WCP1697

[1]¹

March 1^st /[18]69

Sir C. Lyell to Alfred Wallace Esq^r

It may be well to say that in assigning ⁸⁰⁰⁰⁰⁰ nearly a million years for the commencement of the glacial period, Sir C. Lyell may not have sufficiently considered the vastness of the sub-aërial denudation which the continents would suffer in that period even if the sediment of the Mississippi be taken as a basis of calculation but it would be a mistake to allow Croll² or Geikie³ to assume that they can obtain any measure of geological time by simply ascertaining the rate at which land would be lowered if the operation of the subterranean forces which are constantly restoring the inequalities of the earth's surface were suspended. The very existence of dry land throughout all time is due to these ^subterranean forces which have always counteracted the levelling effects of running water and we have no right to suppose that they are less active now than at all former periods. We can [2] [p. 2] certainly proove that they have been very energetic since the beginning of the glacial period, prob[ably] in both directions both upward and downward ^{since the beginning of the glacial epoch} but as they have probably been most effective in mountainous regions, we cannot often do justice to their energy as it is not in the interior but in regions comparatively near the sea that we can point to positive proofs of submergence as in Moel Tryfane⁴ [sic] where we have ^marine shells of recent species to attest the fact at the height of 1400 feet.

In the present state of our knowledge we cannot tell what lands are increasing in height in spite ^{of the lowering effect} of atmospheric waste, or in what cases the upward movement ^simply balances the abrasions, but we must remember that as the most dense ^copious and coarse sediment is either deposited by rivers by ⁱⁿ lakes or in deltas and shallow seas, the upward movement is continually uplifting the modern as well as the ancient sediment which prevents the regions where land abounds from being converted as rapidly into seas.

[3] [p. 3] What measure of geological time have Geikie & Croll discovered. What relation have they hit upon between three kinds of change, namely 1^st levelling power of running water. 2^ndly the forces of upheaval & subsidence, and 3^rdly the change of species.

We must always bear in mind that the change of the latter estimated by the mollusca is very small indeed ^{almost nil} since the beginning of the glacial period in which time there there has been a vast amount of change of levels both of sinking accompanied by deposition, and of re[-]elevation attended by the re-excavation of valleys cut through the marine sediment accumulated during glacial submergence[.] As to the denudation of solid rock being greater when there is much ice & snow, this is a point which ought not to be hastily assumed. There will almost always be much bare rock in mountainous regions where the slopes are great and a ^{more or less dense} covering an of aluvium [sic] in the river plains ^{where the slope is much less} but the rate at which the rivers will [4] [p. 4] ^{be able to} clear the valleys will depend on the change of levels or ^{w[hic]h is going on}. If the continent be sinking are the upper parts of a great valley like those of the Mississippi or Amazons sinking at a different and greater rate than the lower parts or that ^{of the same or those} near the sea, or if the whole basin of the river be rising is that part near the mountains moving up less rapidly than the sea-ward part?

2. {It ought to be stated that Mess^r[s] Geikie and Croll may be jumping too hastily to conclusions seeing how very complicated is the problem, and full acknowledgement may be made of the importance of their showing that one kind of change or atmospheric waste can at present be measured more accurately than any other in reference to the time which it takes for its accomplishment and yet it may be shown that the bearing of this rate on geological chronology depends on our discovering its relation to the change of species and rising and sinking of land in ascertaining w[ich]^h we have ^{as yet} made ^scarcely any progress[.]

[5] 1 {We cannot safety assume that these continental movements are uniform. & according as they affect the fall of the waters per mile will be the rate at which the alluvium of the plain is delivered into the sea for much of the detritus of the higher lands may be intercepted even where no lake basins are formed.