The
_Cauda-Galli Grit_ is so called from the abundance of a peculiar
spiral fossil (_Spirophyton cauda-Galli_), which is of common
occurrence in the Carboniferous rocks of Britain, and is supposed
to be the remains of a sea-weed.
(2) The _Corniferous_ or _Upper Helderberg Limestone_.--A series
of limestones usually charged with considerable quantities of
siliceous matter in the shape of hornstone or chert (Lat. _cornu_,
horn). The thickness of this group rarely exceeds 300 feet; but
it is replete with fossils, more especially with the remains
of corals. The Corniferous Limestone is the equivalent of the
coral-bearing limestones of the Middle Devonian of Devonshire
and the great "Eifel Limestone" of Germany.
(3) The _Hamilton Group_--consisting of shales at the base
("Marcellus shales"); flags, shales, and impure limestones ("Hamilton
beds") in the middle; and again a series of shales ("Genesee
Slates") at the top. The thickness of this group varies from
200 to 1200 feet, and it is richly charged with marine fossils.
(4) The _Portage Group_.--A great series of shales, flags, and
shaly sandstones, with few fossils.
(5) The _Chemung Group_.--Another great series of sandstones and
shales, but with many fossils. The Portage and Chemung groups
may be regarded as corresponding with the Upper Devonian of
Devonshire.
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