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 débris, 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 cron 
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 Jower 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 
