RECORD: Darwin, C. R. Geological diary: Port Desire (appendix). (5.1834) CUL-DAR33.243-244 Transcribed by Kees Rookmaaker, edited by John van Wyhe (Darwin Online, http://darwin-online.org.uk/)

REVISION HISTORY: Transcribed from the microfilm by Kees Rookmaaker, corrections and editing against the manuscript by John van Wyhe 7.2010. 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.


243

1834 May Port Desire (Appendix). —

Having read in Mr Lyells 3d Vol1 respecting the metamorphic rocks; & having at S. Cruz seen proofs of several elevations; many facts are now much more easily explicable than they formerly were. —

If we suppose all the Porphyries to be altered stratified rocks, we at once account for the numerous obli semi-obliterated lines of aqueous deposition: the rocks which have undergone most alteration not being always the inferior ones, may perhaps be accounted for by beds of different mineralogical nature being differently affected: the same method will account for the dykes; & there being no effect alteration on the sides, where these dykes & the beds of Porphyries are adjacent. — We may perhaps imagine that the beds of white earthy matter (1651) with granules of quartz, is the same substance collected which dispersed forms the earthy & siliceous crystals in the Porphyries. — There beds are generally the superior ones. — The substance being fusible, leads one to suppose the heat could not have been great. — it would be interesting to try the earthy crystals in the Porphyry. —

The existence of pebbles renders it very improbable that these beds are inferior members of the great oyster formation altered by Heat. —

I think the series are worth an accurate examination.

With respect to the valleys thet yet appear to me of rather difficult explanation. — Many of the ravines in the Porphyry are from 100 to 200 feet deep. — it is certain they were completely covered up, anterior to the 250 f foot plain V being elevated: granting any number with shells retaining their color

1 Lyell 1830-3, vol. 3. Darwin read Lyell's third volume in April-June 1834.

243 verso

[sketch] Diagram (Z)
Great oyster format
D A C F D A C F
B E
Porphyry

The uneven bottom of Poprhyry being covered (E) by horizontal strata: at whatever point those valleys (A B C) commenced finally they would assume the forms (D E F) & be more or less coincident with the old vents & irregularities of the Porphyry. —

244

1834 May Port Desire (Appendix)

of elevations & consequent sea-action; I cannot can hardly think such ravines have been cut in so recent a peryod period. — I did not obtain many sections, but they were sufficient to prove that the hills of Porphyry were present anterior to the deposition of "oyster formation." —

from horizontal stratification near the highest 440 hills

it is proable that the volcanic-like. ravines are of the same date. — There is no just cause to suppose. The coincidence of present valleys (a) with old valleys & cracks is easily accounted for. as represented in the Diagram (Z). —

With these cracks as a base, the action of the sea at different levels & earthquakes (almost proved by the Lava cliffs at S. Cruz); the valleys are just what one might expect. —

That most of the valleys have been formed by the sea. is probable from entire insufficiency of present terrestrial causes, in the most abrupt ravines, old thorny bushes grow which attest that for many years no gus flood has passed down them. —

The difference of 2 75 feet, between the $ 250 & 325 plains I am strongly inclined to attribute to two elevations; although as there ate no proofs in other places on the coast of such a plain it must remain doubtful. —

[sketch in margin]

The 100 feet cliffs, with great bed of superior gravel. & cliffs of Porphyry. backed at some distance by the regular plains, are most probably owing to same causes, which at St. Julian produced the 90 foot plain: — There are therefore here signs (b), which render it probable, that the 325

244 verso

(b) plains, have been elevated by at least three upheavals. — At the head of the creek, where the river entered, upon reflection, I believe there exists a series higher than the 325. plain. —

Not being aware of all the proofs (z). which subsequently have occurred of repeated elevations. I did not pay sufficient attention either to the relative level of the plains or structure of the valleys. —

(a). It may be questioned whether in submarine tracks, valleys can be formed; rents clearly can. — if I could had paid more attention & proved the existence of old valleys. it would have been a presumption, in my opinion, that the Porphyry had once been dry land. —

Where did the conglomerate derive the large pebbles of crystalline Porphyry? —

(b) The successive lines of cliffs, most beautifully explains my puzzle, about the presence only of littoral shells on the Plains.


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