HELDERBERG DIVISION. 167 



western terminus at, Manlius, it is succeeded by the Onondaga limestone, the rocks which 

 interjjose at the east between it and the onondaga in tlie valley of the Hudson being dis- 

 continued (See PI. XX. Main section 1 ; also sections 2 and 5) . 



Disturbances which the Pentamcrus limestone has suffered. It is principally in the valley 

 of the Rondout that this rock has been disturbed to the greatest extent, in common with 

 its associates. Upon Catskill crei>k, three miles west of the village of Catskill, near tiie 

 railroad, the rock is not only elevated, but curved as represented in PI. IV., forming an 

 interesting and rather picturesque view. The cfTect is due to lateral pressure and the joint 

 operation of an elevating force, which has fractured this thick rock, and separated it from 

 its continuity. It appears in the face of a precipice 250 - 300 feet high. The exposed 

 rock at lire top of the eminence is the Pentamerus limestone. 



Terrace and outcrop of the Water and Pentamcrus limestones, as they appear iri the Schoharie 

 range (PI. viii.) . The semi-panoramic view in Plate 8 gives a better idea of the outcropping 

 rocks of the Helderberg, than can be conveyed by description alone. The series of rocks 

 in this plate are the same as those of PI. I. of the Helderberg range, except that the Old 

 Red sandstone does not occur within the field of view. The naked perpendicular cliffs are 

 formed of the pentamerus, which is the superior and prominent rock, and the thin-bedded 

 waterlimes which are directly beneath. The superior masses are the Onondaga limestone 

 and Marcellus shales ; the inferior, the thick-bedded Hudson-river series, which extends 

 north to the valley of the Mohawk. 



§.3. Delthyris shaly limestone (PI. XX. Sec. 1). 



The passage of the Pentamerus limestone into the Delthyris shaly limestone is rather 

 abrupt or indistinct. The entire mass of the shaly limestone is argillaceous ; some layers 

 consisting of slate, which disintegrate : others resist the action of the weather for a long 

 time, and are extremely tough and difficult to break ; they all, however, become drab- 

 colored externally on exposure to the weather, while internally they retain a bluish color. 

 The grain is fine, and unlike the pentamerus ; in fact, the harder layers are nearly com- 

 pact. The limestone throughout is impure, mixed with argillaceous matter and silex, 

 and for this reason it is unfit for lime. It is useless too as a flagging stone, and is only 

 good for stone fences. 



Distinctive characters. It is not difficult to distinguish this rock from the preceding lime- 

 stones, when once we have become familiar with its aspect ; still, the best characters are 

 derived from its fossils. Several species of Delthyris are confined to it (See Vani;xem's 

 Report, p. 120, and p. 122 for figures of a few of its characteristic fossils) . 



Extent and limits of the Delthyris shaly limestone. It requires to be traced in a line, as, 

 with the exception of one or two limited patches, it appears only in an outcropping edge. 

 At Becraft's mountain it is one of the principal rocks : this is its eastern limit. It forms 

 a north and south outcropping edge near the west side of the Hudson, from Kingston point 

 to Coeymans ; and from thence west, it accompanies the Pentamerus limestone as far as 



