No. 4.] SPEiNCER — SURFACE GEOLOGY. 217 



90-fathom chaonel. Here and there is a deeper sounding — the 

 deepest being 123 fathoms or 738 feet. The long channel, sur- 

 rounded by the 90-fathom contour line, is situated at a mean 

 distance of not less than twenty miles from the Canadian shore, 

 whilst its southern side approaches in some places to within six 

 miles of the American shore, with which it is parallel. This 90- 

 fathom channel varies from three to twelve miles in width. It.s 

 broadest and deepest portion is south of the Canadian peninsula 

 of Prince Edward's County. 



The mean slope of the lake bottom, from the Canadian shore 

 to this deep channel just pointed out, may be placed at less than 

 twenty-five feet in a mile, with variations from twenty to thirty 

 feet in that distance. The mean slope from the New York shore 

 line to the 90-fathom channel may be placed at sixty feet in a 

 mile, but varying generally from fifty to ninety feet. On examin- 

 ation we find that the greater portion of this slope belongs to 

 a belt which descends much more rapidly than the ofi-shore 

 depression. 



That the southern side of Lake Ontario has a submerged 

 series of escarpments or one moderately steep and of great dimen- 

 sions, is manifest when we come to study the soundings. In fact, 

 if the bed of Lake Ontario were lifted out of the water, this sub- 

 merged escarpment would be more conspicuous than the greater 

 portion of the present one, known by the name of the Niagara. 

 In many places the descent from the table-land above the Niagara 

 escarpment is no more precipitous than the slopes of the sub- 

 merged Cambro-Silurian (Hudson River, in part, if not through- 

 out the entire length) rocks, with its sloping summit, in part 

 crowned by a gently sloping surface of Medina shales. Nearly 

 north of the mouth of the Genesee river, we find that within 

 a single mile the soundings var}' from forty-three to seventy- 

 eight fathoms (between contour lines). This gives a sudden 

 descent in one mile of 210 feet. As the soundings are not taken 

 continuously to show to the contrary, most of the change of 

 levels may be within a few hundred yards. 



In the region of these soundings the deepest water outside of 

 the 78-fathom line is 84 fathoms, whilst from the shore to the 

 43-fathom sounding the least distance is four and a half miles, 

 thus giving the greatest mean slope of the lake bottom at sixty 

 feet in a mile, before the escarpment is reached. 



YoL. X. 2 No. 4. 



