408 
GEOLOGY OF THE FOURTH DISTRICT. 
The amount of fresh water contained in all these lakes has been estimated at ten thousand 
five hundred cubic miles, being more than half the fresh water on the surface of the whole 
globe. 
The basin of Lake Ontario is excavated in the Medina sandstone, the Grey sandstone, the 
Husdon river group of shales and sandstones, and towards its eastern extremity in the Trenton 
limestone. It appears probable, however, as stated by Mr. Yanuxem, that the Trenton lime¬ 
stone forms the bed of this lake, its estimated general depth reaching to about the surface of 
that rock. On its southern side the Clinton and Niagara groups of rocks rise in a high escarp¬ 
ment, and the excavation from this point northward is doubtless due to the same cause which 
eroded that portion now occupied by the lake. 
The basin of Lake Erie is in like manner in the bottom of a deep valley of erosion; and 
on its eastern and southeastern side, at the distance of four to eight miles, an escarpment of 
the higher rocks rises to an elevation of from five hundred to eight hundred feet above the level 
of the lake. It is quite evident that the strata, whose outcropping edges now appear on the 
southeast of Lake Erie, once extended much farther to the north and west; and since there 
is no evidence on the northern side of the lake of any sinking down of the strata, it would be 
an unnecessary supposition to bring in any such agency to account for the existence of this 
valley. 
The Corniferous limestone forms the northern margin of this lake for many miles, and 
dipping southward, probably forms its bed, as the small depth of the lake appears to indicate. 
Had the eroding agency removed this limestone, the soft rocks of the Onondaga salt group 
coming beneath would allow of a deep excavation with much less power than the limestone 
above ; and if this had occured, Lake Erie would probably have been the deepest lake of the 
whole, while now it is the most shallow. 
Elevation and depression of the water of the Great Lakes. 
The fluctuating level of the water of these lakes has long excited attention, and many 
speculations have been hazarded to account for the phenomenon. The somewhat general 
belief, that the periodical rise and fall in the water of these lakes occupy seven years, appears 
not to be founded upon authentic observation. Sandbars and beaches, or the inlets to certain 
bays,, are regarded as the landmarks; and these being liable to fluctuation from accumulation or 
removal, it follows that no hypothesis founded on such observations can be of any value. 
Of this character were many of the early observations, where the harbors were entered 
with a certain depth of water, which at some subsequent period was found to have diminished. 
It is nevertheless true, that there are important fluctuations in the lake levels, which are 
unconnected with the influence of winds. 
The only rational explanation of these changes yet offered, is that depending on the waste 
and supply of water. From the immense surface exposed to the sun’s rays, it is plain that 
the amount of water evaporated is immense ; and if by any means this process becomes re¬ 
tarded, the water is elevated. Again, the greater quantity of snow falling during certain 
