RECORD: Darwin, C. R. Geological diary: [Valparaiso] Rock about the town generally gneiss. (7.1834) CUL-DAR35.218-226 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)

REVISION HISTORY: Transcribed by Kees Rookmaaker, corrected and edited by John van Wyhe, corrections by Gordon Chancellor 4-5.2011. RN3

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.

Darwin was at Valparaiso in July-November 1834 and March-April 1835. See the Beagle Diary, ed. Keynes, pp. 249ff. Valparaiso is mentioned in South America, pp. 31-35.

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

See the introduction to the Geological Diary by Gordon Chancellor.


218

1 July. 24th. — (arrived 23d) [1834]

Rock about the town generally gneiss. — much traversed by veins, are rather beds with vein-like appearance, because they preserve one direction & are hence often parallel, to one another & where I could trace it to the cleavage of the gneiss. — This direction varies from SE by E to E. (without variation). ESE perhaps commonest. — The gneiss is generally very much disintegrated; of a bright red color; in many places it would be mistaken for earth, if it was not a vein yet retaining its hardness:

does not resemble Brazilian Lithomarge: rock remarkable excessive number of veins or beds generally of a mixture of quartz, feldspar; often times each separate, go frequently irregularly & coarsely granitic from which passes into [Eurite]. —

These veins enlarge into masses & thin out, therefrom send off irregular veins; but generally rather compose beds.

The character of bed & veins is here quite ambiguous: the veins — bed was remarkable. (— the veins are slightly dislocated & a little curved, of course more so where partake most of character of vein). — from containing a mass of true porphyry in a greater vein

this was surrounded by granitic material & passed into the of the rest of the bed. which traversed the usual. —

218 verso [blank]

219

(29.th) [July 1834]

All the highest hills are composed of gneiss which can hardly be distinguished from granitic mica quite black. —

[sonitine] syenite. — I found in one plain a disintegrated (therefore sand) mica slate or gneiss with its laminae running S by E & N by W. — There were not far off parallel quartzose veins running SE & NW

August 3d. I find the concretionary masses mention[ed] the first day are no way uncommon: they contain mica — are very hard. —

Found great mass of rock dipping at 35° to NNW 1/2 W. — ؟ true dip?

Generally gneiss, sometime contains little hornblende

Petrified wood ؟ Mr Cood ??1

1 Henry Cood, an English merchant residing in Valparaiso. Cood is also mentioned in the Galapagos notebook.

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220

(3) Saturday [2 August 1834?]

Walking to the West — I found some fine gneiss the laminae thickly coated with scales of black mica. — the direction was in two good examples situations varying from SSE to S 1/2 E (in one case even to the West of South. — near to this was much gneiss, where waving lines of mica showed on the grand scale the line of cleavage ( [blank] ).

Generally the rock is soft gneiss, with much black mica. — I found however one mass of syenite, where the crystals of hornblende were many one inch long — in another site angular pieces of syenite, such as as the hornblendic rocks at St Salvador. — were imbedded in a coarse gneiss. & this again was mingled with great amorphous masses of white feldspar & quartz crystallized in large crystals. —

I saw the mineral ( [blank] ) forming parallel layers in the gneiss & not as before contained in a vein-bed. — In a steep ravine I found a true dyke of grey compact sonorous greenstone porphyritic with white feldspar, traversing the gneiss layers of gneiss at right angles: it is about 12 feet wide. I do not think long it extends far: the junction

220 verso

small faults abound in the gneiss

221

(4)

of the two rocks is remarkably distinct & fine. — the breadth of a line drawn by a pencil would touch both perfect rocks kinds. — they adhaere firmly. — The line of junction in some places for several inches is very straight, then there will come small (1/4 to 1 inch long) angular points of the gneiss, round which the greenstone folds. — this convinces me that the greenstone has never flowed through the dyke, but merely been injected in it (or old contemp dyke all in the upper parts of the dyke the rock singularly differed from the lower; it is soft & reddish & can only be distinguished from the surrounding gneiss by its massive forms, the absence of cleavage & porphyritic structure. — Is not imperfect mica likewise present? —

I believe The gneiss surrounding the dyke, was a little contorted perhaps owing to its presence. —

It is curious finding the rock ( [blank] ) with acicular crystals of feldspar forming a part of a contemporaneous red vein. & this a dyke cutting through the the layer of the same gneiss. — (on the hypogene doctrine, it might be explained by supposing an anterior contemp vein or dyke, which do traverse slate at different angles) but I am doubtful how far it will explain appearance

221 verso [blank]

222

(5)

In very many parts of the hills, the solid rock is coated with an indurated red clay, containing small fragments of gneiss & some few larger blocks. (& very rarely pebbles) It has the appearance of having been formed beneath water, & clearly from the degradation of the neighbouring rocks

It so closely resembles the disintegrated gneiss, which remain in situ that they are often only with difficulty to be distinguished.

This substance is covered by a thick bed of reddish clayey earth & vegetable mould. —

(x)

As every one has heard of the upheaval of the coast of Chili, I took particular pains to see if any signs of former beaches were to be discovered. — I saw in many places at an elevation from about 12 to 15 feet at high water [mat], that the gneiss was worn as if by the waves of the sea: this must have been produced, anteriorly to the small elevation of 1822. — Also at about 50 ft. elevation there was in one cliff of gneiss one or two or 3 small caves which had the an somewhat of the appearance of a similar origin

222 verso

(x) 29th. [July 1834] — The highest bed of shells which I found was 1300 feet above the sea. — there was the Conchlepas, [Trochus] with its color quite perfect. [Fissurella] & a Patella:

But certainly the shells are not as nearly so numerous in the higher parts as beneath the level of 1000 feet: they also show more signs of age. — Generally in all these higher limits, they are only to be found, when partly preserved in a (not below) thin covering of vegetable mould. —

The shells often times are covered with Balanus & perforated & I should think without doubts have been old shells cast up on the beach. —

The highest land in the vicinity is 1626 feet, this being covered with same hardened clay (above which the shells always lie). I have no doubt it likewise has risen in period of colour-retaining shells. — During elevations the present hills formed outlying rocks, they became smaller islands with beaches, these beaches converted into vegetable mould from present hills. — the very summit of hills common site for the shells & small turf valleys. — Looking at the whole country from the 1626 hill, it resembles in form what the present streams would form; no one could imagine the recent residence of a sea. — Yet who can doubt that the ravines are a succession of the small inlets found on every coast. — they now drain the country. —

223

(6)

In the form of the land, with the above exceptions, I could find no evidence, that it had lately been raised, in the presence of recent shells However there was abundance. — I first found these these shells in a bed on a little flat piece of ground about 100 ft above the sea. — they were in great numbers; after this I saw them repeatedly in many places, generally very near the summits of hills, one of which attained a height 648 feet; indeed the whole face of the country may be said to be sprinkled over with these marine remains. — The shells are recent; the commonest is an Concholepas it is found [sketch] in the proportion of 50 to one. — a [coloro] Trochus is in groups common, it yet retains some of its color. —

A Fissurella was encrusted with a Corallina, also with its color. — there was a fragment of the great Balanus of these coasts & of an Echinus. —

These shells at the present day I have seen lying in great numbers on beaches; hence I suppose they live near the coast. — for the greater parts of the shells are merely seen in small pieces, excepting Concholepas, which from its strength has best withstood the injuries of

223 verso

Concholepas — littoral. Mr Stokes

The indurated clay very unfavourable for retaining organic remains. ؟ Why so? —

The number of Balaenidae & circular perforations, besides the broken condition make the resemblance with beach shells very perfect. —

224

(7)

time. — The shells occur either above the solid rock or the indurated clay; they are either exposed entirely to the atmosphere or imbedded in a covering of (a color) few inches thick of damp vegetable mould

They may be often seen with roots of grasses clinging growing around them, & partly showing above. —

There was no difference in any respect in position or state the shells from the highest station & the lowest; the upper ones retained some of their color. It appears to me a most extraordinary fact, that shells thus situated could last a century. —

Valparaiso has been built & [blank] years, & doubtless was placed originally near the sea: therefore we know of so many years duration.

From the littoral habits of these shells, their broken & loose uncovered position their occurrence in groups such different heights, they clearly have been left in their present position by a succession of elevations, each of which has left its tidal spaces with broken shells; the beach has subsequently been converted into vegetable mould. —

224 verso

That the ravines are formed by sea proved by shells

(Z)

so lately here have taken place

225

(8)

Indeed the shells are too numerous to allow the supposition, that any one (or even half a dozen) elevations could have raised the ground; at one period because a space of sea coast of this extent would not probably support so many, as we now see over the face of the country. With these facts in my mind, I was surprised at not being able on the coast to find traces of the former beaches, in the sloping valleys. no where was there a step-like form or line of pebbles (the gneiss does not form many pebbles, in the cliffs the outlines were angular & there were no horizontal lines of particular degradation. — On the [coast] the numerous valleys were sometimes sloping, but generally abrupt, narrow & very steep; they gave one the idea of being formed at an indefinite period since, & nothing called to mind as long as the fragm recent (z) & gradually receding action of the waves could be forgotten: they might well be supposed to only by streams,

The shells must soon perish: the indurated clay affords no certain presumption of the former residence of an ocean, & without these, the most discriminating eye might in vain (I think) look for the history of the changes which

225 verso

(z) The bank at the foot of which the town stands, as seen from the anchorage, is so [blank] feet high. We know then, that within a recent period the sea has reached up [blank] its height. — Yet viewing the curiously numerous deep gullies worn on its surface, they appear as regular as if worn by water on soft clay. — who can doubt, with the above statements that has played a great part in forming the deep furrows in the rock.

226

8 bis

I found a black gneiss alternating with a pale one, which layers had a direction about S by E & N by W. — but on the other hand another irregular cleavage which ran about E with & W. —

Found an immense mass of steep rock, which was greenstone with pyrites. — I do not know whether contemp. formation; this rock in places contained great crystals of hornblende. —

some signs of cleavage. —

At Cascade [pecipia] is composed of hornblende & feldspar with a distinct cleavage, which seems owing to laminae of chlorite (?) (2154)

The country I think ought to be considered as a sloping platform of gneiss cut into valleys of the same shape rather steeper. as at Coast of Patagonia. — the resemblance is very striking

the valleys are so numerous, that the platform is only seen in the flat tops of the hills, sloping at small angle to the sea. — In patches, which

226 verso

which must probably have formed, shallow bays when sea had different level. there are beds of soft sandstone, slaty clay- slate & conglomerates.

A small plain on St Jago road called Plonilla is elevated feet has a mean elevation of [blank] feet above the sea. — it is composed of beds of slightly agglutinated granitic sand. — pale clay, green sandy do & masses of coarse conglomerate, where many of the pebbles are large. the beds are horizontal. — this plain most probably formed a bay in some former period

In several parts of the neighbourhead. large one but chiefly in the water courses large masses of petrified wood are found. — these are universally supposed to be found formed by the present cause. upon examination however I found several rounded pieces amongst the conglomerate of granitic rock which forms the lower beds in this plain: the water courses have simply exposed them to view. — Perhaps these were formed when [sinuous] springs existed in the country

Saw many more concretions. — decomposed gneiss, 30 & 40 feet thick cleavage, very irregularly SSE & NNW. — some plain, in others SW & NE. —

The covering of hardened red clay I imagine to have been formed by simple alluvial action. The quantity of iron is chief difference in the concretion they being always pale. —


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