x EARTH CHANGES & EVOLUTION 177 



When this mode of estimating the rate of subaerial 

 denudation was first applied to well-known regions, geolo- 

 gists themselves were surprised at the result. For I foot 

 in three thousand years is 1000 feet in three million years, a 

 period which has always been considered very small in the 

 scale of time indicated by geological changes. When we 

 consider that the mean height of all Europe (according 

 to a careful estimate by Sir John Murray) is a little under 

 i ooo feet, we find, to our astonishment, that, at the average 

 rate of denudation, the whole would be reduced almost to 

 sea-level in the very short period of three million years, 

 while all the other great continents would be reduced to 

 the condition of " pene-plains " (as the American geologists 

 term it) in about six or eight million years at the utmost. 

 It is quite certain, therefore, that there must be some counter- 

 acting uplifting agency, either constantly or intermittently at 

 work, to explain the often-repeated elevations and depressions 

 of the surface which the whole structure and mechanical 

 texture of the vast series of distinct geological formations 

 with their organic remains, prove to have taken place. 



The exact causes of these alternate elevations and 

 depressions, sometimes on a small, sometimes on a gigantic 

 scale, have not yet been satisfactorily explained either by 

 geologists or physicists. Two of the suggested causes are 

 undoubtedly real ones, and must be constantly acting ; but 

 it is alleged by mathematical physicists that they are not 

 adequate to produce the whole of the observed effects. They 

 are both, however, exceedingly interesting, and must be 

 briefly outlined here. We require first, however, to trace 

 out what becomes of the denuded matter that lowers the 

 continental surfaces at so rapid a rate, and is poured into 

 the sea at various points around their coasts ; and this is the 

 more necessary because recent researches on this matter 

 have led to results as surprising as those of the measurement 

 of the amount of denudation by rivers. 



During the voyage of the Challenger round the world for 

 the purpose of oceanic exploration, not only was the depth 

 of the great oceans determined by numerous lines of sound- 

 ings across them in various directions, but, by means of 

 ingenious apparatus, samples of the sea-bottom were brought 



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