EAETH CHANGES AND EVOLUTIOiV 191 



square miles of surface that we are able to understaucj ami 

 appreciate the tremendous power of rain and rivers, greatly 

 assisted by frost, in the disintegration of rocks, which lower 

 the whole surface of the land at such a rate that, if we had 

 means of accurate comparison with its condition a few thou- 

 sand years ago, we should see that in many places the whole 

 contour and appearance of the surface was changed. 



When this mode of estimating the rate of subaerial de- 

 nudation was first applied to well-known regions, geologists 

 themselves were surprised at the result. Eor 1 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 esti- 

 mate by Sir John Murray) is a little under 1000 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 counteracting uplifting agency, either 

 constantly or intermittently at work, to explain the often-re- 

 peated 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 depres- 

 sions, 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, ex- 

 ceedingly interesting, and must be briefly outlined here. We 

 require first, however, to trace out what becomes of the de- 



