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