538 Trof. */. TF. Judd — On Volcanos, 



same estimates, in giving us the means of realizing the amount of 

 work effected by those subterranean forces, the results of the opera- 

 tion of which are even less easily detected than those of subaerial 

 waste. 



That the ordinary processes of the earth's economy have been 

 maintained in constant operation throughout a long succession of 

 geological periods we have the clearest proof. And, as a necessary 

 consequence of this, we are bound to conclude that something like 

 the same general balance between the elevated and submerged areas 

 of the earth's surface has been preserved during those periods. 



Now nothing comes out more clearly from the ingenious estimates 

 to which, we have referred than the conclusion that — in a period of 

 time of comparatively short duration, geologically speaking — the 

 whole of the existing continents would be washed away, and their 

 materials deposited on the bed of the ocean by the action of the 

 agencies of waste and transport now in operation — were these 

 destructive forces not counteracted by the constant elevation of 

 portions of the earth's crust. 



Assuming then, as we have seen there are good grounds for doing, 

 that the general balance between land and water has been maintained 

 during very long geological periods — the subterranean movements of 

 the earth's crust, if all in the direction of elevation, would produce 

 effects that would be exactly represented by the total effects of 

 subaerial and marine waste during the same period. 



But seeing that a large amount of the effects of elevation must of 

 necessity be neutralized by those of depression, we are forced to 

 conclude that the results produced by subterranean are far greater 

 than those brought about by surface denudation. That, indeed, the 

 results of the latter are actually compensated by the excess of the 

 elevatory movements over those of subsidence. 



To enable us therefore to establish anything like an actual relation 

 between the work performed by subterranean and surface forces, in 

 modifying the earth's crust, we should require to obtain a measure 

 of the effects of movements of depression. Here Mr. Croll's calcula- 

 tions, based on the thickness of the sedimentary rocks, may be of some 

 use, for, as Mr. Darwin has so well shown, the stratified rocks must 

 almost without exception have been deposited in subsiding areas, 

 and the thickness of the rocks thus becomes a measure of the 

 amount of subsidence that the areas on which they were deposited 

 have undergone. But as many areas are undoubtedly subsiding, 

 without receiving sediment at all, even this will only give an un- 

 known fraction of the quantity required. 



From all these considerations then we are driven to conclude that 

 the work performed by surface agencies, in modifying the forms of 

 the earth's crust, vast as it is, bears but a very small, though indeter- 

 minable, proportion to that effected by the subterranean forces. 



THE END. 



