epidote? (2140). — This latter substance also occurs in layers parallel to cleavage of gneiss; we shall hereafter see it abundantly in the metamorphic breccias & the greenstones, which has been injected amongst them.

371 verso

This is very different from the Brazilian lithomarge. — We do not here have those brilliant colored clays, each spot marking where original mineral would have stood. —

At the mouth of the Maule I saw had specimens of a syenite fine grained grabite & granite porphyritic with large crystals of feldspar.

[8 words illeg] Fitton's appendices King's Australia1

Vide about concretions granitic R.N. p. 52

& p. 6 about [astedote] in metamorphic rocks Anglesea

Playfair p. 285 prophenes & greenstone will be discoveries from altered slate3

Daubuisson on my porphyritic breccia vol. II p. 309 & 3104

There are some heights given in review of Meyers travels in Geographical Journal5

Humboldt vol IV p. 59 tells of dionite in veins of gneiss6

On auriferous alluvium, Daubuisson vol. ii p. 479

1 William Henry Fitton was the author of the geological appendix to King 1827.

2 Red notebook.

3 Playfair 1802.

4 Aubuisson de Voisins 1819.

5 A review of Meiers 1826.

6 Humboldt 1819-1829.

372

Valparaiso (2)

I was surprised to find in one of the large veins a mass of blackish sonorous conch: fract: porphyry with acicular crystals of feldspar; this was surrounded by granitic material, into which it by transition the whole vein traversed the gneiss, parallel to others. — It was highly interesting finding a rock of so truely an igneous character origin in the veins, which clearly had not been injected. — In a steep ravine I found a true dyke of grey compact greenstone porph: with white feldspar (2143) traversing the layers of gneiss at rt ∠r it is about 12 feet wider.

I do not think it extends far: the junction of the two rocks is remarkably distinct & fine, the breadth of a line drawn by a pencil would touch both perfect kind; they adhaere firmly. — The line of junction, for several inches, may be quite even, then there will be small ∠r projections of gneiss, round which the gneiss folds.

This convinces me, the greenstone has not flowed in a continued stream, but merely injected in it. —

In the upper parts of the dyke the rock singularly altered differed from the lower (2144): it is soft & reddish & can only be distinguished from the surrounding gneiss by its massive form, the abscence [sic] of cleavage & porphyritic structure. It is curious fining two somewhat similar rocks

372 verso

(a) Granting gneiss to be a metamorphic rock: perhaps we may suppose that a contemp vein

Section of country from St Jago to Valparaiso (1835) very imperfect (being unwell)

Having crossed the plain of St Jago, the first low hills consists of porphyries (which from their nature have probably been injected. The commonest kinds are those with feldspar plates (2757) which is associated with feldspathic stone amygdaloid with agate & quartz & very much epidote (2850): —

These hills a little way further within contain four copper veins running NNW & SSE. — To the West of this to the Pacific, the whole country is granitic. — The Cuesta of Prado (pass in road [blank] ft) consists of rocks of harsh decomposed nature either siliceous or siliceo-feldspathic, containing grains of quartz of specks of chlorite or mica or ferruginous matter (2851: 52: 53). — They are traversed by an infinity of thin veins & vein-like masses of decomposing trappean rocks, some of which are porphyritic with feldspar. — Some of these appear to be greenstones (2857: 58)

At the foot of the Cuesta of Zapata (pass in road — ft) there is a decomposing gneiss, where the mica is replaced by a decomposed green mineral, contains hard & large concretions, cleavage exceedingly obscure. Above these we have a hill such as Prado, with the same class of veins. I am quite in doubt concerning the nature of these veins; in form they

P2

373

Valparaiso (3)

one occurring in a dyke, the other in contemp: vein. — In several places where the gneiss was disintegrated, I w observed large globular masses of a rather different mineralogical appearance & nature standing out.

They are granitic in structure & composed of feldspar, hornblende & mica (2141); are excessively tough & hard.

These masses are nearly sphaerical — 2 to 3 feet in diameter & appear & are like boulders: the layers of gneiss are curvilinear & fold round the balls. I nearly at first thought they were rolled masses of rock imbedded in the gneiss. From closer examination of their surface, I have no doubt they are of concretionary origin similar to those we have seen in slate of T del Fuego & the great tertiary beds. — Is not this another point of analogy between the crystalline (metamorphic) rocks & those of the sedimentary rocks. —

I have said the rock is gneiss, it sometimes in few places contains hornblende, & looses its mica. I saw in one place a true (injected) granite with crystals of hornblende nearly an inch long — near to this there were angular pieces of syenite — (in same manner as at St [Salandra]) imbedded in gneiss mostly & this again now mingled with great amorphous masses, of white quartz & white feldspar in large crystals. —

373 verso

(a) continued. The stone which is closely included between these veins is of a different nature. they resemble contemporaneous quartz in veins, splitting uniting twisting & thinning out. — In nature to injected veins. Near top of hill I think the veins certainly belong to the former class. — specimens (2854: 55: 56). — Mem: the porph. greenstone included in contemp vein at Valparaiso. gold mines have been worked in C. of Prado. —

I cannot conjecture whether the substance of these hills belongs to the gneiss or to the granite & signite which doubtless underlie this formation. — I incline to the latter. —

Section from Valparaiso to Bell mountain (1835). —

Passed over to Limache an undulating country thickly covered with alluvium, which is highly auriferous, certain strata contain most & are hence mined. — The same fact occurs near Plazilla. — The alluvium here resembles that of the Plazilla. Is hence stratified & probably marine. — Passed two small paps one of granite &other syenite, their distinctness gave probability to my idea that nearly all the granite rocks in this district are gneiss. — Close South of Campaña there is a larger hill of syenite containing mica, which is remarkable from having contained a silver mine. —

From the S. foot of Bell to village of Quillota all rock either decomposing gneiss or granite (later probably gneiss without its cleavage apparent) many small veins of trappean rocks. —

374

Valparaiso (4)

Cleavage

At the Laguna found a great mass of rock of greenstone with specks of pyrites & occasional large crystals of feldspar & hornblende

At the Waterfall (near Plazilla) it is greenstone with distinct cleavage (owing to chlorite?) (2154)

In many parts of the hills, the solid rock is coated with an indurated red clay containing small fragments of gneiss & some few large blocks (& very rarely pebbles). — I believe however it was deposited beneath the sea? —

To the North I found granitic rock till I crossed the Cerro de Chilicauquen. To the South to the neighbourhead of R. Maypo: beyond this they were covered by the great tertiary formation. South of the R. Rapel however I crossed one ridge, composed chiefly of feldspar & quartz; also with mica: an extensive dyke of porphyritic greenstone (abounding with iron) th traversed this rock in a N & S. direction. — Near Colchagua there was an isolated pap of quartz. —

I tried very carefully in all parts to ascertain the direction of cleavage: the disintegrated gneiss not often allowed this & the direction was certainly variable. I have seen it E & W & again WNW 1/2 N & ESE to S. dipping at an angle of 35°. — But certainly the prevailing dip direction in the greater proportion of cases, was within a point

374 verso [blank]

375

Valparaiso (5)

of each side of S by E & N by W. —

The hills, or bank, beneath which Valparaiso is built has a height of about 13 to 1500 ft. the highest being 1626 ft. In waling about the country, it is evident all the lines of hills are nothing more than what is left, unremoved, between the valley, which slope to the sea. — The country is in fact a gently sloping platform of gneiss, traversed by numerous broard deep flat valleys of same figure as those eaten into plains of Patagonia: & probably by

The summit of the hills, (or intervals between valleys) are quite flat topped: a person approaching some of them would suppose he would find there now a plain on summit.

Reason will presently be given to show

The sea has probably been the agent in leveling & during the gradual retreat of scraping out the valleys.

I collected, during my excursion, geological notices of several localities.

I at present see no better order of relating them than that in which I observed them. —

The Cerro de Chilicauguen lies between the great valley of Quillota & the lesser ones of Quintero, near to its base we loose the gneiss & in its place find porphyritic greenstones. The general basis is (2263) is a compact (∠r semi=conch. fracture) dark-

375 verso [blank]

376

Chili Valparaiso (6)

slate colored feldspathic rock. — this is oftentimes contains (a) acicular or ordinary crystals of feldspar. — The bass at other planes becomes slaty & passes into a greenstone. — Perhaps the porphyritic varieties are most abundant high up the Cerro. — The range appears to run E & W. but I believe it is merely formed by the removal of the immense masses, which have formed hollowed the great flat bounding valleys, & is in short a hill of the same class as those near Valparaiso, but with this difference that here probably there have been several N & S. lines of upheaval. — although the present form is that of E & W. —

In the South of valley of Quillota, the Campana, a mountain 4716 (measured barometrically by Mr Eck)1 stands conspicuous. In ascending by the most gradual line of ridges I passed an intricate alternations of porphyritic granite, altered slate, breccias & porphyries: descending on foot by a steep front ravine on West side at right angles to direction of mountain I obtained a good section of the upper 2/3d of the formation.

The lowest rock appears to be granite, composed solely of feldspar & quartz: its connection with the superimposed masses I did not see; but in a like manner, when riding up the towards Aconcagua, I passed repeatedly this variety mingled alternating with porphyritic greenstones & red porphyries; the upper parts of the mountains judging from color & appearance being of same nature as the Campana. M. Gay (p. 8. Ann. de Science)2 states that near Mancagua, a true granite supports a sandstone alternating with "arkose"? which passes into a pudding stone

1 Frederick Andrew Eck (1806 or 1807-1884), Swiss-born banker and collector of mineral specimens in South America, 1825-1852.

2 Gay 1833.

376 verso

(a) This basis becomes strong or earthy & looses occasionally its crystalline structure. —

1835. From a more extended view, I feel no doubt but that this is a true E & W line of upheaval, or injection of the above porphyries. — The confusion caused by such a line crossing the N & S line to the South probably first determined the excavation of ground valley of Quillota. —


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