THE SILURIAN PERIOD 543 



and thin where they are thickest. It contains few fossils, and it 

 is thought that it may be a chemical precipitate. Its relations to 

 the shales and salt beds are such as to indicate that the areas of 

 accumulation of the several sorts of rock material were shifted 

 from time to time, as if by gentle changes of level of land or water. 



Above the Salina proper of New York, there is a thin (150 feet 

 maximum) series of limestones, the Waterlime (Cobleskill, Roncl- 

 out, Manlius, etc.), generally regarded as a part of the Silurian 

 system. The name Waterlime has reference to the fact that the 

 limestone is the source of hydraulic cement, though it is by no 

 means everywhere useful for this purpose, and many other lime- 

 stone formations are similarly used. The Waterlime is more wide- 

 spread than the Salina, extending westward through Ohio to 

 Indiana and Wisconsin. Both its distribution and its character 

 show that the eastern interior was more generally submerged than 

 during the deposition of the salt-bearing series which preceded. 



In the northern Appalachians there is a conglomerate formation 

 (Shawangunk, N. Y., Green Pond Mountain, N. J.), formerly 

 classed as Oneida, which is now regarded as contemporaneous with 

 the Cayugan series. Until recently, this conglomerate was not 

 known to contain fossils, but their discovery has shifted the classifi- 

 cation of the formation from the early Oswegan to the later Cayugan 

 series. 1 The materials of this conglomerate are mostly quartzose, 

 and seem to have been derived from lands to the east. They have 

 been thoroughly indurated by cementation, so that the formation 

 is exceedingly obdurate. The outcropping edges of its tilted beds 

 constitute the crest of the Kittatinny range in New Jersey, and its 

 continuations in New York and Pennsylvania. Silurian rocks have 

 been recognized recently among the gneisses of Connecticut. 



The Helderberg formation, formerly regarded as a part of the 

 Silurian system, is here classed with the Devonian. 



Silurian of the West 



At various points in the West, there is a series of sedimentary 

 beds, poor in fossils, between the known Ordovician below and the 

 Devonian above. The character of the fossils being indecisive, 



1 Hartnagle, Bull. 107, N. Y. State Mus. 



