OF DENISON UNIVERSITY. 
65 
They maintain, however, an average thickness, indicating that, what- 
ever were the causes which had disturbed the bed of the Lower Silu- 
rian sea, these had disappeared by the time the strata of the Clinton 
Group were deposited. 
In other words, at the close of the Lower Silurian age, the ocean 
became shallow; large valleys and gullies were washed out from the 
beds of the Cincinnati Group. The silt was deposited as a blue clay, 
which terminates the strata referred to this age. On this uneven and 
very undulating bed, the rocks of the Clinton Group were deposited. 
At this time a portion of the Lower Silurian rocks must have 
been exposed, as the researches of the last geological survey revealed 
evident shore markings in several places, containing the pebble-washed 
fragments of the Lower Silurian strata. 
In fact, the very fragmentary condition of most of the fossils found 
in this group indicates the action of shore waves upon the accumulated 
deposits of this sea. The slight variation in the thickness of the 
group may readily be explained by the conditions brought about by 
its uneven bed. The washings of an otherwise shallow sea would 
naturally be very unequally deposited near the shore line. At a dis- 
tance from the land the fine silt would be almost equally deposited, 
but near the shore the washings would accumulate most in- the depres- 
sions of the ocean bed. And, although the inequality of deposition 
would of course not be very great over any large extent of territory, 
nevertheless it is sufficient to account for the variation observed. At 
Soldiers’ Home, where there is a dip of seven feet in one hundred, 
the rock increases one foot in thickness. This is the most marked 
instance which has fallen under my observation. 
The Clinton Group consists of a crystalline, crinoidal limestone of 
variable color, sustaining a high polish, extremely fossiliferous in places, 
differing in this particular from the Niagara strata immediately overly- 
ing it. 
Between it and the Niagara Group is a fine clayey or marly 
bed, about nine inches thick, which in some places becomes quite hard, 
and in others is replaced by a soft blue clay. In connection with the 
Dayton limestone it usually attains the hardness of stone and is char- 
acterized by a number of minute species, which, considering the 
small attention hitherto paid to this course of stone, is unusually great. 
For the present it will be called the Beavertown marl, on account of 
