Correlation of the Lower Silurian — Ulrich. 39 
then formed, but they are v6ry obscure, not Hkely to be noticed 
by anyone who does not know exactly what he is looking for 
and where to look for it. Along most of that ancient shore no 
such traces can be detected. In the neighborhood of the river 
the country must have been greatly transformed, since shores 
existed at that hight, but on the opposite side of the bay and 
the opposite border of the island mentioned, such changes 
might be expected to be less extreme. 
From the fact that shore marks there are so obscure I infer 
that they were never strongly marked, or else that the earth in 
this region was then in a condition too loose and unstable to 
retain their traces. 
Note. — Since the above was written, I have discovered evidence 
proving the existence of shore lines at two different levels, both passing 
through the site of Ann Arbor. The upper one of these coincides with 
the delta plateau above described, while the lower has left its trace in an 
obscure terrace noticeable at intervals along near the base of that plat- 
eau at a level about thirty feet lower. It is further evident that at the 
epochs of these shore lines the water margin in this region was a suc- 
cession of deeply indented, land-locked bajs, flanked and intermingled 
with an archipelago of islands, which have served greatly to complicate 
and obscure the traces now remaining of these shores. C. W. W. 
A CORRELATION OF THE LOWER SILURIAN HORIZONS 
OF TENNESSEE AND OF THE OHIO AND MISSISSIPPI 
VALLEYS WITH THOSE OF NEW YORK AND CANADA. 
BY E. O. ULRICH. 
IV. 
Beds XII. The strata comprised in this division are a little 
more than 200 feet thick in the vicinity of Cincinnati. Com- 
pared with beds XI, it is found that they differ considerably in 
the much greater abundance of calcareous material, the layers 
of limestone, particularly in the lower portion, being compara- 
tively much more numerous, the ratio of limestone and shale 
being on an average about one foot of the former to two feet of 
the shale. The latter also appears more calcareous and weathers 
to a yellow or drab color. 
