Chap. XVIII.] SILUKIAN EOCKS IN THE UNITED STATES. 
429 
creased. Besides the large Isotelus gigas, the characteristic fossil of the stra- 
tum, Trinucleus concentricus occurs, with the genera Cheirurus (Ceraurus 
of Green), Lichas, Phacops, and Calymene, all of different species from those of 
England j hut among the Shells, Orthis striatula, 0. biforata or lynx, 0. porcata 
(occidentalis, Hall) (Foss. 36. f. 5. p. 193), and Bellerophon bilobatus (PI. VII. 
f. 9) are Lower-Silurian fossils, characteristic of the same rocks in Britain, 
Scandinavia, and Russia. The geological position of this limestone is main- 
tained, too, by the presence of numerous species of Lituites, Orthocerata, and 
large-plaited Orthides, which resemble those of Europe without being identical. 
Such, for example, are the two large-plaited species, Orthis pectinella and 0. 
tricenaria, which represent the British forms O. flabellulum and 0. Actonire 
(Foss. 35. f. 1, 2. p. 192), and yet are perfectly distinct. 
The Trenton Limestone is overlain, throughout a considerable tract in North 
America, by the Hudson-river Group, at the base of which lie the Utica Slates, 
full of their characteristic Graptolites. These are covered by, and often pass 
into the schist and sandstone of the Hudson-river Group, also containing many 
Graptolites with other fossils, and now regarded as part of the American equi- 
valent of the Caradoc or Bala formation *. 
The conglomerates and sandstones of Oneida and Medina, which succeed, are 
believed to be the representatives of the Lower Llandovery rocks of Britain ; 
and, like them, they may, according to my view, be classed as the uppermost 
part of the Lower Silurian rocks. Though containing few fossils in the con- 
tinent of America, beds occupying the same position in the large Island of 
Anticosti, at the mouth of the St. Lawrence, are charged with Shells which 
have led Sir W. Logan to refer them also to this date f. Besides Pentameri, 
they there contain the following Silurian types — namely, Leptsena sericea, Orthis 
striatula, and Strophomena alternata, with the remarkable elongate Coral (?), 
Beatricea of Billings %. 
According to Hall, Logan, and most American authors, as well as Dr. Bigsby, 
the Clinton Group has been considered the base of the upper division in the 
United States. It is characterized by the typical Pentamerus oblongus, which, 
as in England, is never found in any of the succeeding strata. 
The same authorities subsequently grouped these rocks containing Penta- 
merus oblongus, and their inferior sandstones, as ' Middle Silurian ; ' for, like 
the Llandovery rocks, they essentially occupy the central part of this natural 
system. For general purposes of classification, I prefer, however, to adhere to 
* M. de Verneuil was the first European geo- 
logist who, having examined the collections and 
visited the localities of the American fossils, suc- 
cessfully compared them with those of our con- 
tinent. See his most instructive memoir, Bull. 
Soc. G-e'ol. Fr. 2nd ser. vol. iv. p. 646. The Lower 
Silurian had not then been subdivided as it now is 
(see Table, p. 446). 
t See G-eological Survey of Canada, Eeport of 
Progress 1853 to 1856 inclusive,Toronto,1857,p.248. 
This volume, with its accompanying illustrations 
and book of maps and plans, is highly instructive, 
and does infinite credit to the Canadian Survey. 
It makes great additions to our acquaintance with 
the physical geography and geology of large 
tracts never before mapped, as examined by Sir 
W. Logan and Mr. Alexander Murray. The re- 
port on the wild and unexplored large island of 
Anticosti by Mr. James Richardson, the descrip- 
tions of the Canadian fossils by Mr. E. Billings, and 
the reports on the minerals and mineral waters 
of the region by Dr. T. Sterry Hunt are all ex- 
cellent. Sir W. Logan and his associates divide 
the Lower Silurian into seven deposits, viz. Pots- 
dam Sandstone, Calciferous Sand-rock, Chazy 
Limestone,Bird's-eye and Black-river Limestones, 
Trenton Limestone, Utica Slate, Hudson-river 
Group. Subsequently he has indicated the inter- 
polation of the Quebec group between the Calcife- 
rous Sand-rock and the Chazy Limestone, as will 
presently be shown in a description of these rocks 
in Canada. Then follows the Anticosti or Middle 
Silurian group, succeeded by Upper Silurian, 
Devonian, and Carboniferous (Eeport, p. 248). 
The still more comprehensive ' Eeport ' of 1863, 
with its Atlas of Maps and Sections, completes the 
exposition of these remarkable labours of the Sur- 
vey to that date. 
I Specimens of this fossil, from six to fifteen 
feet long, were at first supposed to be Sea-weeds ; 
but Dawson, Chapman, and Billings regard Bea- 
tricea as a ' Eugose ' Coral (Canad. Natur. and 
G-eol. new ser. vol. ii. p. 405). Mr. Salter believes 
that it may be a gigantic Annelide-tube, allied to 
Cornulites; its cellular structure leads him to 
this view. Amphitrite has a thick shelly tube 
some feet in length. 
