32 Messrs. Foster and Whitney on the Geology, etc. 
The rock is fine grained, of a light yellow color and very 
friable. Some of the superior beds, which are thin, have been 
wrought for grindstones. The friable character of this sandstone 
is one of its most prominent features, and, owing to this cireum- 
stance, the escarpments are not usually high, or abrupt, unless it 
has been protected by the overlying rock. In its want of co- 
hesion, it differs, in a very marked degree, from the prevailing 
character of this rock, as developed in New York and Canada, 
where it is usually, though not always, compact. It is not, how- 
ever, unlike the sandstone of the Pictured Rocks, and is less 
friable than that of the Mississippi and St. Croix region. 
The almost uninterrupted continuity with which this rock can 
be traced, even from its eastern extension through Canada and 
along the northern shore of Lake Huron to the St. Mary’s river, 
and thence westerly, leaves no doubt as to its true position and 
identity in age with the Potsdam sandstone of New York. 
we were at a loss in thus tracing it continuously, we have still 
the evidence of the succeeding fossiliferous strata, which show, 
conclusively, the same relations to this sandstone as they do to 
its equivalent in New York. With both these evidences com- 
bined, we cannot hesitate for a moment in our conclusion 
regarding its age and place in the series. z 
bands, or the more calcareous portions of the group, and it is to 
this modification that we should look for the development of the 
d “ 
From all this evidence, we regard the question of the age of 
this rock as settled—that the Potsdam sandstone of New York is 
stone, or lower magnesian limestone. It is a thin mass, evidently 
due to a recurrence of the same causes which produced the 
inferior deposit. This has been well elucidated by Dr. Owen in A 
his reports on the upper Mississippi, in which he has shown that, . 
t 
SNRs ae: ems 
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