i 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 IOOO feet in three million years, a 

 oeriod which has always been considered very small in the 

 scale of time indicated by geological changes. When we 

 ;onsider that the mean height of all Europe (according 

 :o a careful estimate by Sir John Murray) is a little under 

 tooo feet, we find, to our astonishment, that, at the average 

 ate of denudation, the whole would be reduced almost to 

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

 vhile all the other great continents would be reduced to 

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

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

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

 cting uplifting agency, either constantly or intermittently at 

 /ork, to explain the often-repeated elevations and depressions 

 If the surface which the whole structure and mechanical 

 •jxture of the vast series of distinct geological formations 

 'ith their organic remains, prove to have taken place. 



The exact causes of these alternate elevations and 

 epressions, sometimes on a small, sometimes on a gigantic 

 rale, have not yet been satisfactorily explained either by 

 2ologists or physicists. Two of the suggested causes are 

 ndoubtedly real ones, and must be constantly acting ; but 

 is alleged by mathematical physicists that they are not 

 •lequate to produce the whole of the observed effects. They 

 - e both, however, exceedingly interesting, and must be 

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

 ut what becomes of the denuded matter that lowers the 

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

 1e sea at various points around their coasts ; and this is the 

 iore necessary because recent researches on this matter 

 l.ve led to results as surprising as those of the measurement 

 c the amount of denudation by rivers. 



During the voyage of the Challenger round the world for 

 te purpose of oceanic exploration, not only was the depth 

 c the great oceans determined by numerous lines of sound- 

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

 ii^enious apparatus, samples of the sea-bottom were brought 



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