GLACIAL DRIFT. 335 



The next stage of the process would naturally be the deposition of blue 

 clay, derived from the ice twenty or fifty miles away, and resting upon 

 either of the tills where the glacier had once been present. When all 

 the clays and sands had been supplied to the second area, the scene of 

 action was transferred to a locality still nearer the ice, and so on till the 

 whole had disappeared. Throughout the valley the clays overlie the 

 till, although the stratified beds low down may have existed while the 

 lower ground-moraine was forming among the mountains. The order of 

 the conditions and of the deposits was strictly uniform, although at the 

 present time it is difficult to realize that a clay at Newburyport should 

 have been formed earlier than the same material at Hooksett. Some 

 have imagined that no clay was formed till after the entire disappearance 

 of the ice. In that case, we could not have had two kinds of clay. The 

 waters washing away cliffs would mix together the protoxide and sesqui- 

 oxide debris, and the resultant deposit would be unlike either of the beds 

 now situated beneath the many brick-yards. 



This view is in agreement with previously expressed suggestions about 

 the rapidity of the accumulations of modified drift. The principal work 

 of depositing the modified drift belongs to the epoch of the melting of 

 the ice; and, following the analogy of spring freshets, the time must 

 have been comparatively short, whether as compared with the antecedent 

 glacial or the subsequent alluvial period. 



Mr. Hawes expressed different views about the origin of these clays 

 when the analyses were returned. I subjoin extracts from his letters : 



My interpretation of those analyses was something as follows : Finding only iron 

 protoxide in the lower till, I thought the analyses indicated that, at the time of its de- 

 position, the whole till contained iron only in the lower state of oxidation ; that, being 

 formed beneath the glacier by the grinding and pulverization of the rocks, it had no 

 opportunities to oxidize. The upper till is lighter in color on account of the oxidation 

 of its iron by the subsequent action of the atmosphere and percolating waters, which 

 action has proceeded only to a certain depth. In being washed down into the valleys, 

 a portion of the iron was oxidized, — and hence the lower clays contain iron oxide ; and 

 after the clays were deposited, the external agencies of air and water, acting on them 

 as on the till, oxidized more iron in the superficial portion, making them again lighter 

 in color than the lower clays. Thus the two colors of the till and of the clay are both 

 referable to one cause, — the action of external agencies that have been in operation 

 since the time of the deposition of these deposits, and are still in progress, driving the 



