8 4 



is rarely more than six inches (15-2 cm.) to one foot 

 (30-4 cm.) and it is frequently absent. Where found 

 it usually grades into the overlying deposits without a 

 break. Southeast of Hagersville these overlying deposits 

 are beds of true Oriskany sandstone (10). At most other 

 places they belong to the Onondaga limestone. The 

 Oriskany (3) is usually composed of moderately coarse- 

 grained quartz sand but it is often much coarser and some- 

 times even pebbly, the individual grains of which are as 

 much as an inch in diameter (5). Some parts of the rock 

 are so closely cemented by silica that it resembles a quart- 

 zite in appearance. The rock is usually massive and some- 

 times the entire formation appears as a single bed. The 

 total thickness of the Oriskany sandstone rarely exceeds 

 20 feet (6- 1 m.) and much less is the usual rule. Although 

 a large part of the deposit is almost barren, fossils are 

 often abundant and in a good state of preservation, even 

 the spires of certain brachiopods and the finest external 

 markings being preserved. 



Where the true Oriskany sandstone is absent, there is 

 sometimes found a deposit of several feet of chert which 

 has been assigned to the same formation as the sandstone 

 (5). However, the fossils so far found in it are so rare and 

 fragmentary that its true age is somewhat in question. 



ONONDAGA. 



The Onondaga limestone usually rests on an eroded 

 Silurian surface, but occasionally it lies unconformably 

 on the Oriskany sandstone. Where the latter is the case, 

 the unconformity is marked chiefly by the change in fossils, 

 but at some places there may be found a well developed 

 conglomerate in which fossiliferous pebbles of Oriskany 

 sandstone are mingled with the remains of Onondaga 

 corals and fishes. 



Where the Oriskany is absent the conglomerate 

 persists, but at such places the pebbles are of Silurian 

 dolomites. The lower portion of the Onondaga limestone, 

 in this region, is usually arenaceous and very cherty. 

 Sometimes the sand is so abundant that the deposit 

 becomes an arenaceous chert, and again a true sandstone. 

 This latter is the case at Springvale, where the lower 

 portion of the Onondaga takes on such a marked resem- 

 blance to the true Oriskany that it has often been confused 



