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 corresponding Skiddaw Slates of the North of England.  It is also to be noted that numerous Trilobites of a distinct Cambrian facies 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 limestones 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 Graptolitic shales of Quebec are wanting; and the base of the Silurian is constituted by the so-called “Calciferous Sand-rock” and “Chazy Limestone."[11] The first of these is essentially and typically calcareous, and the second is a genuine limestone.

[Footnote 11:  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.]

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 important.  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[12]).—­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, sandstones, and limestones (the “Cincinnati” rocks proper).  The exact parallelism of the Trenton and Cincinnati groups with the subdivisions of the Welsh Silurian series can hardly be stated positively.  Probably no precise equivalency exists; but there can be no doubt but that the Trenton and Cincinnati groups correspond, as a whole, with the Llandeilo and Caradoc groups of Britain.  The subjoined diagrammatic section (fig. 35) gives a general idea of the succession of the Lower Silurian rocks of North America:—­

[Illustration:  Fig 35.  GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF NORTH AMERICA.]

[Illustration:  Fig. 36.—­Licrophycus Ottawaensis a “Fucoid,” from the Trenton Limestone (Lower Silurian) of Canada. (After Billings.)]