140 
GEOLOGY OF THE THIRD DISTRICT. 
be relied upon alone ; for all the limestone rocks, from that which forms the bed of the Niagara 
falls, to the one under consideration, contain flints in one or more localities, and arranged in 
the same manner. No difficulty now can take place in its use, the position of this limestone, 
including the Seneca, being well defined; resting invariably upon the Onondaga limestone, 
and covered by the Marcellus shales, the shales and the corniferous and Onondaga limestones 
being coextensive deposits in the district and State. 
But little if any of the layers afford a pure limestone, being usually impure, the lower layers 
especially. Its color varies from black to ash grey, brownish, and light dull blue. The layers 
which contain flint are usually very compact; others show a crystalline grain, but more rarely ; 
and there are many in which the mixture of shale is very evident, and even in great excess. 
The color of the rock is more dark at the west than at the east end of the district. 
The joints or vertical divisions in this rock are extremely well defined ; so that in quarrying, 
it presents walls or sides of the greatest regularity. Quarries are usually opened where the 
nodules of flint exist, the layers immediately below the flint being esteemed the best. The 
joints are nearly at right angles to each other, the one which is commonly observed in this 
rock, forming the back of the quarry, is north about twenty degrees east. 
There are many fossils which are peculiar to the corniferous limestone, and by them it is 
readily recognized in the district; but the individuals are few, and are not found in every 
locality, and therefore are not so useful practically as many which are not exclusive, and 
which are numerous in the rock. They are, however, so different as a group from those of 
all its contiguous members, that it is readily known by them. 
The most characteristic fossil of this rock is the Odontocephalus selenurus, figured as No. 
1 of the preceding wood-cut. It is quite numerous in some parts of the limestone, but the 
heads and tails are separated. Only recently a specimen was found entire ; previous to which 
they were supposed to belong to distinct trilobites, and known by distinct names ; the head 
being the Calymene odontocephalus ( Tootli-head calymene ), and the tail part the Asaphus 
selenurus ( Moon-tailed asaphus ). They were united, and the genus and species constructed 
by Mr. Conrad from their conjoined specific names. 
The next characteristic fossil is the Cyrtoceras trivolvis of Mr. Conrad. There are four 
or five species of this peculiar genus in this rock, all which look like ram’s horns ; they are 
from about four to seven inches in diameter. No. 2, the species figured in the wood-cut, is 
different, being more expanded towards the mouth, and the surface markings undulating upon 
the sides and back, whence its name of undulatus. 
The corniferous limestone also contains two species of strophomena, which were formerly 
considered to be the same, and known by the name S. depressa or rugosa. No. 3 is the 
Undulated strophomena, and differs chiefly from the other in having about eighteen ridges 
upon the upper valve, while the rugosa or depressa has but about eight: the size of the 
shells is about the same. So far, the Undulated strophomena has only been seen in this rock. 
No. 4 of the wood-cut is a small orthis, called Lenticular orthis (0. lenticularis), about 
half an inch in diameter; the striae neither fine nor coarse, but sharp-edged, increasing late- 
