WCP1698

[1] [p. 1]¹

Lyell to Wallace June 16. 1869²

Dear Wallace³

— It strikes ^me that some way could be found of expressing the departure ^of from of the present shape of the globe from a perfectly even spheroid assuming that the average height of the land is 1000 feet & the average depth of the sea 15,000 (see Principles ^{Vol I} p 265)[.]⁴ Suppose the portion subjected to subaerial denudation[,] i.e. the land[,] to be about 1/4 of the whole & the portion which suffers submarine denudation to equal half the land. Then in order that the subterranean movements should counteract the levelling effect of water & keep up an average quantity of unevenness equal to that now exhibited by the planet, we should not expect the dry land to undergo an amount of movement sufficient to counteract the lowering effect of rain, rivers[,] waves & currents because more than half of the movement wanted to keep up the required inequality would be spread over the whole terraqueous globe. Submarine volcanoes [2] & volcanic chains would be formed in some places & ^{in others} submarine platforms by upheaval and as abysses by depression. If therefore the rate of waste proceed at one foot in 5000 years for all the land it does not follow that we should require proof of an amount of movement of the land corresponding to that which would be expressed by the conversion of so much rock into sediment & its transportation to the sea.

M^r. Hilgard⁵ of the Hydrographical Office of Washington U.S. tells me that their chief hydrographer Pierce [sic]⁶ thinks that the late earthquake waves have proved, what he suspected before, by the time which they took to cross the Pacific, that the depth of the ocean is not on an average 15,000 feet as calculated by Laplace?⁷ & others from the tides but nearer 10,000 feet. I am afraid nothing is yet published of this.

The permanence of continents once formed must depend greatly on the manner in which the sediment [3] is removed by denudation. All the pebbles for example & coarser sand tend to be deposited partly in lakes & partly on the bottom of the sea within a few miles of the coast. The consequence is that oscillations of levels on the ^{borders of continents} bring up all deposits of recent date & convert them into land & cause them to be again exposed to denudation whereas similar oscillations would not produce land for nor shoals exposed to submarine denudation if they took place in that far from continents & in that part of the submarine area which is of the average depth of the ocean[.]

It may be said that the more energy I attribute to submarine denudation the more I Iessen the chance of the emergence of recently formed deposits such as deltas & shoals; but on the other hand we must remember that the shelter of a continent already raised protects deltas & shoals from the action of marine currents in most directions. It is only in exceptional cases when shoals are trying to emerge [4] [p.4] in an open ocean that the full play of the waves has full effect.

If the movements of depression in the Gulf of Mexico exactly equal the upheaval this would not tend to keep things the same in the area of a delta like that of the Mississippi for supposing the depth to be ^{delta to sink} 500 feet so as to allow an additional sediment of that thickness to be superimposed a reupheaval of the original area to its old level would cause the emergence of the modern deposit to the extent of 500 feet. (see [sic] my letter to you March 1. 1869[.]⁸)

May not a great error be made by taking the average waste of a continent from certain rivers like the Mississippi[,] Ganges & Po & omitting other vast areas, the basins for example of the Amazon & Orinoco, where there may been a movement of elevation going on in those parts of the basin which are nearer the ocean causing the waste of the Andes to raise the lower parts of the hydrographical basin instead of allowing the rivers [5] [p.5] to transport the materials to the sea. It may perhaps be answered that such movements occurring in some basins would be balanced in others by such a distribution of the upheaving & depressing force as would tend to increase the fall or steepness of the river channels. This I admit but the danger is that we shall form our estimate of fluviatile denudation from rivers which are carrying down an excess of sediment & omit from our calculation rivers like the Orinoco where there may be a deficit of the average quantity of mud, sand etc in proportion to the vast area drained. In this latter case there are no lakes to intercept the sediment in the upper country.

June 17.

If the movements of upheaval & depression were suspended the effect of denudation would be to carry down sediment from the land to the sea, the transported matter displacing a certain quantity of salt water & diminishing the capacity of the ocean by so much [6] [ p. 6] & causing it to stand higher & to submerge the low margin of the land[.]

If this could go on for six million years[,] the time required according to previous assumptions for the delivery over of the whole land to the sea; would not the ocean cover the whole globe & be 250 the planet have a larger average diameter by 500 feet. In other words would not the waters rise everywhere 250 feet vertically in consequence of the diminished capacity of the basin of the ocean, the floor of which would be raised by the layer of solid matter carried into it from the waste of the land.

One of the mistakes often made in considering the manner in which subterranean movement counteracts the levelling effect of denudation, arises from forgetting that it is not upward movement alone which is effective in this way. If the capacity of the sea be diminished by the carrying into it of 1000 cubic feet of [7] [p. 7] sediment; it may be increased by a subsidence adding to the dimensions of the oceanic basin, a capacity of 1000 cubic feet. It is not therefore upward movement alone that is required to prevent a continent which is suffering denudation from losing area. Downward movement in any part of the submerged or oceanic area may be just as instrumental & what is required may be stated to be the keeping up of a constant amount of inequality of the earth's surface[,] the planet continuing to be of the same diameter[.]

It is perfectly true we cannot assume that the proportion of land & sea is a constant quantity throughout geological time. It no doubt varies within certain limits from one geological period to another but in speculating on this subject in reference to geological time it is safest to take the present quantity as an average. There must be some reason for the congregating of all the land in great masses or continents as ^{well as} for the length & enormous height of mountain chains. & this is no doubt connected As these chains form to a certain extent backbones of continents & as they are often volcanic like the Andes & Rocky Mountains the laws which govern earthquakes & volcano[e]s may have to do with this congregating of lands in great continental [8] [p. 8] masses; & the mode in which in the course of ages the chief theatres of volcanic action shift from one site to another may govern this great feature in the physical geography of the globe. I think we may perhaps make some useful speculations on the relation of denudation & the subterranean movement considered as forces counterbalancing each other, if we assume that the force of upheaval & depression has to perform the task of exactly counteracting the effect of the other force or the levelling power of water. This power consists of subaerial & submarine denudation. Unfortunately the submarine is an unknown quantity; the subaerial according to Tylor⁹ Croll¹⁰ & Geikie¹¹ may be obtained by careful measurement of the solid matter transported to the sea by rivers. ^{They would suppose} Assuming a mass of rock equal to the land in area & 1000 feet in average height if put into an hour-glass & would run through it in a pulverized state in six million years & the subterranean forces may be compared to a gigantic power which is capable of turning the hour-glass upside down & leaving it to run out in another six million years[.]

[9] [p. 9] River sediment The Nile is a good case of the mouth or delta not receiving a quantity of sediment proportioned to the area wasted because of inland lakes & the shape of the basin: for the last 15,000 miles of its course it receives no tributary great or small. This has to be considered. But we need not Geikie & others do not cite the Nile. As to your remark about the lakes¹² it is of course important but it would only show that they underrate the amount of waste. In the case of the Po it seems very remarkable that it should be eight times as great as the Mississippi is, seeing the great lakes of the north of Italy must receive so much of the sediment which would otherwise be carried to the Adriatic by the Po. I do not clearly see when ^we the attempt to measure the rate of waste in past time, that ^how the Glacial Period would tell. You would attribute an enormous force of erosion to ice, but are we to conclude that the quantity of rock thereby transported from the land to the basin of the sea by glaciers is greater than previously by rivers which were not frozen. ^{Perhaps your} I do not see any objection to taking part of the average from rivers in which the low lands may be encumbered with glacial drift is sound. But a considerable portion of the Mississippi basin is encumbered with drift, the basin [10] of the Ohio for example & so is the basin of the Danube yet the proportion of sediment given in Geikie's table¹³ is trifling in these rivers compared to the proportion in the Ganges, or Chinese river, or the Po. And in the case of the Po you have much sediment intercepted by lakes which is not the case with the Mississippi or Ganges.¹⁴