SILURIAN STRATA. 167 



boniferous series. The sandstone, which rises on the flanks 

 of the Rocky Mountains to the height of 8,000 feet above 

 the sea, is referred by some geologists of the United States 



such as thin shaly sandstones, shales, conglomerates, thin-bedded 

 impure limestones, with iron ores. 



11. Niagara group ; consisting of dark-bluish shale and dark lime- 

 stone, and taking its name from the cataract, where a section of 

 both its members is exposed. 



C. Helderberg Division. 



12. Onondago salt group, is an immense development of argillaceous 

 shales and marls with shaly limestones, veins and beds of gypsum, 

 giving origin to copious and very rich salt springs. This forma- 

 tion re-appears near Slave River, on the 60th parallel of latitude, 

 and also, I believe, on the shores of the Arctic Sea. 



13. Water-lime group, consists generally of two layers of drab- 

 coloured water-limestone, with an intervening layer of blue 

 lime-rock. 



] 4. Pentamerus limestone ; named from its characteristic fossil, and 

 rarely a pure limestone, being more or less mixed with black 

 shale. 



15. Delthyris or Catshill shaly limestone; composed of beds of sub- 

 crystalline grey limestone, slaty limestone, and slaty argillaceo- 

 siliceous limestone. 



16. Oriskany sandstone; a whitish sandstone, composed of sand 

 derived from granitic rocks or mica schist. 



17. Caudagalli grit; named from the feathery forms in which it 

 abounds. It is a drab-coloured or brownish calcareous and 

 argillaceous sandstone. 



18. Shoharie grit ; a fine-grained calcareous sandstone, from which 

 the calcareous matter may be washed away by long exposure, 

 leaving the siliceous skeleton of the rock. 



19. Onondago limestone; recognisable by its crystalline structure, 

 toughness, and numerous organic remains. 



20. Corniferous limestone ; a fine-grained compact limestone, which 

 is very durable, and produces cascades where the smaller streams 

 traverse it. It is the uppermost of the important limestone beds 

 of the New York or silurian system, being succeeded by shales 

 in which the limestone fossils give place to others of a different 

 character. 



D. Erie Division, referred by most English geologists to the De- 



vonian or to the carboniferous series. 

 m 4 



