Silurian Strata of the Continent of Europe.—When we turn to the continent of Europe, we discover the same ancient series occupying a wide area, but in no region as yet has it been observed to attain great thickness. Thus, in Norway and Sweden, the total thickness of strata of Silurian age is considerably less than 1000 feet, although the representatives both of the Upper and Lower Silurian of England are not wanting there. In Russia the Silurian strata, so far as they are yet known, seem to be even of smaller vertical dimensions than in Scandinavia, and they appear to consist chiefly of the Llandovery group, or of a limestone containing Pentamerus oblongus, below which are strata with fossils corresponding to those of the Llandeilo beds of England. The lowest rock with organic remains yet discovered is ”the Ungulite or Obolus grit” of St. Petersburg, probably coeval with the Llandeilo flags of Wales.

The shales and grits near St. Petersburg, above alluded to, contain green grains in their sandy layers, and are in a singularly unaltered state, taking into account their high antiquity. The prevailing Brachiopods consist of the Obolus Shells of the lowest known Fossiliferous Beds in Russia.

or Ungulite of Pander, and a Siphonotreta (Figs. 565, 566). Notwithstanding the antiquity of this Russian formation, it should be stated that both of these genera of brachiopods have been also found in the Upper Silurian of England, i.e., in the Wenlock limestone.

Among the green grains of the sandy strata above-mentioned, Professor Ehrenberg announced in 1854 his discovery of remains of foraminifera. These are casts of the cells; and among five or six forms three are considered by him as referable to existing genera (e.g., Textularia, Rotalia, and Guttulina).

Silurian Strata of the United States.—The Silurian formations can be advantageously studied in the States of New York, Ohio, and other regions north and south of the great Canadian lakes. Here they are often found, as in Russia, nearly in horizontal position, and are more rich in well-preserved fossils than in almost any spot in Europe. In the State of New York, where the succession of the beds and their fossils have been most carefully worked out by the Government surveyors, the subdivisions given in the first column of the table below have been adopted.

Subdivisions of the Silurian Strata of New York.
(Strata below the Oriskany sandstone or base of the Devonian.)

New York NamesBritish equivalents
1. Upper Pentamerus Limestone
2. Encrinal Limestone
3. Delthyris Shaly Limestone
4. Pentamerus and Tentaculite Limestones
5. Water Lime Group
6. Onondaga Salt Group
7. Niagara Group		Upper Silurian (or Ludlow
and Wenlock formations
8. Clinton Group
9. Medina Sandstone
10. Oneida Conglomerate
11. Gray Sandstone		Beds of Passage, Llandovery Group.
12. Hudson River Group
13. Trenton Limestone
14. Black-River Limestone
15. Bird’s-eye Limestone
16. Chazy Limestone
17. Calciferous Sandstone		Lower Silurian (or Caradoc and Bala,
Llandeilo and Arenig Formations).

In the second column of the same table I have added the supposed British equivalents. All Palæontologists, European and American, such as MM. De Verneuil, D. Sharpe, Professor Hall, E. Billings, and others, who have entered upon this comparison, admit that there is a marked general correspondence in the succession of fossil forms, and even species, as we trace the organic remains downward from the highest to the lowest beds; but it is impossible to parallel each minor subdivision.

That the Niagara Limestone, over which the river of that name is precipitated at the great cataract, together with its underlying shales, corresponds to the Wenlock limestone and shale of England there can be no doubt. Among the species common to this formation in America and Europe are Calymene Blumenbachii, Homalonotus delphinocephalus ([Fig. 544]), with several other trilobites; Rhynchonella Wilsoni, [Fig. 531], and Retzia cuneata; Orthis elegantula, Pentamerus galeatus, with many more brachiopods; Orthoceras annulatum, among the cephalopodous shells; and Favosites gothlandica, with other large corals.