The Niagara group, as a whole,
corresponds unequivocally with the Wenlock group of Britain.
(4) _Lower Helderberg Group_.--The Upper Silurian period in North
America was terminated by the deposition of a series of calcareous
beds, which derive the name of "Lower Helderberg" from the Helderberg
mountains, south of Albany, and which are divided into several zones,
capable of recognition by their fossils, and known by local names
(Tentaculite Limestone, Water-lime, Lower Pentamerus Limestone,
Delthyris Shaly Limestone, and Upper Pentamerus Limestone). As
a whole, this series may be regarded as the equivalent of the
Ludlow group of Britain, though it is difficult to establish any
precise parallelism. The summit of the Lower Heiderberg group
is constituted by a coarse-grained sandstone (the "Oriskany
Sandstone"), replete with organic remains, which have to a large
extent a Silurian _facies_. Opinions differ as to whether this
sandstone is to be regarded as the highest bed of the Upper Silurian
or the base of the Devonian. We thus see that in America, as
in Britain, no other line than an artificial one can be drawn
between the Upper Silurian and the overlying Devonian.
As regards the _life_ of the Upper Silurian period, we have,
as before, a number of so-called "Fucoids," the true vegetable
nature of which is in many instances beyond doubt.
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