368 REVIEWS 
it affects profoundly nearly all of the larger questions of glacial 
history. 
The distinction between the ages of the several glacial sheets 
is founded upon careful estimates of the amounts of erosion they 
have respectively suffered, upon the depths and extent of the weather- 
ing process as exhibited alike in the clays and in the pebbles and 
bowlders, upon the degree of constructive mineralization in the form of 
segregates and general induration of the deposits, upon the extent of 
interglacial accumulations of soil, peat and similar deposits, and upon 
the nature of the life which occupied the region between the glacial 
stages, together with incidental criteria of more special nature and 
limited application. When it is considered that the broad sheet of 
Kansan till, which shows indubitable evidence of having been spread 
out as an approximately plane sheet, has been so thoroughly eroded 
over very large areas that only remnants of the original plane remain 
_ here and there, it is impossible for the candid mind to resist the con- 
viction that it is very widely separated in age from the later drift- 
sheets which have been merely ditched by the water courses, leaving 
scattered over the broad, scarcely modified surfaces, multitudes 
of shallow basins which a few feet of cutting would completely 
drain. 
While not new, the monograph brings out into sharp definition the 
lobate character of the ice margin at all of its stages. At the same 
time it shows that there was a change in the configuration of these 
lobes at different stages. It is perfectly clear from the general nature 
of these configurations that they are fundamentally dependent upon 
the topography of the region they occupy and of that which lies back- 
ward along the line of glacial invasion. At the same time there are 
some anomalies which, while not defiant of topography, do not clearly 
show their dependence upon it and indicate that other factors than 
topography were involved in determining the development of the ice 
lobes. These other agencies are very likely climatic, but they have 
not yet been deciphered. The most notable of these anomalies are 
the peculiar forms assumed by the Iowan drift and the shifting in 
the contours of the lobes between the earlier and later Wisconsin 
stages. 
Closely allied to this variation in configuration is a remarkable 
variation in the mode of action of the ice at different stages to which 
the monograph contributes a large mass of data. The earlier drift-sheets 
