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 /aczes 
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 Zrenton 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. 
+ 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 ‘* Blue Limestone Series.” 
