616 TRANS. OF THE ACAD. OF SCIENCE. 
have occupied. It is never (?) more than ten feet thick, 
thinning to a knife-edge, and disappears in some directions. 
* Third Limestone, Millstone Grit Series.—This mass varies 
from twenty to fifty feet in thickness, the upper part of 
it separated from the fourth sandstone by a bed of marly or 
aluminous shale of greater or less thickness. The whole 
mass is sometimes found as alternate beds of marly shales, 
thin-bedded limestone, with sometimes a bed of limestone 
Bryozoa. 
_ g. Locally, the third limestone has a considerable develo 
feet, the united thickness of both beds would be over one 
hundred feet. A locality showing both beds f and g largely 
developed is of rare occurrence. Ay 
h. Third Sandstone of the Millstone Grit Series. — This 18 4 
well defined mass of sandstone from twenty-five to forty 
ing blackened by fossil tar. The beds of the whole mass ay 
not all charged with the bituminous deposit, t 
charged with it are intercalated between beds in which va 
of it is visible; the beds charged with tar vary1n from an ine 
to two feet in thickness and alternating with eds varying 
in thickness that appear to contain none of it. Some loc 
ties exhibit the outcropping edge of the mass sien a hee 
y the bituminous matter flowing from the beds which co 
tain it over those which do not. 
Some parts of this mass are frequently oolitic 
it contains regular plates or beds of chert ney pros 
The lower part of the mass is frequently formed 0! 
while, the 
i i iti puff aluminous 
middle being oolitic, the upper part 18 4 Pentremites pyri 
