WCP4883

[1]

9, St. Mark’s Crescent

June 16th [1869].¹

Dear Sir Charles

I have read over Geikie’s paper on "Modern Denudation"², and your MSS. and will make a few remarks on both.

1. Mr Geikie³ has a curious way of sometimes saying exactly the opposite of what he means: — at p. 8 l[ine] .8 he says the proportion of solid matter "by weight is probably on an average abouthalf that by bulk;" — but on p. 9 l.7 he gives two examples in which the proportion of earthy matter by weight is just double that by bulk as of course it must be, rock being heavier than water.

Again at p. 36 l. 9 he says that there is reason to believe "that the variations on the whole have tended rather to the increase of denudation," — or [2]⁴that the present rate "is rather below the mean than above it". He evidently means by increase, increase backwards into past time, but this seems to me a most unintelligible way of writing. I merely notice this in case you might quote either of these passages.

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2. There are two important facts in relation to denudation entirely omitted in Mr Geikie’s paper.

a. The quantity of matter carried into the sea, he takes as the measure of sub-aerial denudation, — but this is evidently a most insufficient one, since a large portion of the denuded matter will often be deposited in lakes or on all low lands subject to inundation. How imperfect would be [3] the estimate of the denudation of the Nile basin, be if founded only on the amount of sediment carried into the Mediterranean! The great lakes and the alluvial valley of the Nile, probably receive annually an amount of sediment many times greater than that which reaches the Mediterranean.

This will apply in a less degree to all valleys, since all have in some portion of their course or of their tributaries either lakes or flat lands subject to inundation, so that in every case the amount of denudation that is affected in a river basin is will be under estimated by the quantity of sediment carried into the sea

b. He has also omitted to notice the fact (to which I called attention in my article)⁵ viz. that the present rate of denudation is probably [4] above the average in all extra-tropical countries, and for all high mountain regions even in the tropics owing to their having been so recently a glacial epoch, by which immense deposits of boulder clay and loose drifts have been left over a large portion of the surface of many river-basins. The denudation of such accumulations of loose materials can afford no measure of the rate of denudation before the glacial epoch, or at any period when denudation had to be effected mainly on solid rock of various degrees of resistance. The enormous amount of sediments of the Po is probably due to this cause.

[5] Notes on MSS

p. 4. "balance of sea & land remaining constant," &c..

I doubt if we have a particle of evidence of this constancy. The enormous depth combined with the great area of the oceans would render it quite possible that a tolerably deep ocean may have ^{at some periods} existed all over the globe, with a few comparatively small masses of land rising here and there above it.

On the other hand such a mass of land as Central Asia would furnish the materials for a large addition to the present area of land; — and to me it seems quite as probable [6] that these variations should have occurred, as that there should ^have been any approach to constancy in the proportion that now happens to exist.

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p. 4. "relation of rate of change in organic nature to rate at which inorganic changes operate."

This "relation" will not give any increased accuracy or certainty in the estimate of Geolog[ical]. time unless the rate of change of each factor during a given epoch be calculated independently. There are three such rates of change to ascertain and compare: 1st. Change of species in each division of animals and plants, since some given epoch [7] (such as the beginning or end of the glacial epoch). 2nd. The amount of upheaval in the same time.

3rd. The amount of denudation, as measured by the quantity of sediment carried away by a river, apportioned, not over the whole area, but over that hilly portion of the basin from which it must have been almost entirely derived; — also by independent measures of the growth of deltas of lakes, and of the erosion of river-channels through deposits of known age.

If these qualities or rates of change were each estimated at [8] many different parts of Europe and N. America, the comparative rate of change deduced from the mean of them all, would probably ^{enable us to} form a fairly reliable estimate of geological time, from either of the three data, — organic change, upheaval or denudation.

[9] Note.

Are not the latter passages — on submarine denudation, elevations and depressions going on in deep sea &c. — rather out of place under "Subaerial denudation."

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I have sent these few rough notes, as I can hardly say much, till I see what is already written in Chap. VI. and what part of it the MSS. now sent is intended to replace.

I shall be out of town from Saturday to Monday, but home again next week.

Yours very faithfully | Alfred R. Wallace [signature]