RECORD: Darwin, C. R. Geological diary: Guasco. (5.1835) CUL-DAR36.592-596 Transcribed by Kees Rookmaaker, edited by John van Wyhe. (Darwin Online,

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. See the Beagle Diary, pp. 572-575.

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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.


1835 Coquimbo — to Guasco 109

Upon this again we had the ordinary granite containing three thin parallel layers of the other kind. — Although in appearance this is very regular stratification: yet we find on close inspection in the common granite, veins of compact euritic granite, others of a black. hornblendic variety, angular imbedded fragments of same. —

Also same kind form of fragments of the white close grained kind, which alternated. — And in this latter again there are angular fragments of the common granite (themselves including the black pieces). — So that in place of simplicity, there is most intricate confusion. —

Proceeding in a NE direction: we come on a grand formation of mica slate, which appears to form a band parallel to the granite mountains. — Mica slate either with large contorted plates of quartz; or with thinner & more even ones, in the latter case the micaceous mineral passes very much into the character of clay slate. — The cleavage is all high about 70° the dip

592 verso [blank]


1835 Coquimbo — Guasco 110

not quite uniform, either about ENR or NE by E. — Near Sauce we again have granite in which are some copper mines. And on road from Sauce to Freirina (in valley of Guasco) we again come to a great band of micaceous schists. — The district of these slates, a short distance to the South abounds with copper mines. — The matrix of many of the veins is talcaceous: some pure soapstone, which I have seen curiously interlaced by thin plate of metallic copper. — The soapstone is thought a favourable sign. — Much of the ore are carbonates. — I saw in one specimen some fine sulphate of copper. — In these mountains, as well as the syenitic & greenstone ones to the North of Coquimbo, the few springs are all partially saline. — It may be suspected that these waters do not owe their origin to the very scanty rains of this country. — In these second mountains of micaceous slate; the proper rock is associated with some black glossy kinds. like

593 verso [blank]


1835 Guasco 111

ampelite, other brown glossy ones & some blue clay slate. — The cleavage irregular, perhaps N & S. the ordinary mean direction. —

V. before. The constitution of the rock is identical with the formations of Concepcion, Chiloe & Chonos. Archipelago. When in the morning, a horizontal stratum of white mist winding amongst the mountains represented an ocean. — I might have believed that there were islands in that latter Archipelago of Chonos. —

Near Freyrina an extensive trappean formation. At the Port of Guasco there is a great variety of curious slaty rocks; some very ferruginous, others jaspery (a). or feldspathic (ate pounded & used to make bricks for the furnaces). the latter often porph. with feldspar. — These lie to westward & I believe conformably to some perfect mica slate as before. — to which formation I feel no doubt they belong. — The dip is various in one part SW by S. & then ESE! — There are very many dykes & injected masses of brownish trappean rocks, probably belonging to hills above alluded to. —

594 verso

(a) Near Freyrina, there are some hills of what appears an altered hard slate. The laminae have an irregular N & S direction. —


1835 Guasco 112

[the page is numbered 596 and 112, and immediately follows 594 and 111. There is no sign of 595.]

V. All this maritime district of granitic rocks & various slates is highly auriferous & contains many good minerals of copper. —

In the Cordilleras. I hear of extensive gypseous. formation. — Fossil shell, chiefly Terebratulae are very abundant, near Guasco Alto. V. specimens; judging from a fragment they occur in a pale coloured calcareous certy rock. (a) — I saw specimen of the R. Claro Pecten (unequal) Bivalve & heard of Ammonites. —

From Guasco to the Hacienda of Potrero Seco, which is in the valley of Copiapò half way between the Villa & the Cordilleras. — From the country being a desert it is necessary to travel fast & hence the geology was nearly unintelligible.

About 2 leagues to the East of Ballenas saw the Escarpement of Porph. Breccia the road enters on this formation very obliquely. — The rocks being the red fine grained sandstone with cryst particles of carb of lime & the white calcareo siliceous stones, abo already mentioned, containing fragments of shells. (B) — The structure of the first low ranges, appeared of this nature

[sketch] W E

596 verso

(a) In the vicinity of Guasco Alto very many silver mines

(B) Part of this structure, namely the strata dipping internally is common on the face of the outer ranges & central Cordilleras. — We must suppose either, that a fault commonly takes place, or the ridge or crest of an anticlinal band — or that one side only is elevated

(1) [sketch] (2) [sketch]

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