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.
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