Ulrich on Correlation of the Lower Silurian. 311 
searched for by him I cannot doubt since in a conversation 
with him, he expressed himself as greatly disappointed because 
of his failure to discover certain species belonging to this hori- 
zon in the counties in which he was engaged. 
Nor is this horizon established in Fayette Co., but in going 
farther northwaixl I have observed, but not studied, certain beds 
at Georgetown and Rogers Gap which may represent them. 
At the last locality, especially, the Cincinnati southern R. R. 
has made several extensive cuts which would, I believe, throw 
much light upon the point. 
XI^. This sub-division is exposed in numerous runs and 
creeks that empty into the Ohio and Licking rivers in the 
vicinity of Cincinnati. The upper half forms the base of the 
hills surrounding that city and Covington and Newport, two 
cities directly across the river in Kentucky. Over three-fourths 
of the entire mass, which here has a thickness of 235 feet, is 
composed mainly of soft shales. These vary slightly in color 
as we go upward, being greenish-gray, drab or yellowish, for 
the first twenty feet and of various shades of blue throughout 
the rest. Limestones are few and generally thin in the lower 
ninety feet, more abundant and in heavier courses (3' to 8' ) in 
the succeeding forty feet. Above this horizon for forty feet 
or more the shales again predominate greatly over the lime- 
stones, the latter being generally less and rarely more than three 
inches thick and separated from each other by beds of shale 
three feet or more in thickness. From this horizon which 
is often marked by two six-to-twelve-inch layers, to the top of 
the sub-division, a distance of fifty feet, the limestones, though 
still thin-bedded, become more frequent, the separating shale 
being usually less than one foot in thickness. Both the shales 
and limestones in this portion of the section gradually assume a 
sandy character, but this is never so pi'onounced as in the lower 
portion of the succeeding division. 
At the base of these shales and resting directly upon the 
heavy crinoidal limestone top of XI(7, there are often about 
two feet of hard drab shales having a concretionary structure. 
These are unfossiliferous, l)ut in the next ten feet quite an 
abundance of species may be found free or on the surface of 
thin limestones slabs. Crepifora vcnusta and Stlctoforclla 
