Page:A Treatise on Geology, volume 1.djvu/160

Subdivisions of the Rocks of the New York System.			Subdivisions of the Silurian and Old Red System in Great Britain.
14. Delthyris shaly limestone			$\left.{\begin{matrix}\ \\\\\\\ \end{matrix}}\right\}$	Wenlock Rocks.
15. Pentamerus limestone
16. Waterlime group
17. Onoudaga salt group
18. Niagara group
19. Clinton group			$\left.{\begin{matrix}\ \\\\\\\ \end{matrix}}\right\}$	Caradoc sandstone.
20. Medina sandstone
21. Oneida conglomerate
22. Grey sandstone
23. Hudson River group
24. Utica slate				Llandeilo flags.
25. Trenton limestone			$\left.{\begin{matrix}\ \\\\\\\\\ \end{matrix}}\right\}$	These formations are not as fully recognised in Great Britain as in New York
26. Bird's eye and Black River limestones
27. Chazy limestone
28. Calciferous sand rock
29. Potsdam sandstone

The 'lowest of the fossiliferous rocks'—the Potsdam sandstone—like some of the lower 'Cambrians,' contains only or almost only a Lingula. In this rock, however, Mr. Logan reports 'traces' of a Chelonian reptile (1851). It requires further study. In the next groups 2, 3. the number increases, but is still small. In the Trenton limestone (4.) organic remains of the Lower Silurian types become plentiful.

The distinctness of the organic fossils of the Silurian rocks from those of the carboniferous formation, as far as regards the marine races, is an important truth which has received further and exact confirmation from Mr. Murchison's researches. To what extent the few fossils of the slate system are analogous to the Silurian reliquiæ is not accurately known; but there appears a sufficient resemblance between them to justify a belief that the physical conditions of the ocean were not greatly changed, though evidently rendered more favourable to the development of a varied system of organic life. Mr. Murchison showed that each of the four formations of the Silurian system contains distinct suites and characteristic species of fossils. The following are among the most common or remarkable:—

Ogygia Buchii, fig. 1. Pentamerus Knightii. fig. 2. Dalmannia caudata, fig. 3. Euomphalus rugosus, fig. 4. Leptæna depressa, fig. 5. Calymene Blumenbachii, fig. 6. Orthoceras pyriforme, fig. 7. Orbicula rugata, fig. 8. Palaeopora interstincta, figs. 10, 10 a magnified. Cateniporalabyiinthica, fig. 11. Cyathophyllum cyathus, fig. 12.