lieview of Recent Geological Literature. 'A'll 
nearly completed during the existence of lake Agassiz while the ice 
sheet formed a barrier to prevent free drainage to Hudson bay from 
lake Winnepeg and the Red River of the North. A slight change in 
continuation of this uplift has, however, taken place since the ice sheet 
ceased to be a barrier. In discussing the causes for the changes of 
levels it is shown that two causes, gravitation of water toward the ice 
sheet and changes in temperature of the earth's crust due to the ice 
sheet, are of minor consequence, and that epeirogenic movements are 
the chief cause. The author concludes that crust deformation by the 
ice sheet is an important, if not the main cause of the changes of level, 
there being downward movement beneath the thicker part of the ice 
followed by a partial return to the pre-Glacial attitude upon the disap- 
pearance of the load at the close of the Glacial period. He inclines to 
the view that there is plastic material at depths of but a few miles 
which was displaced somewhat by the sinking of the earth's crust 
under the ice weight and which flowed back by gravitation when the 
load was removed, thus restoring the crust approximately to its former 
position. In this connection attention is called to the fact that epeiro- 
genic movements have been close accompaniments of glaciation 
throughout the world, and a causal relationship between these move- 
ments and glaciation is inferred. 
The duration of lake Agassiz is estimated from the size of the outlet 
and the strength of the beaches to have been included within a period 
not more than one-seventh the length of the period since the ice sheet 
disappeared from the Laurentian highlands. This period the author 
thinks to be but 6,000 to 8,000 years in length, his estimate being 
based upon various measurements and computations by independent 
investigators concerning the length of post-Glacial time. The duration 
of lake Agassiz is, therefore, placed at 1,000 years or less. The great 
deltas on its borders, formed by inflowing streams, which in some cases 
contain several cubic miles of material, are thought to have been built 
up largely from the deposits contributed by the melting ice sheet, 
while still covering the head water portions of the valleys with which 
they are connected. Their bulk in some cases is more than sufficient 
to fill these valleys. It is, therefore, difficult to make an estimate of 
the duration of the lake by a measurement of these deltas. In the re- 
viewer's opinion few of the time estimates upon which Mr. Upham's 
calculations are based are substantiated by such accurate data as are 
brought forward in support of his estimates and conclusions on other 
subjects. The treatment of time relations is at present necessarily 
somewhat defective both because of the meagerness of available data 
for computation and the imperfections in method of calculating. The 
wide range in recent estimates of the duration of Niagara falls (from 
3,500 years to 32,000 or more) illustrates the uncertainty of such com- 
putations as are used by Mr. Upham in determining the duration of 
lake Agassiz. 
Thus far shells of but five species of mollusks have been found in the 
beaches discussed in this monograph. These are all fresh water 
