432 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Sir William Logan are at issue as to the real position of the mass of 

 slate and conglomerate which extends from Gaspe to the Shawan- 

 gunk Range, a distance of 800 miles, — whether these strata are of 

 the Hudson-River or Oneida Conglomerate. 



The distinction between this Group C and its predecessor B is 

 broad in its mineral conditions, in the sudden diminution (in Utica 

 Slate) of the abounding Trentonian life, and in the accession through- 

 out Group C of new and interesting invertebrates, the majority of 

 them being typical. Utica Slate, comparatively unfossiliferous, has 

 only ten typical species ; but the Hudson-River group has sixty, 

 although the whole population of the latter is only three-tenths of 

 that of Trenton Limestone. Their Graptolites are numerous, and 

 usually typical. 



This Group C, then, is a distinct epochal centre ; but it is not 

 without its affinities to Group B, and through their Brachiopoda 

 principally, — this, however, not being so great as once thought ; for 

 the fossils common to both groups are only one-fourteenth of the 

 whole. 



Oneida Conglomerate, Group D of the Middle Stage. — The sepa- 

 ration of this stratum from every other (save one, perhaps) is justi- 

 fied by its mineral structure and constitution, and by the nearly total 

 absence of life. 



It consists of an alternation of three kinds of material : 1st, a con- 

 glomerate of quartz-pebbles ; 2nd, a siliceous sandstone ; and, 3rd, 

 of a hard red and green shale. When the thin limestones of the 

 Hudson-River section are wanting, and when conglomerates of white 

 and black quartz-pebbles become frequent, that section can scarcely 

 be distinguished from the Oneida Conglomerate. 



Medina Sandstone and Clinton Rocks, Group E. — This is entitled, 

 for useful purposes, to be treated as an independent formation, as will 

 be seen from the following remarks. Firmly as the Clinton strata 

 are linked to the Niagara group, they are still more intimately and 

 extensively united with the Medina, and especially in Pennsylvania 

 (H. D. Rogers). We are speaking with sufficient precision for our 

 present object, when we say that Group E stands apart from all its 

 neighbours in its lithological aspect, — an aspect which is perpetually 

 changing, and which becomes calcareous in the west. These two 

 sections contain large and beautiful Fucoids in great quantities (which 

 are exclusively their own), together with several animal organisms in 

 common, and typical of this group. 



Niagara Group and Schoharie Coralline Limestone, Group F, like 

 its kindred group H (Lower Helderberg), consists of limestones and 

 shales, which vary in purity over the large spaces they occupy, — 

 constantly returning, however, to their normal composition. 



The main distinction of the Group F resides in its characteristic 

 forms of animal life ; and this is amply sufficient to vindicate its in- 

 dependence. The fossils of the Niagara section are mostly new, are 

 extremely abundant in certain spots, and comprehend 145 typical 

 forms out of 180. There are here three times as many species of 

 Zoophyta, Bryozoa, and Echinodermata as there are even in the rich 



