74 The American Geologist. February, 1902. 
of the forests and the laying down of the interglacial beds is 
then about 1,300 years. 
The question of interglacial water levels also has an impor- 
tant bearing on the duration of interglacial time. The evi- 
dence is conclusive that the water stood as low as in lake On- 
tario at present, if not lower, at the begirning of the waim 
climate beds; for streams were flowing and cutting channels 
in the Hudson River shale at the time. ‘Lhe water then rose 
to Go feet above Ontario during the warm climate period, and 
finally to 152 feet during the deposit of the Scarboro beds. 
Afterwards the water sank much below the level of Ontario so 
ihat wide valleys could be excavated. 
Of what nature was the barrier toward the northwest? 
he only possible causes of the rise of water are the forma- 
tion of an ice dam or a rise of the land at the outlet by reason 
of epeirogenic movements. The first supposition can scarce- 
ly “hold water” under all the circumstances. It is incredible 
that the glacier should steadily advance during the period of 
warm climate when the rich Ohio forest was flourishing along 
the Don so as to push a wall of 200 or more feet of ice 
across the Thousand Island region into the state of New York, 
damming up the waters of a powerful river. If it did so ad- 
vance during the warm period why did it withdraw again 
during the cooler climate that probably existed after the for- 
mation of the Scarboro beds? 
The supposition of a slow epeirogenic uplift toward the 
northeast, such as appears to be under way in the same re- 
gion at present, is a much more probable one than that of a 
glacial advance of 50 miles or more into a climate like that 
of Pennsylvania. Afterward there must have been an equally 
slow sinking of the land to the northeast to a level considera- 
bly below the present. These changes of level were in all 
hkelihood very deliberate processes, if Dr. Gilbert's estimate 
of the present rate of differential elevation of the Great Lakes 
region be taken as the standard. Certainly thousands or tens 
of thousands of years would be necessary to accomplish them, 
at least double the time during which the present uplift has 
been going on, 
The cutting of river valleys through 190 feet of intergla- 
cial beds after the draining off of the water should also be 
