Geology of the Lake Superior Land District. 29 
whose greatest depression is in the lower peninsula of Michigan, 
where the surface is occupied by rocks of the carboniferous 
epoch. It is only in a northern and northwestern direction, 
however, that we are enabled to trace the strata in a descending 
order quite to the lowest members of the series, and even to the 
non-fossiliferous series beneath them. In other directions, we 
find the most elevated portions of the border exposing only mem- 
bers of the upper, or at most, of the middle, portions of the Silu- 
rian system,’’ 
Potsdam Sandstone—This was the first formed deposit in 
the Silurian basin, and attained, in its greatest development, a 
thickness of about two hundred and fifty feet, if we exclude the 
conglomerate-bands associated with the trap of Keweenaw Pcint 
and Isle Royale 
Range and Extent.—The bed of Lake Superior, embracing 
an area of about 32,000 square miles, is occupied almost ex- 
clusively by this rock, if we may judge from the islands which 
dot its surface, and the isolated patches which occur along its 
shores. On the north, the granite ranges approach near the 
coast and confine the sandstone within narrow limits; on the 
south, it occupies a broader area, and has been traced continu- 
ously around the axis, which divides the waters respectively 
flowing into Lake Superior, Lake Michigan, and the Mississippi. 
While the granite ranges attain in places an elevation of 1200 
feet above the lake, the sandstones, except in the vicinity of the 
trap, do not reach higher that 350 feet, and rest in a position 
nearly horizontal. Consequently, the granites and slates rise up 
like islands through this great waste of sandstone. 
This sandstone does not exhibit, at remote points, a homo- 
geneity of character, or uniformity in thickness; but appears to 
have been modified, to a great extent, by local causes. Thus, 
in the vicinity of the trappean rocks, which afford ample evidence 
of intense and long-continued voleanic agency, the beds attain 
the enormous thickness of 5000 feet, and often consist of con- 
glomerates, composed of trappean pebbles, cemented by a vol- 
canic sand. Away from these lines of disturbance, and where it 
abuts against the azoic rocks, the mass consists of nearly pure 
silicious sand, enveloping pebbles of quartz and patches of slate. 
here granite forms the adjoining rock, an equally marked 
change is observed in the character of the pebbles. 
* * #; * * 
‘The Potsdam sandstone of New York is a quartzose rock, 
whose particles are firmly aggregated, while the same rock, on 
ne northern slope of Lake Michigan, is so slightly coherent, tha 
it may be crushed in the hand. The calciferous sandstone 
New York, when traced west, passes into a magnesian lime- 
ieee ay 
