314 Ulrich on Corr elation of the Lower Silurian. 
widely separated, with bryozoa the principal fossils. Above 
these there are seventy or eighty feet more of shales and lime- 
stones, differing from the preceding eighty feet in having the 
limestones, especially in the lower part, heavier and the shale 
bands thinner. 
This portion of the section contains several "wave layers"' 
like those that occur in the same beds at Cincinnati. The up- 
per one, which in central Kentuck}^ comes in at about 170 feet 
above the top of beds X, is a marked feature at many localities 
in Washington and other counties. 
Above this wavy layer there is a succession of limestones and 
shales 100 or more feet thick, the limestones, occurring in 
courses from two to eight inches thick, being generally impure, 
but in some cases semi-crystalline. The shales are more or less 
sandy and form beds of from two to twenty feet in thickness.. 
The sandy character of the strata increases toward the top 
causing some difficulty in drawing the line between this division 
and the arenaceous base of beds XII. 
Fossils belonging to the same species as those mentioned in 
the description of the Cincinnati exposures are very abundant 
in both the shales and limestones, particularly so at several hor- 
izons in the lower and middle thirds of the sub-division. In 
these they are also well preserved and readily washed from the 
matrix; but in the sandy upper layers they are neither so well 
preserved nor so plentiful. 
Of the 1S6 species that are mentioned in the list as occurring 
in these beds, loS, or more than one half, are restricted to them,, 
eighty are found in both the upper and lower portion, fifty pass 
into them from lower beds, and fifty-two pass into beds XII. 
The late discovery of natural gas and oil in northern Ohio 
and Indiana has given us excellent opportunities for comparison. 
Innumerable wells have been drilled, and of many of them 
fairly reliable data have been preserved. These are often im- 
portant and generally most interesting. In this place, however, 
I shall restrict myself to a few remarks on the northward ex- 
tension of the 300 feet of shales and hmestones which above 
are described and designated as beds XI. 
In tracing the beds northward from Cincinnati it appears that 
the calcareous material gradually decreases till at Findlay, Ohio, 
