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Darwin Correspondence Project

From J. B. Jukes   30 May 1862

Geological Survey of Ireland, | Office, 51, Stephen’s Green, Dublin,

May 30th 1862

My dear Darwin

Many thanks for your letter.1 It is of course impossible for any one to see the proof of my case without maps & sections & even then difficult without knowing the ground.—2 In my paper I confine myself to Ireland & merely ask if the principle be not applicable to the Weald & all other cases.3 Your account of the form of residuum of the insoluble matter of the Chalk however is very interesting.

Concerning marine and atmospheric denudation I am inclined to lay down the following canons.—

Marine denudation acts horizontally, like a wide plane, & cuts back into the land so as to make long lines of vertical cliffs if the land be stationary, or broad slopes if it be rising or any combination of the two according to rates of movement &c.

It does not make narrow winding valleys, except as passes on the crests of rising mountain chains, just while these passes are straits between islands & the arm of the sea made to run back & forward with a river-like action.—

(I do not see how the Sea can make mere inlets or Fiords, or even wide valleys if they be closed at the end, though it might cut a little at the mouths of such valleys).—

Atmospheric denudation acts vertically by weathering over the whole surface of land degrading its whole surface at different rates according to the nature of the rock. The matter removed is carried off ultimately by the rills, brooks & rivers either in solution or mechanical suspension.

The rivers (of all sizes) also erode the rocks by their own action, continually cutting their own channels deeper, and also widening them where those channels wind much from side to side. Now I think the question whether the river make a wide channel or a ravine depends on the ratio between the rate of the motion of its water and the nature of the rock.— If the river does not wear the rock very fast its channel may shift very much & eventually make a wide open valley. If however the river once cut a deep channel it can only make a ravine & that can only be widened by the slipping of the sides of the cut into the bottom & therefore its form depends on the nature of the rock whether it will stand perpendicular or at what slope.

I take these as tolerably obvious & certain rules.—

All that I propose to do is to push the amount of atmospheric & river action to a greater extent than most people have propos〈ed〉 to do, because I can, I think, prove it to have gone to that extent in the S. of Ireland.

As to the valleys you mention in Australia I looked at some of them & have often considered your notions respecting them & on the whole agree with Dana.—4 Look at Sydney Harbour with its numerous arms all successively joining and going out by one narrow entrance.— I do not see how the Sea can possibly have made those ravines. Lay them down on a map & they have exactly the form of a number of brooks uniting to form a river—

Look at the slope of the ascent of the Blue Mountains above the Hawkesbury   it is all gullied by similar ravines successively meeting, often emptying their brooks into one another through the narrowest & most precipitous gorges. I cannot but think they have all been cut by brooks when the land was higher & Sydney Harbour above sea level, perhaps a long way, & much more rain pouring down perhaps than there is now.

But if so then those huge circular valleys you describe so well must nevertheless have been formed also on dry land however inconceivable it may be.— If the others are furrows worn in a sheet of sandstone those are only bigger holes worn perhaps by the union of some of the furrows. It is however quite possible that the erosion may have been helped a bit by the sea after the land had been down again & during its last rise.—

So in S. Ireland the Sea during the Glacial period doubtless has left its mark, but the main modelling of the “form of ground” below the original surface of marine denudation has been done on the dry land.

Who is that goose that reviews your book in the Athenæum.5 I had a good read at it yesterday coming up in the train.

Yours very truly | J. Beete Jukes

Footnotes

CD’s letter has not been found, but it was presumably a reply to the letter from J. B. Jukes, 25 May 1862.
Jukes had formerly believed that the topography of modern Ireland resulted from marine denudation of Upper Carboniferous strata. However, in a paper first delivered before the Geological Society of Dublin in May 1862 (Jukes 1862), Jukes announced his rejection of the marine erosion theory of landscapes, adopting instead the view that Ireland’s drainage system had been formed by the action of fluvial processes. See letter from J. B. Jukes, 25 May 1862, and Davies 1969, p. 326.
In a postscript to his paper (see n. 2, above), Jukes suggested that the Weald district in Kent might lend itself to a form of explanation similar to that he had provided for the drainage pattern of the south of Ireland (Jukes 1862, p. 400). See also letter from J. B. Jukes, 25 May 1862.
CD, in attempting to explain the formation of the ‘grand valleys’ penetrating the mountains of New South Wales, dismissed as ‘preposterous’ the idea that they could have been formed by fluvial action (Volcanic islands, p. 136), arguing instead that they resulted from the action of the sea during a period of slow elevation of the land. James Dwight Dana took issue with CD’s conclusions and argued in favour of fluvial action as the main agent involved (Dana 1850).
According to the publisher’s marked copy of the Athenæum (City University Library, London), John R. Leifchild was the author of the anonymous review of Orchids that appeared in the issue of 24 May 1862, pp. 683–5 ([Leifchild] 1862). CD’s copy of this review is in DAR 226.1: 11 (Scrapbook of reviews).

Summary

Elaborates his denudation theory: marine denudation works horizontally, atmospheric works vertically.

Answers point in CD’s letter on Sydney Harbour, N. S. W.

Who is the "goose" who reviews CD’s books in the Athenæum [review of Orchids, 24 May 1862]?

Letter details

Letter no.
DCP-LETT-3579
From
Joseph Beete Jukes
To
Charles Robert Darwin
Sent from
Geol. Surv. Ireland, Dublin
Source of text
DAR 168: 91
Physical description
8pp

Please cite as

Darwin Correspondence Project, “Letter no. 3579,” accessed on 22 March 2019, http://www.darwinproject.ac.uk/DCP-LETT-3579

Also published in The Correspondence of Charles Darwin, vol. 10

letter