DEVONIAN AND OLD RED PERIOD. 135 



North America. It seems quite possible, also, that the so- 

 called " Carboniferous Slates " of Ireland correspond with 

 this group, and that the former would be more properly re- 

 garded as forming the summit of the Devonian than the 

 base of the Carboniferous. 



In no country in the world, probably, is there a finer 

 or more complete exposition of the strata intervening be- 

 tween the Silurian and Carboniferous deposits than in the 

 United States. The following are the main subdivisions 

 of the Devonian rocks in the State of New York, where 

 the series may be regarded as being typically developed 

 (fig. 67):- 



(1) Cauda-Galh Grit and Schohane Grit. Considering the 

 " Orfskany Sandstone " as the summit of the Upper Silurian, 

 the base of the Devonian is constituted by the arenaceous 

 deposits known by the above names, which rest quite conform- 

 ably upon the Silurian, and which represent the Lower 

 Devonian of Devonshire. The Cauda-Galli Grit is so called 

 from the abundance of a peculiar spiral fossil (Spirophyton 

 cauda-Gatti}, which is of common occurrence in the Carbon- 

 iferous rocks of Britain, and is supposed to be the remains 

 of a sea-weed. 



(2) The Corniferous or Upper Helderberg Limestone. A 

 series of limestones usually charged with considerable quan- 

 tities of siliceous matter in the shape 'of hornstone or chert 

 (Lat. cornu, horn). The thickness of this group rarely exceeds 

 300 feet ; but it is replete with fossils, more especially with 

 the remains of corals. The Corniferous Limestone is the 

 equivalent of the coral-bearing limestones of the Middle De- 

 vonian of Devonshire and the great " Eifel Limestone " of 

 Germany. 



(3) The Hamilton Group consisting of shales at the base 

 (" Marcellus shales ") ; flags, shales, and impure limestones 

 ("Hamilton beds") in the middle ; and again a series of shales 

 ("Genesee Slates") at the top. The thickness of this group 

 varies from 200 to 1200 feet, and it is richly charged with 

 marine fossils. 



(4) The Portage Groiip. A great series of shales, flags, and 

 shaly sandstones, with few fossils. 



(5) The Chemung Group. Another great series of sand- 

 stones and shales, but with many fossils. The Portage and 

 Chemung groups may be regarded as corresponding with the 

 Upper Devonian of Devonshire. The Chemung beds are 

 succeeded by a great series of red sandstones and shales the 



