1(36 HELDERBERG DIVISION. 



igneous injected mass, similar in its formation to a trap dyke, inasmuch as there are no 

 appearances of ignition upon the adjacent rock. 



§2. Pentamerus limestone (Plates xx. and viii.). 



This is a gray crystalline limestone, with thick beds from its beginning. Its beds, 

 however, are uneven, and some are concretionary and extremely rough ; in fact, as will 

 be seen, this concretionary mass is the most persistent and extensive. Its name is derived 

 from the great abundance of a fossil called the Pentamerus galeatus (See Vanuxem's Re- 

 port, p. 117, fig. 1) . It is constant in the rock as far west as Herkimer county ; but at its 

 extreme western limit, it is rare, if it exist at all. 



Points where this rock may be examined. The most eastern limit of the rock is Becraft's 

 mountain, about three miles southeast of Hudson, on the road to Catskill : it forms two 

 or more high bluffs on the east side of the road. Again, west of Catskill, or on the rail- 

 road between Catskill and Leeds, or on the turnpike between the two places. So also it is 

 an outcropping mass on the west side of the Hudson river, forming the first of a series of 

 cliffs from Kingston point to Coeymans. At numerous places also nearer the main hills of 

 the Helderberg ranges, this rock is constantly present in heavy beds, whose aggregate 

 thickness is about thirty feet. Another extensive outcrop of this rock forms the northern 

 brow of the Helderberg hills : it exists in cliffs, formed by its outcrop, from Knox to Her- 

 kimer count)', where it begins to lose this character, and to become more depressed. At 

 Cherryvalley, at the head of a ravine, it seems to have attained its maximum develop- 

 ment : from this point westward, it begins to thin out ; and when it has reached Manlius, 

 Onondaga county, it is only a few feet thick, and destitute of its characteristic fossils or 

 other marks except its peculiar concretionary structure. At Tyler Post-office, and at 

 Geddes, five miles west of Syracuse, the Onondaga-salt group is in contact with the Onon- 

 daga limestone, this whole mass having thinned out entirely. 



Uses. This rock is totally unfit for purposes of construction : it is rough, and makes an 

 indifferent wall. A part of it forms the cement rock of Ulster county. As a limestone, for 

 ordinary purposes, it is not esteemed ; for agricultural purposes, it may be well adapted, 

 but has not been tried. 



Range and extent. It will not be difficult to trace this range of limestone, from what has 

 been said already. It begins at Becraft's mountain, near Hudson ; forms one of the highest 

 outcropping rocks in the first tier of hills which bound the Hudson valley west, from Coey- 

 mans to Kingston, and thence from Kingston to the falls of the Rondout. The same may 

 be traced from Coeymans west to Manlius, and disappears entirely a few miles west of 

 Manlius. Tlie belt it forms is every where inconsiderable, and would be more properly 

 represented by an outcropping edge. 



Relations and connection of the Pentamerus limestone. Below, every where in New-York, 

 it is succeeded by that part of the Onondaga-salt group which is called the Manlius water- 

 limes. Above, and at the east, it is succeeded by the Catskill shaly limestone. Near its 



