INTRODUCTION. - 33 



The Lower Helderberg group, which constitutes the more important 

 portion of the strata from which are derived the fossils of the present 

 volume, has been so termed from its very complete development along 

 the base of the Helderberg mountains; constituting, in this part of 

 New- York, an important fossiliferous group. In some parts of the 

 Helderberg mountains, and along the Hudson river at Rondout, and 

 at Schoharie and elsewhere, the lowermost beds of this group rest 

 directly upon the Waterlime beds, which we regard as the uppermost 

 member of the Onondaga-salt group, indicated as a separate formation 

 by reason of its economical importance, and likewise characterised by 

 certain peculiar fossils, while the marls of the Salt group are usually 

 nonfossiliferous. 



The lowest member of the Lower Helderberg series is a thin-bedded, 

 often thinly laminated, dark-blue limestone, which, from the abundance 

 of its tentaculites, has been termed the Tentaculite limestone. Its 

 color, texture and composition, contrasts strongly with the rock below. 



The second member of this group is a thin mass of limestone, con- 

 sisting almost entirely of the coral Stomatopora, and constitutes a very 

 persistent member of the group ; to this succeeds a limestone charged 

 with great numbers of the broken shells of Pentamerus galeatus, and 

 known as the Pentamerus limestone. This graduates above into a shaly 

 formation, which was designated in the New- York Reports as the 

 Delthyris shaly limestone, from the abundance of this genus of fossils. 

 It is the most fossiliferous member of the group, as will be seen by 

 consulting the position of the fossils described in the following pages. 

 This shaly limestone, in physical character and composition, corre- 

 sponds nearly with the shaly member of the Niagara group, and contains 

 numerous similar or representative forms. 



To this succeeds a compact crinoidal limestone, and above this is a 

 mass of bluish gray limestone, charged with Brachiopoda, among which 

 a Pentamerus similar to P. galeatus is so abundant that the rock has 

 been termed Upper Pentamerus limestone. 



[ Pal JSONTOLOOT III.] 5 



