118 MAN AND THE GLACIAL PERIOD. 



mate which characterised the preceding period, the rocks 

 must have been extensively disintegrated through the 

 action of subaerial agencies. The extent to which this 

 disintegration takes place can be appreciated now only 

 by those who reside outside of the glaciated area, where 

 these agencies have been in uninterrupted action. In the 

 Appalachian range south of the glaciated region the gra- 

 nitic masses and strata of gneiss are sometimes found to be 

 completely disintegrated to a depth' of fifty or sixty feet ; 

 and what seem to be beds of gravel often prove in fact to 

 be horizontal strata of gneiss from which the cementing 

 material has been removed by the slow action of acids 

 brought down by the percolating water. 



Now, there can be no question that this process of 

 disintegration had proceeded to a vast extent before the 

 Glacial period, so that, when the ice began to advance, 

 there was an enormous amount of partially oxidised and 

 disintegrated material ready to be scraped off with the 

 first advance of ice, and this is the material which would 

 naturally be transported farthest to the south ; and thus, 

 on the theory of a single glacial period, we can readily ac- 

 count for the greater apparent age of the glacial debris 

 near the margin. This debris was old when the G-lacial 

 period began. 



3. With reference to the argument for two distinct 

 glacial periods drawn from the smaller apparent amount 

 of glacial erosion over the southern part of the glaciated 

 area, we have to remark that that would occur in case of 

 a single ice-invasion as well as in case of two distinct ice- 

 invasions, in which the later did not extend so far as 

 the former. 



From the very necessity of the case, glacial erosion 

 diminishes as the limit of the extent of the glaciation is 

 approached. At the very margin of the glacier, motion 

 has ceased altogether. Back one mile from the margin 

 only one mile of ice-motion has been active in erosion. 



