798

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1835 November

Tahiti

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, &

798v

(a) I may remark that the form of the land renders it probable, that no Crater yet could at the present day be found. Mr Ellis, whose opportunities for ascertaining such a point, must have been so good, states the same facts.

799

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1835 November

Tahiti

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 ! —

800

3

1835 November

Tahiti

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. —

conjectural

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.

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4

1835

Tahiti

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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801 verso

(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 Is^d 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?