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Darwin Correspondence Project

To A. C. Ramsay   21 December [1846]

Down Farnborough Kent

Dec. 21st

My dear Sir

I am much obliged by your interesting & friendly letter, which I shd. have answered sooner had I not been unwell during the whole of last week.— I am delighted that you have thought parts of my Book worth reading:1 it is in my opinion much more difficult to get a geologist to read a book than to write one.—

I have been much interested at what you say about traces of Terraces;2 I could never see any signs of such in Snowdonia the only part of N. Wales I have of late years visited. This subject has always been a hobby-horse of mine; simple as the subject may be, it was long before I understood it.— I see to the present day, (for instance in Murchison’s paper on Scand. drift)3 that the fundamental error (in my opinion) is made in speaking of the successive terraces as the direct effects of so many elevations, instead of looking at them as the indirect effect of an elevation, & the direct effect of sea’s destroying power at those levels. This leads me to a subject, mentioned in your letter; namely whether the old Tertiary beds of S. America were under water during the whole period intervening between their deposition & that of the superficial recent layers: on this point I have no knowledge; but it is certain that they were very slowly uplifted from under the recent sea, & therefore the less high parts, at least, must have been long under water. I declare I think this absence of any considerable recent fossiliferous deposits on both E. & W. coasts, the most remarkable thing I observed there.4 I hope you will turn this in your mind & take the trouble to reflect over my remarks at p. 135. I have found it a most fertile subject for thought; it helps to explain the breaks in Geological chronology & has disabused my mind of a prejudice that durable fossiliferus formations are in most places now accumulating. As it appears to me I see this error prevailing or implied in the writings of most geologists.—

Your trappean deposits, I can well believe are very perplexing; but I do not think I have ever seen anything quite analogous. As far as my memory serves, I think ejected volcanic crystals of glassy feldspar are always broken.— Near the Stiper Stones, I went to look at some of Murchison’s thin Silurian lava-streams; but I could not make them out; all that I traced far enough seemed to have been injected: in my book on Volcanic Islds. (p. 109. & 103:) I give the measurements of the thinnest aërial streams, I ever heard of. You must have an interesting field, where you are now at work; & Forbes told me of some very curious points.—5

Where lavas are vesicular & have become decomposed, I have seen the most marvellous transitions into sedimentary beds; partly caused by the compression & movement of the once solid & vesicular lava; so that it was quite impossible to say where lava ended & tuff began, though neither had been in the least metamorphosed. I suspect, however, slight degrees of metamorphic action are usually overlooked.—

I will only allude to one other point in your letter, namely to the long continued action of gravel on the underlying rocks beneath the sea: now this first supposes a very thin layer of gravel, for we well know how great is the inertia from mutual friction of a very thin layer of sand.— Secondly the very frequent presence, as stated in my book, of delicate corallines on gravel in very turbulent seas & in not very deep water;6 this appears to me opposed to much grinding action of the gravel; & the more I think of the subject, the more inclined I am (putting out of the question beds of mud or fine loose sand) to doubt the sea having much destroying power, more than two or three fathoms below low-water mark.

You apologise for the length of your letter, but I fear I have much greater cause to do so.—

I direct this to Charing Cross;7 thinking it probable that you may have returned from Bala & I beg to remain | My dear Sir | Yours sincerely | C. Darwin

Footnotes

South America.
Ramsay later read a paper to the British Association on the terraces of Cardiganshire (Ramsay 1847).
Murchison 1846.
CD discussed the absence of geologically recent conchiferous deposits in South America, pp. 135–9, and offered his explanation of why, because of denudation during periods of elevation, ‘there is but a small chance of durable fossiliferous deposits accumulating’ (South America, p. 139).
Both Ramsay and Edward Forbes were employed by the Geological Survey in Wales.
South America, p. 23.
The postal area of London in which the offices of the Geological Survey were located.

Summary

Discusses the geological origin of terraces.

Mentions R. I. Murchison’s paper ["On the superficial detritus of Sweden", Q. J. Geol. Soc. Lond. 2 (1846) pt 1: 349–81].

Comments on Tertiary beds of South America. Doubts that durable formations are now accumulating. Discusses formation of trap-rock. Notes effect of decomposition of lava; discusses action of submerged gravel on underlying rock.

Letter details

Letter no.
DCP-LETT-1038
From
Charles Robert Darwin
To
Andrew Crombie Ramsay
Sent from
Down
Source of text
American Philosophical Society (52)
Physical description
11pp

Please cite as

Darwin Correspondence Project, “Letter no. 1038,” accessed on 22 July 2019, https://www.darwinproject.ac.uk/letter/DCP-LETT-1038.xml

Also published in The Correspondence of Charles Darwin, vol. 3

letter