stones, shales and some layers of limestone. The seas 
which existed at the time these layers were formed, must 
have abounded with life, for the remains of corals, brachi- 
opods and lamellibranchs are found in the greatest pro- 
fusion. Occasionally the remains of the earliest fishes are 
found, while terrestial plants are now more numerous than 
heretofore. Insects, too, make their appearance in this 
period. 
The four groups into which this period has been 
divided, the Marcellus Shale, the Hamilton Beds, the 
Tully Limestone and the Genesee Slate, are all well 
represented in our county. Two of them, the Marcellus 
Shale and the Tully Limestone, here reach their culmina- 
tion. 
The [arcellus Shale.—Directly succeeding the upper 
layers of the Corniferous Limestone, we find an extremely 
soft, black, fine-grained shale with an occasional layer of 
limestone. This is the oldest member of the Hamilton 
Period, the Marcellus Shale. It can readily be distin- 
guished from any of the preceding groups by its black 
color, by the extreme thinness of the shale, and by the 
fact that when rubbed it gives off a bituminous odor.” As 
stated, itis for the most part a thin, slaty and extremely 
brittle mass, which quickly disintegrates when exposed to 
the weather. Nevertheless, a few feet above the base of 
the shale a thin layer of limestone, usually not more than 
two or three feet in thickness, occurs. This limestone is 
very impure, and readily breaks up into small irregular 
fragments when hit with the hammer. It is often called 
by the popular name, ‘‘shell rock”, not because it con- 
tains an abundance of shells, but because to the popular 
mind the small irregular masses into which it readily 
breaks somewhat resemble shelled corn. Local geolo- 
gists, however, term this rock by a more appropriate 
name, the Goniatite Limestone. The shales in the upper 
34 
