DISTINCT GLACIAL EPOCHS. 73 



characteristic of low latitudes and warm climates is significant. 

 If therefore a soil developed on the surface of one sheet of drift 

 and buried by another, be found to possess, in addition to unmis- 

 takable marks of long exposure, the peculiar marks which seem 

 to be characteristic of soils developed under high temperatures, 

 the argument gains in strength. 



This argument from oxidation and weathering has another 

 application. If in a later advance, following a protracted reces- 

 sion, the ice-sheet failed to reach the limit of its earlier advance, 

 there would remain an area of drift deposited by the first ice - sheet, 

 outside the drift deposited by the later. Now if the time 

 interval between these two advances was great, and especially if 

 during this interval the climate was mild, the oxidation and 

 weathering of the older drift surface would be markedly different 

 in degree from that of the newer. If, under these circumstances, 

 the surface of the older sheet were found to be weathered and 

 oxidized and reddened up to the border of the newer drift sheet, 

 and if here there were found to be a sudden change in the charac- 

 ter of the surface of the drift so far as depth and degree of 

 oxidization and weathering is concerned, we should have strong 

 evidence that the one sheet of drift was much older than the 

 other. The statement sometimes urged that the drift which was 

 deposited near the edge of the greatest ice advance would be 

 largely made up of the residual materials which occupied the 

 surface invaded by the ice, would not meet the case. For if it be 

 granted that this statement is qualitatively good, we should find 

 the greatest degree of weathering and oxidation at the extreme 

 margin of the drift, and it should be found to be less and less on 

 receding from this margin. There would in this case be no 

 sudden transition from a deeply weathered and oxidized surface, 

 to one which is fresh and unoxidized, along a definite line. We 

 maintam that if the whole of the drift deposits are referable to 

 one epoch, there should be no sudden transition in the surface 

 of the drift from that which is deeply weathered to that which 

 is not, the one surface being separated from the other by a defi- 

 nite and readily traceable line. 



