CONCLUSIONS. 63 
in water, and till originates largely on the lands above the water level. Unless 
the lands on which tills are deposited sink relatively soon after the withdrawal 
of the glaciers which formed them, such tills would be eroded and lost, so far 
as any geological record of them is concerned. It is impossible to know how 
much of the till of the Pleistocene period will be preserved as tillite. It is safe 
to say, however, that unless submergence operates on a large scale, and rela- 
tively soon, a very small proportion of the Wisconsin and earlier Pleistocene 
tills will be preserved as tillites. The chances of the exposure of such prob- 
‘lematical Pleistocene tillites would also be small. It is even possible that no 
tillite at all of the Pleistocene period will be exposed as evidence of that period. 
The chances of the preservation of the record in Pleistocene clay, however, 
would be greater, even though the area covered by the Pleistocene glacial clays 
is much less than the area covered by tills of the same age. 
As previously stated many shales and slates with banding occur which 
have not been investigated, in regard to either glacial origin or seasonal deposi- 
tion, and until such an investigation is made it will not be possible to say 
how important the ancient glacial clays will be in the study of ancient climates 
of the earth. It is probable, however, that Glacial periods of the past, of which 
we have no clue at present, will be discovered by means of ancient glacial clays, 
even though no tillites of such periods will ever be found. 
