dstone series.
Both in Britain and in America the Lower Devonian beds repose
with perfect conformity upon the highest Silurian beds, and the
two formations appear to pass into one another by a gradual and
imperceptible transition.

The Upper Silurian strata of Britain vary from perhaps 3000 or
4000 feet in thickness up to 8000 or 10,000 feet. In North America
the corresponding series, though also variable, is generally of
much smaller thickness, and may be under 1000 feet. The general
succession of the Upper Silurian deposits of North America is
as follows:--

(1) _Medina Sandstone_.--This constitutes the base of the Upper
Silurian, and consists of sandy strata, singularly devoid of life,
and passing below in some localities into a conglomerate ("Oneida
Conglomerate"), which is stated to contain pebbles derived from
the older beds, and which would thus indicate an unconformity
between the Upper and Lower Silurian.

(2) _Clinton Group_.--Above the Medina sandstone are beds of
sandstone and shale, sometimes with calcareous bands, which
constitute what is known as the "Clinton Group." The Medina and
Clinton groups are undoubtedly the equivalent of the "May Hill
Group" of Britain, as shown by the identity of their fossils.

[Illustration: Fig. 57. GENERALIZED SECTION OF THE UPPER SILURIAN
STRATA OF WALES AND SHROPSHIRE.]

(3) _Niagara Group_.--This group consists typically of a series of
argillaceous beds ("Niagara Shale") capped by limestones ("Niagara
Limestone"); and the name of the group is derived from the fact
that it is over limestones of this age that the Niagara river
is precipitated to form the great Falls. In places the Niagara
group is wholly calcareous, and it is continued upwards into a
series of marls and sandstones, with beds of salt and masses
of gypsum (the "Salina Group"), or into a series of magnesian
limestones ("Guelph Limestones"). 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, th