XV.] CAMBRIAN AND SILURIAN IN NORTH AMERICA. 421 



palaeozoic day, the divisions of which were designated by 

 names taken from the sun's apparent course through the 

 heavens. (Geology of Penn., I. vi, 105.) So far as regards 

 the three great groups which we have recognized in the lower 

 palaeozoic rocks, the later names of Rogers, and his earlier 

 numerical designations, with their equivalents in the New 

 York system, were as follows : 



Primal (I.). This includes the mass of 2,500 feet or more 

 of shales and sandstones, which in Pennsylvania and Virginia, 

 and farther southward, form the base of the palaeozoic series, 

 and rest upon crystalline schists. The Primal division was 

 regarded by the Messrs. Rogers as the equivalent both of the 

 Potsdam and the still lower members of the Cambrian. 



Auroral (II.). This division consists in great part of lime- 

 stones, often magnesian, and corresponds to the Calciferous, 

 Levis, and Chazy formations. Its thickness in Pennsylvania 

 varies from 2,500 to 5,000 feet, and, with the preceding divis- 

 ion, it includes the first fauna of Barrande. The representa- 

 tives of the Primal and Auroral divisions attain a great de- 

 velopment in southwestern Virginia and also in eastern Ten- 

 nessee, where they have been studied by SafFord. 



Matinal (III.). In this, which represents the second fauna, 

 were comprised the limestones of the Trenton group, together 

 with the Utica and Hudson River shales.' 



Levant (IV.). This division corresponds to the Onedia and 

 Shawangunk conglomerates and the Medina sandstone. 



Surgent, Scalent, and Pre-Meridional (V., VI.). In these 

 divisions were included the representatives of the Clinton, 

 Niagara, and Lower Helderberg groups of New York, making, 

 with division IV., the third fauna of Barrande. 



The parallelism of these divisions with the British rocks 

 was most clearly and correctly pointed out by H. D. Rogers 

 himself, in an explanation prepared, as I am informed, with 

 the collaboration of Professor William B. Rogers, and pub- 

 lished in 1856, with a geological map of North America by 

 the former, in the second edition of Keith Johnson's Physi- 

 cal Atlas. The palaeozoic rocks of North America are there 



