306 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Regarding the strata above the Tully limestone, Hall's remarks ^^^' "prelimi- 

 nary to the following rocks and groups" may stUl be read to advantage: 



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 readily recog- 

 nized. This rock forms a strong line of demarcation not only in this respect, but also as regards 

 fossils, verj^ 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 toward 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 

 beUastriata, the Modiola concentrica, and some others, and I have even detected the Calymene 

 hufo and Dipleura dekayi m the same association. Still farther east there is a greater mingling 

 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 there- 

 fore that guide to the line of division between the lower and higher groups is wanting. The 

 absence of this rock and the similarity of lithological products, as well as the mingling of the 

 organic remains of the lower rocks, render 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 it will be seen that the Genesee slate, a black carbonaceous mass, is succeeded by shale 

 of a deep-green color and well defined above. This is succeeded by flagstones, 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 presenting 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 1,000 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 separation of this 

 portion of the system and the adoption of Portage or Nunda as the locality designating the 

 group. Farther east the green shale (Cashaqua shale) is often darker in color and more sandy, 

 becoming largely interstratified with flagstones which have much the character of those which 

 lie above it on the Genesee, except in the absence of a peculiar species of Fucoides. At the same 

 time the small Avicula, which is so abundant in the Genesee slate, is almost equally abundant 

 in the green shale and flagstones at Penn Yan. 



Again, when we go westward as far as Lake Erie, we find that instead of the flagstones 

 succeeding the green shale, as on the Genesee, a black shale is the first mass above. This soon 

 gives place to green shale, and we have alternations of green and black shale for many hundred 

 feet, before coming to the thin-bedded sandstones. 



