IO NEW YORK STATE MUSEUM 



lentils the largest of which are east of Cayuga lake and attain a 

 thickness of 20 to 30 feet, and sometimes are quite fossiliferous. 



There are exposures of thin lentils at Phelps, Honeoye Falls and 

 Morganville and a vertical crevice in the Cobleskill waterlime in the 

 cement quarry at Buffalo was filled with the characteristic gray 

 sand of this formation, and at some localities where it is absent as 

 a distinct stratum the basal tier of the succeeding limestone contains 

 a considerable proportion of the same material. 



At most exposures where the sand is not present this horizon 

 is marked by a thin, uneven seam of black shaly matter, in which 

 small pebbles of waterlime or black sand are imbedded forming 

 a coarse conglomerate a few inches thick. The sandstone is absent 

 and the horizon is but slightly marked on this quadrangle. 



ONONDAGA LIMESTONE 



This formation is composed of dark bluish gray limestone, 

 bedded in layers from 3 inches to 2 l / 2 feet thick in which there is 

 usually a quarter or less proportion of chert or hornstone in nodules 

 or nodular layers. It was formerly known as the " Corniferous " 

 limestone, and included in the " Upper Helderberg Group," a few 

 layers at the base in which chert is absent being distinguished as 

 the Onondaga limestone. The terms " Corniferous," and " Seneca " 

 limestone were at one time used interchangeably. The term Onon- 

 daga limestone is now adopted to cover all the strata in central and 

 western New York, which lie between the Oriskany horizon and 

 the Marcellus black shale, here aggregating in thickness about 125 

 feet. 



The limestone contains considerable black, carbonaceous matter 

 that appears in the shaly partings and on the surface of the tiers. 

 It is removed by slow decomposition on exposure and the rock 

 weathers to a light bluish gray color. 



The chert is usually very dark and but slightly translucent, but 

 is sometimes much lighter colored and chalcedonic. It is very 

 unevenly distributed in the beds predominating in some parts and 

 frequently occurs in nodular layers that are continuous for long 

 distances and the separate nodules from I to 3 inches in diameter 

 occupy much of the remaining space. 



Weathered outcrops and loose blocks from the cherty beds have 

 a peculiarly ragged and scraggy appearance owing to the superior 

 resistance to decomposition of the chert over the limestone. This 

 chert, commonly called flnt, was of the greatest value to our Indian 

 predecessors as it is the material from which their spears, arrows 

 and other implements were formed, but until recently its presence 



