THE LOWER SILURIAN PERIOD. 95 



regarded as constituting an exceedingly full and typical series 

 of the deposits of this period. The chief groups of the Silurian 

 rocks of North America are as follows, beginning, as before, 

 with the lowest strata, and proceeding upwards (fig. 35) : 



1. Quebec Group. This group is typically developed in 

 the vicinity of Quebec, where it consists of about 5000 feet of 

 strata, chiefly variously - coloured shales, together with some 

 sandstones and a few calcareous bands. It contains a number 

 of peculiar Graptolites, by which it can be identified without 

 question with the Arenig group of Wales and the correspond- 

 ing Skiddaw Slates of the North of England. It is also to be 

 noted that numerous Trilobites of a distinct Cambrian fades 

 have been obtained in the limestones of the Quebec group, 

 near Quebec. These fossils, however, have been exclusively 

 obtained from the limestones of the group ; and as these lime- 

 stones are principally calcareous breccias or conglomerates, 

 there is room for believing that these primordial fossils are 

 really derived, in part at any rate, from fragments of an upper 

 Cambrian limestone. In the State of New York, the Grapto- 

 litic shales of Quebec are wanting ; and the base of the Silurian 

 is constituted by the so-called " Calciferous Sand-rock" and 

 " Chazy Limestone."* The first of these is essentially and 

 typically calcareous, and the second is a genuine limestone. 



2. The Trenton Group. This is an essentially calcareous 

 group, the various limestones of which it is composed being 

 known as the " Bird's-eye," " Black River," and " Trenton " 

 Limestones, of which the last is the thickest and most import- 

 ant. The thickness of this group is variable, and the bands of 

 limestone in it are often separated by beds of shale. 



3. The Cincinnati Group (Hudson River Formation t). 

 This group consists essentially of a lower series of shales, often 

 black in colour and highly charged with bituminous matter 

 (the " Utica Slates "), and of an upper series of shales, sand- 



* The precise relations of the Quebec shales with Graptolites (Levis 

 Formation) to the Calciferous and Chazy beds are still obscure, though 

 there seems little doubt but that the Quebec Shales are superior to the 

 Calciferous Sand-rock. 



t There is some difficulty about the precise nomenclature of this group. 

 It was originally called the "Hudson River Formation; " but this name 

 is inappropriate, as rocks of this age hardly touch anywhere the actual 

 Hudson River itself, the rocks so called formerly being now known to be 

 of more ancient date. There is also some want of propriety in the name of 

 " Cincinnati Group," since the rocks which are known under this name in 

 the vicinity of Cincinnati itself are the representatives of the Trenton 

 Limestone, Utica Slates, and the old Hudson River group, inseparably 

 united in what used to be called the " IMu- Limestone Series." 



