GEOLOGY AND PALEONTOLOGY OF THE SCHOHARIE VALLEY 193 



Onondaga limestone 



The Schoharie grit passes upward by imperceptible stages into 

 a moderately pure limestone of grayish color and arenaceous or 

 muddy texture. Chert nodules are common but fossils are not 

 very abundant in the outcrops of this formation about Schoharie. 

 The rock is usually thin bedded, specially in its lower portion, 

 but massive strata, producing good quarry stone, are not uncom- 

 mon. The most extensive quarrying operations in this formation 

 are within the limits of the village of Cobleskill, but only the 

 upper beds of the formation are exposed here. Perhaps the best 

 exposure of the formation is along the bed of the small stream 

 which cuts the northern face of Sunset hill at East Cobleskill. 

 The total thickness here, according to the measurements of Pro- 

 fessor Prosser, is 95 feet, but only something over 60 feet of this 

 thickness is exposed in the bed of the stream. Numerous falls 

 are caused by the heavier beds, while joint fissures everywhere 

 traverse the rock. These fissures are frequently widened by 

 solution, and along them the stream is in places drawn off into 

 underground drainage. Other good exposures of the rock are 

 on the summit of West hill, where from 20 to 30 feet of this 

 formation remain. Though heavily forested, the ledges are well 

 exposed, and are generally broken into huge blocks by the 

 numerous widened fissures which traverse it. Along the road 

 between Dann's hill and Sunset hill, ledges of the Upper Onon- 

 daga crop out abundantly. Near the northeastern end of the 

 outcrop the rock lias the characteristics of a coral reef with 

 species of Favosites, Zaphrentis and Eridophyllum occurring in 

 abundance.. The coral reef structure is characteristic of this 

 formation in other parts of the State, and it is most probable 

 that in tins region numerous other reefs occur, which were the 

 source of the lime sand and mud from which these clastic lime- 

 stones were made. (Joort exposures of this limestone are also 

 found on the small creek which descends the east face of the 

 depression between Sunset hill and South hill, opposite Fris- 

 hy's mill and again on the slope of East mountain, where 



