CANANDAIGUA AND NAPLES QUADRANGLES 



25 



eastern limit of this map and here the exposure shows not only 

 the black Gorham shales above but the soft Moscow shales beneath. 

 It is separated by a thin shaly seam into two layers, and in the 

 lower layer iron pyrites is highly abundant in nodules, probably 

 representing the commencement of the pyrites layer, which from 

 here westward is the sole representative of this formation. The 

 place of the Tully where the limestone is wanting and the pyrite 

 layer not clearly apparent is always well defined by the sharp 

 line of contrast between the gray Moscow shales beneath and 

 the overlying black Gorham shales. 



The Tully limestone, as has been recognized since the observa- 

 tions by Conrad in 1836-37, is distinguished by the presence 

 of the species Rhynchonella or Hypothyris cuboides 

 and the equivalency of this geologic horizon with the Cuboides 

 zone of Europe has been a fact of general recognition for more 

 than half a century. This fossil is very abundant in the outcrops 

 in Ontario county but the rest of the fauna is essentially that of 

 the underlying beds of the Hamilton group, specially the Moscow 

 shale. We shall presently note in more detail that the fossils 

 contained in the pyrite layer have all been singularly dwarfed by 

 the unfavorable conditions of growth and are regarded as repre- 

 senting stages of arrested development of Hamilton species, the 

 characteristic Hypothyris cuboides not having been 

 found therein. 



Genesee shale 



This term was originally applied by Hall to a division on the 

 Genesee river section consisting of very dark bituminous beds at 

 the base becoming lighter colored and more sandy upward. The 

 highly bituminous beds are distinctly defined by their character 

 and their definition at the top by the Genundewa limestone. It 

 was clearly this excessively black mass of shale that it was in- 

 tended to distinguish by the name Genesee and as it is now im- 

 portant to refine the subdivision of this series of sediments for 

 more exact correlation, it is here proposed to restrict the term Gen- 

 esee to this lower member only. 



Directly over the Tully limestone, or its horizon when absent, 

 lies a mass of densely black bituminous shale becoming very 



