RECORD: Darwin, C. R. Geological diary: [Valdivia] New form[ation] Sandstones (like 2576 rather harder) ferruginous acicular. [2.1835] CUL-DAR35.351-353 Transcribed by Kees Rookmaaker, edited by John van Wyhe. (Darwin Online, http://darwin-online.org.uk/).

REVISION HISTORY: Transcribed by Kees Rookmaaker and John van Wyhe, edited by van Wyhe 8.2011. 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.


351

(1)

new form. sandstone (like (2576 rather harder) ferruginous acicular plates in concretionary structures also veins of cyst. carb of line; passes into a quite white & softer sandstone & thence again irregularly into white aluminous. —

x frequently rest on a hard greenish black sandstone, distinct grains of quartz cemented together (2581): there is much ferruginous breccia of the soft white pieces, exactly like at Chiloe. These & white alum beds generally superior. — There is also much very soft earthy stone with minute white imbedded mineral looking like porph earth. — color dark brown (2582) or red: — The covering of the cliff soft dry friable reddish earth — doubtless same as to; South. would not be supposed to be aqueous deposit but capped with shells. — V gel Blackened but to very short depth: —

Contains lignite & silicified wood in abundance. — Also very few shells; either impressions (2583 in micaceous brown sandstone) of Venus Solen &c & sev others vestiges converted into lime. — Shells too imperfect to recognize. — Form same as at Chiloe. — Rather coarser not so many concret: chiefly differs in lines of pebbles being numerous. — some part many lines — generally small, less than egg — Consist of clay-slates passing into porphyries, some red, generally blackish or green — with very few & minute crystals: [sketch] When a person approaches the older slate which forms point — meets great bed of

page in pencil

351 verso

to common horizontal concretion of tertiary strata

Elevation — Recent shells

B. Blanca. ornithology

352

2)

of peb coarse pebbles, angular & rounded — composed of the neighbouring slates & to the quartz veins. — Is seen on other side of Isd to be thick conformable bed. — The form originally reposed on this old first slate —

These latter consist of black glossy clay slate, 3685 which is either smooth & over laminae, or appears abrased is [illeg] & laminae more contorted 2584 more abundant. — In spots immersed immense number of quartz veins. — cleavage here irreg. — There is some light brown stock of same construct 2586: — I imagine I see in these the construction of the ampelite = Cleavage at regular dip strike generally small: runs NW & SE: in the plain W & S.

These rocks are only found on P of Quinquina & the portions of which transverse at Pen: of Lamuy the softer strata to the South: This part is traversed by very many dykes: There is some shade of difference of the mineralogical nature of allum.

One may perhaps conjecture that they are of slightly different age although possessing nearly same direction & most within a yards of each of them: Beginning at the southern one — (they all lie within space of 1/4 of a mile)

(1) a very narrow dyke 4 or 5 inches wide composed of pale slate with feldspathic rock (2581) is vertical, thins out & thickens very suddenly. is seen terminated upwards by a round head. Vide Fig. 1. — (2) an irregular dyke, running NW & SE

of peb coarse pebbles...1/4 of a mile)] in pencil

352 verso

From 9— to 15 ft wide. nearly vertical. — traveling off sending off branches composed V. Fig.2. — composed of dark grey brilliant dolerite (2588) with glassy feldspar. — On the northern wall, the strata dip very regularly to NW as if tilted by this the slate here dips SW: & then have same strike with line of dyke: — (3) — About 18 inches wide composed of feldspathic rock blended with red particles very little smally crystalline (2589) followed for some distance the laminae strata of the calc: of slate & then suddenly bulging out, showed its true dyke-nature. — (4) a mass of narrow reticulated veins (evidently injected) composed of a strong mottled base with large layers of quartz & earthy feldspar (2590). — (5) dykes about 25 ft wide irregular chiefly composed of a very curious snow white stony feldspathic rock with large octagons of quartz (2592): part of the dyke was in a friable state of white friable powder. — Is not such material comes from volcano the origin of the white aluminous beds of Patagonia, Chiloe & here. — part of this dyke is constituted of same substance as the part a partially decomposed feldspathic greenstone base with large irregular crystals of feldspar & layers of quartz (2591). This dyke runs NW & SE, on its northern wall the slate dipped regularly to the NE by N, as if tilted by the dyke. — (6 & 7) dykes for 10 to 20 ft wide inclined composed and the last variety (2593 2591) & (8th) nearly the same

The [word deleted] last was inclined & runs rather more E & W than NW & SE nearly W & E: all these last varieties strongly remind me of some of the dyke at north part of P. Tres Montes traversing the granite & mica slate. — I believe there to be more than these but had not time to examine them. —

353

3) other side

May be when that the extreme point of island has a bluff & [illeg] evidently formed by the upheaval of these slates:

But that the general direction is about NW & SE. & from this & similar position are clearly perhaps are influenced by cleavage of slate, although very often having broken through its restraint:

I have said nothing about the stratification of the tertiary formation. — At northern part the dip is at about ∠ 30° to SW by S — seen in ship — satisfaction of dyke right across Island — seen by single lines of pebbles in fine sediment: some little curvilinear.

Again ridge of thick slate plain — perhaps horizontal & South extreme is formed by N by W dip generally. That [3d] one plain ? up to more than nearly 40° — The upheaved not quite regular dip varies — basset edge curved — Clearly owing to paps such an extreme. — Form of Island protected by mica slate owing to degradation

The fourth band of disturbance in S. America.

[sketch] ؟ Whether irregularity in direction of slate here & at Valdivia may be owing to these having been horizontal & so showing better the results of elevations & violence: the presumption is drawn for small regular dip seen near the some of the dykes

Tilting beneath the sea

All part of tilt at Concepcion: beds deposited on mica slate

page in pencil except 'of island' and 'perhaps are' and 'Whether irregularity ...some of the dykes' in ink

353 verso

Elevation — sorts of shells — at least 400 ft

Trochus. full of carb. Concholepas — Balanus. fissurella. Mytilus. —

The fact that only the northern part of Island is composed of the mica slates seems probably owing to their elevations by this system of dykes — Their direction to a certain degree has been apparently influenced by the strike of the cleavage, although very often breaking through the constraint of the laminae. —

In At northern boundary part of Island the slates dip NE. At Near to the extreme southern dyke it would appear dips to opposite direction S to SW. — It would almost appear like an anticlinal line; the band where dykes are most numerous being the axis. — I do not feel certain, for the dip is so very variable as is seen in proximity with the quartz veins: but the fact is worthy of attention, for in no locality to the South could I find any sort of proof in the older slates of such upheaval. Here the tertiary strata will prove such to have taken place & of course it is probable in every locality. age of dyke diff. mineralogical structure — supposed nearly horizontal laminae of slate. —

necessary to represent elevation.

form of island protection —

Could not see any alteration in neighbouring slates

Elevation — sorts of shells — at least 400 ft] in pencil

form of island protection —] in pencil


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

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