CANANDAIGUA AND NAPLES QUADRANGLES 11 



number. The fossils occurring here, besides the Stromatopora, are 

 Leperditia alt a Conrad and L. s c a 1 a r i s Jones, 

 Cyathophyllum hydraulicum Simpson, Spirifer 

 eriensis Grabau and Whitfieldella sulcata 

 Vanuxem. Fragments of Eurypterus also occur at this horizon. 



DEVONIC 



General observations. The division line between the great Siluric 

 and Devonic systems is very well marked here on account of the 

 entire absence of the Helderbergian limestones, which, in the 

 eastern part of the State, represent the incipient stages of Devonic 

 deposition. There is good reason to believe that the uppermost 

 Siluric beds which we have just considered were for a time ex- 

 posed above water to the action of aerial decomposition and 

 erosion before the later sediments were laid down on them. This 

 has been found to be the case in Erie county, where the eroded 

 upper surface of the Cobleskill dolomite is overlain by a regular 

 deposition of the following formations. 



Oriskany sandstone 



In eastern New York this formation takes on, in certain places, 

 the character of an arenaceous limestone but it is an interrupted 

 deposit in its course across the State from east to west, though 

 in places tremendously abounding in fossils. At Oriskany Falls 

 and at Union Springs it assumes the character of a more or less 

 friable whitish sandstone. The formation constantly thickens 

 and thins, forming lenses, as in Cayuga county, sometimes 20 or 

 more feet thick and then again thinning to actual disappearance. 

 As it becomes thin it usually assumes the character of a hard com- 

 pact quartzite composed of silicious grains cemented by a deposit 

 of silica. Throughout western New York this thin bed frequently 

 contains angular masses, evidently washed from the hydraulic 

 limestone beneath and thus forms a breccia. In Ontario county 

 the exposures of this rock are largely confined to the townshi]) 

 of Phelps a few miles to the east of this quadrangle. From its 

 uneven thickness and general appearance at this ])lace and the 

 fact that it fails entirely within a half mile on the east and a 



