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, 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.