RECORD: Darwin, C. R. Geological diary: Tahiti. (11.1835) CUL-DAR37.798-801 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)
REVISION HISTORY: Transcribed by Kees Rookmaaker, 9.2009. RN1
NOTE: This document, part of the largest scientific document composed by Darwin during the voyage of the Beagle, is written mostly in ink. Where pencil was used instead this is noted in the textual notes. Paper size 20 x 26 cm.
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Reproduced with permission of the Syndics of Cambridge University Library and William Huxley Darwin.
The only part of the Island which I visited is the NW extremity. — I also followed the river which joins the sea at P. Venus to near its source in the lofty central mountains. Nowhere, even in the pebbles, on the sea coast, did I meet with other than volcanic rocks. — M. Hoffman found Granite more to the South — (a) In the central mountains the commonest rock is a blackish grey
olivine. Lava nests of olivine, — its structure is frequently columnar 3376. — There is another abundant kind with large black crystals 3377. — I saw a few fragments of a true Trachyte, with large Crystal of glassy fractured Feldspar. — vasicular rocks, more or less amygdaloid, are found in great quantities. — I noticed one curious variety, when half the cell, was fitted up by calcareous matter & the other half siliceous 3378 3379: 80. — the line of division distinct & horizontal. — With these a few coarse sedimentary strata containing large, angular, & semi-rounded fragments of volcanic rocks, were interstratified. — Nearer to the coast the common varieties 3381 3382: 83, were compact, grey, &
(a) I may remark that the form of the land renders it probable, that no Crater yet could at the present day be found. M. [Ellis], whose opportunities for ascertaining such a point, must have been so good, states the same facts.
mottled stones, containing very few crystals. All the Lavas, but especially these last kind, have undergone decomposition to a very remarkable degree. — Hence we have very great thickness of Wacke. — With these, as before, there were some sedimentary strata. — In many cases, it was impossible from the crystalline structure being so completely lost, & the sedimentary strata. — In many cases, it was impossible from the crystalline structure being so completely lost, & the sedimentary deposits so fine to distinguish one class from the other. — It must have been in such strata that Werner traced a transition from Gravel & Clays to Wacke, & from Wacke to Trappean rock. — It must be remembered, how frequently subaqueous Lavas have a brecciated structure, the cement of which appears partly the effect of heat & partly of aqueous deposits. — if such underwent decomposition (& in vesicular rock generally, an absolute movement in the component particles) it would be impossible ever to decide on its origin. 3384: 85 — Specimens (3384. 85) show the decomposing lavas, & (3386: 3387) those soft stones 3386: 87, which I believe, but am not all sure are likewise Wackes ! —
In every part the strata. dip at a small angle, from the centre. towards the sea. — The outline of the lower exterior part of the mountains, is a uniform smooth slope, which from its nature, has been worn into flat topped sloping ridges, separated one from the other, by profound, steep sided, smooth ravines. —
I cannot doubt, that these
general sloping envelopes were formerly accumulated beneath the sea, & that they have been elevated into their present position. — Or which, is the same thing I believe the greater part of the Lavas have been subaqueous; the structure of the amygdaloids & coarse conglomerates, — is the chief [grounwork] of this opinion. — For any proofs of part of this elevation having taken place recently I looked in vain. — The highest central mountains are worn into some of the most extraordinary, praecipitous outline
conjectural] added pencil in margin.
which I ever beheld. proofs of age, in crater valleys. — The characteristic feature of this scenery is the depth, narrowness, & extreme steepness of the sides of the valleys or rather mountain gorges. — The main one which I ascended, in its highest part,
was scarcely ever exceeded in its breadth twice the width of the torrent. even in the present low state dry seasons. — The sides, in its whole length, with the exception of two or three points, are altogether inaccessible, & of very great height. The lateral streams, joined the principal ones by a succession of Cascades. I looked at this scenery with real interest. — I believed I here saw the effect of running water, continued through so long a succession of ages, as to suffice to wear away, several thousand ft: in thickness of solid strata. — I do not say that the mouth of the valley, & its higher parts were not have never been acted on by the waves of the sea: but often, in the Cordilleras (a) having attributed so much power to
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(a) to this latter kind of action, it was very satisfactory to see that form of valley, where I could believe, that the more ordinary agent of running water had been the efficient one. — It has strengthened in my view of the origin of the structure of the Andes. — From the same facts, I deduce, that the Isd of Tahiti has existed as dry land for a long period. —
Does this agree with any conclusion, which might be drawn from the geographical kinds of its flora or Fauna? Does such indicate a distinct & ancient origin?
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