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.)]

[Footnote 12: 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,