240 
GEOLOGY OF THE THIRD DISTRICT. 
Lake Ontario. 
(Signifying, according to Father Hennepin, “ as much as to say, the pretty lake.”) 
This lake forms a part of the north boundary of the third district. Its origin is of some 
importance as to the original state of the Great level, and the origin of the lakes to the south 
of the level. 
The opinion that Lake Ontario is a lake of excavation, is rendered highly probable from 
the circumstance that its northeastern end on the New-York side towards the outlet is com¬ 
posed of rocks, none of which are higher in the series than the Trenton limestone, whilst the 
rocks which border it on the west are the red sandstone of Oswego and its overlying ones. 
Between these two points, allowing but fifty feet to be excavated in the red sandstone, one 
hundred feet in the grey sandstone under it, thirty for the sandstone shale of Pulaski, two 
hundred for the Frankfort slate, and one hundred and twenty feet for the Utica slate, the 
whole making a thickness of five hundred feet, is about equal to the real depth of the lake, 
the surface of the Trenton limestone being its bottom. That this is but an obvious inference 
from well ascertained facts, admits of no doubt, the aggregate thickness of the different 
masses being undeniable ; and moreover a guide or rule by which to calculate the level of the 
Trenton limestone, is equally well established; a position, however, which may not be so 
evident, and therefore a knowledge of the following facts is necessary to make it so. 
In order to ascertain what was the nature of any given surface as to level, it is but neces¬ 
sary to attend to the materials of which it is composed. Thus, mud, for example, tends to 
spread itself uniformly over any surface upon which it may fall, provided that it be deposited 
in water ; in which case, its particles being kept separate, the mass continues to flow, spread¬ 
ing itself uniformly over the surface until it finds its level. Such also must be the case with 
carbonate of lime, when it separates from its solvent, as is verified in every deposit of marl in 
the State ; the hollows gradually filling up, and the surface becoming horizontal. The Tren¬ 
ton limestone shows a twofold origin: that of marl or carbonate of lime; and that of mud, 
from whence the shale which causes its divisions, and with which it is often intermixed, was 
derived. The fact moreover is universal, that all rocks of sediment, when undisturbed, are 
horizontal; and therefore were the Trenton limestone and its overlying rocks to be returned 
to their original position at any level that was above the ocean, the basin of Lake Ontario 
would appear to be a basin of excavation; the rocks which rise upon the Trenton limestone 
forming its sides, and this latter rock its lowest surface, covered with the sediment of the 
tributaries of the lake. 
The excavation of the lake presents no greater difficulty than the removal of all the over- 
lying masses of the Great level, and those of the Mohawk valley, which must have taken 
place; for the Helderberg range has extended north over a considerable surface, as well as 
its overlying rocks, the whole having thinned out originally, there being no higher elevation 
by the side of which they could have been deposited. 
