Ulrich on Correlation of the Lower Silurian. 315 
we find them represented by an apparently uninterrupted series 
of shales 300 feet thick. These shales, because they are black 
or brownish and contain Leptobolus insignis., have been identi- 
fied by Prof. Orton with the Utica shale of New York. 
Though a little anticipatory, I may still be allowed to say here 
that, judging from the position held by them and from a series 
of drillings kindly sent me by Prof. Orton, I fully concur with 
him in thus placing them. 
On page 117 of his "preliminary report upon petroleum 
and inflammable gas," Prof. Orton concludes that in tracing 
the shales southward from Springfield, to Dayton, then to Mid- 
dleto^vn (\vhere it is only 100 feet thick) and finally to Hamil- 
ton, it rapidly lost in thickness. At the last locality the shale is 
reported as only 37 feet thick. At this rate of decrease it would, 
of course, run out before reaching the Ohio river. We inust, 
hov/ever, bear in mind that it is only the black shale that is de- 
creasing in vokmie, while, if the 300 feet of this shale at Find- 
lay is not represented in the Ohio valley, the Lower Silurian 
strata above it must have increased from 460 feet at Findlay to 
^t least 800 feet near Cincinnati. 
In my opinion, and I hope to show it in a later installment, 
this increase is greater than the facts known and relating to 
the point indicate. For the present it must suffice to say that, 
if not all at least a large proportion of the strata comprised in 
beds XI are equivalent to the black or Utica shales underlying 
Findlay. A study of the tabulated list will show that the Cin- 
cinnati exposures of these beds have furnished not only one 
but at least ten of the species which Walcott and others regard 
as strictly diagnostic of Utica. When we consider the change 
in composition, this is as great a proportion of such species as 
can reasonably be expected. 
{To be continued.) 
