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 ( Tooth-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 dificrs 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 (O. lenticularis), about 

 half an inch in diameter ; the striae neither fine nor coarse, but sharp-edged, increasing late- 



