Notes on the Drift and Alluvium of Ohio and the West. 211 
homogeneous, horizontally stratified ; will effloresce with acids; 
and washes into deep ravines like the Lake Erie blue marl. 
There are found (but only very seldom) blocks of trap rock, 
imbedded, and boulders of native copper. 
This marl and that of Lake Erie were evidently left by com- 
paratively quiet waters. Mr. Hubbard refers the Lake Superior 
deposit to the tertiary. 
To the southward of the trap ranges and the Ontonagan River, 
boulders of trap of immense size and in considerable numbers, 
are seen on its surface, and in the gulleys and valleys of the 
Streams that cut through it. Its surface descends like that of 
the Lake Erie deposits gently towards the lake, favoring the idea 
in both cases, of their being the beds of au ancient and more 
elevated {freshwater lake, descending and thinning out towards 
the centre. 
In the vicinity of Cleveland the rate of or angle of descent, 
would bring down the surface of the lacustrine deposits to the 
surface of the waters, about two miles from shore ; and Judging 
from the shallowness of the lake, which at ten miles out is only 
about sixty feet in depth, it has gained on the land several miles 
Since the waters assumed their present level. This is more ap- 
parent from the form of the coast, which is that of a broad bay, 
having rocky headlands about fifteen miles to the right and left, 
and ten miles advanced into the lake. The present rate of en- 
croachment is about a mile in eight hundred years. 
No. 6. Boulders, or “erratic rocks.’—Mr. Christy of Oxford, 
who has travelled extensively through the west and southwest, 
limits the primitive boulders on the south and east, by a line from 
ercer in Pennsylvania, through Zanesville, (Ohio, ) Cincinnati, 
Princeton in Indiana, to the mouth of the Kaskaskias on the Mis- 
Sissi ppi.* ae 
According to his observations, the materials of the diluvial beds 
South of the above line, are wholly of the Silurian and second- 
ary rocks of the Western States, and primary pebbles are want- 
ing. Proceeding southward these materials become less | ’ 
as might be expected from the fact of the rocks that furnish the 
pebbles being softer, and the distance from the su 
tock, deposited in solitary blocks or in nests over the surface of 
hill and valley, we have blocks of sandstone conglomerate and 
cliff limestone mingled with them. Near the bend of the Cuy- 
* Letter to Verneuil, 1847. 
Secoxp Series, Vol. V, No. 14.—March, 1848. 28 
