REVIEWS 369 
are spread widely over the country without evidences of pro- 
found abrasive action upon the pre-existing surface, not that such 
action was absent, but it was far less vigorous than in the later stages. 
In harmony with this milder action upon the face of the country 
invaded, the drift-sheet itself was spread much more uniformly than 
in later times and pronounced morainic ridges are much more rare, 
and when present are much feebler and less characteristic. At the 
same time, the glacial drainage appears to have been much less vigor- 
ous and in some instances surprisingly lacking in vigor. These phe- 
nomena are among the most suggestive that yet await causal explana- 
tion. 
By far the most careful and trustworthy estimate of the average 
thickness of the drift which has heretofore been made in this country 
is embraced in chapter x1ll of this monograph. Not only are the 
data much more ample and better distributed than those that have 
heretofore been at command, but they have been analytically classified 
and discussed by more critical methods. The most difficult element 
of the problem is the drift embraced in the preglacial valleys, the 
depth and configuration of which it is difficult to estimate. This has 
been attempted, however, along two different lines which give essen- 
tial concordant results, and it is a fair presumption that the total esti- 
mate of the mass of the drift of the region investigated is a not 
distant approximation to the real facts. How far the territory of the 
Illinois glacial lobe is representative of the average thickness of the 
drift throughout the glaciated region cannot now be determined, for 
if the great Canadian tract be embraced, as it should, our knowledge 
is best defined by emphasizing its limitatations ; but the average thick- 
ness in Illinois may rudely represent the average thickness for areas 
similarly situated near the border of the glaciated area, but even this 
cannot be confidently affirmed. 
The work of Mr. Leverett is conspicuous for the judicial attitude 
of mind which eminently controls it. The emotional factor is held in 
marked abeyance and the intellectual factor suffers little trammeling 
from predilections. At the same time the large area covered by 
critical study testifies to an industry which could not have been greatly 
enhanced by emotional enthusiasm. The monograph will be best 
appreciated by those who are most familiar with the ground.—T. C. C. 
