INTRODUCTION. 
S 
It is of much interest that this early sediment presents such a uniform, 
even monotonous physical character, over the wide areas in which it has 
been investigated in New-York, Canada, Pennsylvania, Virginia, Iowa, 
Wisconsin, and Minnesota. 
The proportion of calcareous matter in the Potsdam sandstone at the 
west and northwest, is much greater than in its eastern localities. This is 
particularly manifest in some parts of the trilobite beds, where the rock 
has often, for several feet in thickness, the character of a silico-argillaceous 
limestone. While in its eastern localities, the Potsdam sandstone is usually 
a hard and compact rock, enduring the action of weather in a very high 
degree, it becomes at the west a friable, sometimes incoherent mass; and 
though presenting high perpendicular cliffs, surmounted and protected by 
the cherty beds of the next succeeding formation, it is usually unfit for 
all economical purposes, crumbling into fine sand on exposure to the frost 
and sun. 
Prom what we already know of this rock in the west, we are prepared 
to believe that some more fortunate localities will yet furnish numerous 
and satisfactory examples of its fauna. Thus far, the trilobites are frag¬ 
mentary; the character and condition of the beds in which they occur 
point to westerly or southwesterly currents, by which they were brought 
to their present position. In these directions, therefore, we may probably 
look for the highest evidences of the characteristic fauna of this period. 
I should not omit to mention, that shells of Brachiopoda and crinoidal 
columns have been found in this sandstone in the northwest*. None of 
these have fallen under my observation; and from the fact that they are 
not figured and described in Dr. Owen’s Final Report, we may infer that 
they were in too imperfect a condition to be specifically recognized. 
Among the thousands of Lingula which I have examined from the beds 
on the St. Croix river, I have discovered no trace of any other shell. 
Indeed the character of the sediment generally is such as would apparently 
preclude the existence of other brachiopodous molluscs. 
* See Owen’s Report, 1848, page 15; and 1852, page 499. 
