3IO Ulrick on Correlation of the Lower Silurian. 
than one thousand individuals was found. These occupied the 
surface of a layer of the shale over a space scarcely 3 feet 
square. In Mercer, Boyle, Washington and other counties of 
central Kentucky, the contact between beds X and Xla is often 
exposed. At the base of the latter, though not often seen in 
places because of its easy disintegration, there is generally a 
layer of limestone, one foot or so in thickness, which usually 
contains some rounded limestone pebbles, a little iron, fluor- 
spar, and worn fragments of shells and bryozoa. Resting upon 
this there may be several feet of thin hydraulic limestone, then 
15 feet, more or less, of dark limestone, appearing thick-bedded 
in fresh cuts, but bi-eaking up into small pieces when long ex- 
posed. These are largely made up of the zoaria of an unde- 
termined ramose bryozoan, with now and then an example of 
a massive species of Creplpora. Between these bryozoa layers 
there is also usually included one or more irregularly bedded 
hard crinoidal limestones. 
Overlying the bryozoa beds we usually find several feet of 
soft, drab slightly greenish shales containing hard concretionary 
masses and a few fossils, mainly Alonotty fella briareus. Above 
these shales there is usually a thick layer of limestone, probable 
equivalent to the crinoidal layer which marks the top of the sub- 
division in the Ohio river exposures. This is supjDOsed to be so 
from the fact that the superposed strata have the character of 
XI^. 
Much study is still necessary before we can satisfactorily cor- 
relate the strata of Xla observed in the central Kentucky 
counties south of the Kentucky river with those exposed ii» the 
neighborhood of Cincinnati. One thing seems certain, and that 
is, that the lower or trllobite layers are entirely absent south of 
the Kentucky river. That some member is missing is already 
indicated by the decreased thickness of the subdivision, it 
having diminished to about half the thickness observed at 
Cincinnati, 
In Washington, Mercer and Boyle counties none of the ex- 
posures seen by me exhibit any strata which could be regarded 
as representing the trilobite layers, nor can I find any mention 
of such in Mr. Linney's reports on the rocks of those and other 
counties studied by him. That these layers \vere carefully 
