~ 
440 J. Croll—Examination of Wallace's Modification of the 
periods can hardly be expected, there have nevertheless been 
found in old preglacial buried channels and other sheltered — 
hollows three, four, and in some places five, bowlder-clays, sep- 
arated from one another by immense beds of sand, gravel and 
clay. Some of these beds are found to be continuous for long ~ 
distances. It is true that these intercalated beds have yielded © 
w or no organic remains, but it may well be that further re- 
. . . . - . u . aA 
during interglacial periods mildness and equability of tempera 
ture rather than heat are the characteristics both of summer and 
thickness of 2000 or 3000 feet. All this enormous quan 
