144 
GEOLOGY OF THE THIRD DISTRICT. 
34 . 
View of Howland quarry, from a drawing by R. C. Taylor. 
The Seneca limestone is the terminal part of the corniferous limestone, and of the great 
range of upper limestone rocks. With it, also, the Helderberg division of the New-York 
system ends. Through Onondaga and Cayuga counties it is abundant, and extends into the 
fourth district, where I first observed it. In all its localities, it rests upon the corniferous 
limestone, without any obvious mineral line of separation. The reason for considering it to 
be a distinct rock, was the finding it in Seneca county the first year of the survey, uncon¬ 
nected with any other rock, and subsequently in the third district, containing in both districts 
the Strophomena lineata in great abundance, none of which had then been discovered in 
any part of the district below this rock; but since that time, one specimen has been found 
adhering to the flint of the corniferous in the State Collection. This rock is the geological 
point at which a fossil appeared for the first time in the district, and in myriads which dis¬ 
appear with the rock, not one being found in the marcellus shales above. This fossil, or one 
similar to it, is in great abundance in the Hamilton group, but no comparative examination 
has yet been made to ascertain their relationship. 
Fossils, as a character to designate rocks, are of two kinds: In the one kind or character 
they are limited to the rock, like the Strophomena deltoidea of the Trenton limestone, the 
Pterinea carinata and Cyrtolites ornatus of the sandstone shales of Pulaski, the Pentamerus 
oblongus of the Clinton group, the Pentamerus galeatus of the limestone of that name, the 
Atrypa elongata of the Oriskany sandstone, and a host of others which the facts of geology 
show are fixed to a rock, mass, period or era. The other kind were a longer lived race, 
having had more enduring powers, appearing and reappearing in the same locality ; and as 
a geological character, their utility arises from the limitation of their range, the mineral 
nature of the rock, and its association with others whose existence was limited to the same 
