REMARKS. 
217 
REMARKS, PRELIMINARY TO THE FOLLOWING ROCKS AND GROUPS. 
In the Fourth District, the Tully limestone terminates all those deposits in which calcareous 
matter forms an essential part. In all the higher rocks, this material, when existing, is the 
result of the destruction of organic bodies; and in the few instances where it appears, the 
origin is unquestionable, for the fossils still retain so much of their original form as to be rea¬ 
dily recognized. This rock forms a strong line of demarcation not only in this respect, but 
also as regards fossils, very few forms which are known below continuing into the rocks above. 
The lithological character of the products above this rock are throughout more or less similar, 
while they differ from those below; and with a single exception, lithological character is a 
sufficient guide for distinguishing the different strata. 
This contrast of character is more marked towards the western extremity of the district, 
than it is farther east; and finally, on its eastern extreme, there is a greater similarity in the 
lithological features. This change is likewise attended with the occurrence of some of the 
fossils of the lower group in the rocks of the higher, the nature of the two being very similar, 
although the Tully limestone is in its greatest force; while at the west, where it does not 
exist, no such mingling of the fossils is known. 
At Ithaca, for example, where we are far above the Tully limestone, and where the rocks 
are well marked by an abundance of fossils peculiar to themselves, still we find the Microdon 
bellastriata, the Modiola concentrica and some others, and I have even detected the Calymene 
bufo and Dipleura Dekayi in the same association. Still farther east, there is a greater min¬ 
gling of species of the lower rocks with the upper, and a nearer approach constantly in materials 
of composition. These circumstances, in the eastern portion of the State, render it difficult 
to point out the line of demarcation between the lower and higher rocks of this division. 
At the eastern extremity of the State, also, the Tully limestone does not exist, and therefore 
that guide to the line of division between the lower and higher groups is wanting. The ab¬ 
sence of this rock, and the similarity of lithological products as well as the mingling of the 
organic remains of the lower rocks, renders it impossible to make a distinction in groups with 
the same degree of satisfaction as farther west. By reference to the section along the Genesee 
river (Plate 13), it will be seen that the Genesee slate, a black carbonaceous mass, is suc¬ 
ceeded by shale of a deep green color, and well defined above. This is succeeded by flag¬ 
stones, which alternate with shale of a less deep green color, and often with black shale. 
These again are followed by a greater proportion of sandstone, often thick-bedded, and pre¬ 
senting far less of the characters of those below. 
These rocks are well exposed on the Genesee river, and there can be no possible room for 
error in their examination. The whole thickness here exposed is scarcely less than one thou¬ 
sand feet; and throughout this thickness, there is as yet no fossil known which occurs in the 
Chemung group to the south and above these rocks. These circumstances led to the separa¬ 
tion of this portion of the system, and the adoption of Portage or Nunda as the locality de- 
[Geol. 4th Dist.] 28 
