88 Mr. T. Sterry Hunt 



detected by him in specimens from tlie sandstones of 

 Wisconsin with JDikeUocephalus, which genus has there been 

 found to pass upwards into the magnesian limestones. On the 

 other hand, the sandstones of Bastard in Canada, having the char- 

 acters of the Potsdam, contain Lingula acuminata and Ophileta 

 compacta, species regarded as characteristic of the Calciferous, to- 

 gether with two undescribed species of Orthoceras, and in another 

 locality a Pleurotomaria resembling P. Laurentina. The re- 

 searches of Mr. Billings have extended the fauna of the Calcifer- 

 ous in Canada to forty-one species, and the succeeding Chazy for- 

 mation to 129 species. The thickness of this latter division in 

 the St. Lawrence valley is about 250 feet, and it includes in its 

 lower part about fifty feet of sandstones with green fucoidal 

 shales and a bed of conglomerate. The Calciferous has a thick- 

 ness of about 300 feet, while the Potsdam may be estimated at 

 not far from 600 feet. 



We have then seen that along the north-eastern outcrop of the 

 great American basin in Canada and New York, the base of the 

 palaeozoic series is represented by less than 1000 feet of sand- 

 stones and dolomites, reposing directly upon the Laurentian sys- 

 tem. A very different condition of things is, however, found in 

 the more central parts of the basin. According to Prof. Rogers, 

 the older Primal slates, which form the base of the palaeozoic sys- 

 tem, attain in Virginia a thickness of 1200 feet, and are succeeded 

 by 300 feet of Primal sandstone marked by Scolithus, which he 

 considers the Potsdam, followed by the upper Primal slates, con- 

 sisting of YOO feet of greenish and brownish talco-argillaceous 

 shales with fucoids. To these succeed his Auroral division, con- 

 sisting of sixty feet or more of calcareous sandstone, the supposed 

 equivalent of the Calciferous sandrock, followed by the Auro- 

 ral limestone, which is magnesian, and often argillaceous and 

 cherty in the upper beds. Its thickness is estimated at from 2500 

 to 5500 feet, and it is supposed by Rogers to include the Chazy 

 and Black River limestones, while the succeeding Matinal divi- 

 sion exhibits first, from 300 to 550 feet of limestone, (Trenton), 

 secondly, 300 to 400 feet black shale, (Utica), and thirdly, 1200 

 feet of shales with red slates and conglomerates, (Hudson River 

 group), thus completing the Lower Silurian series. 



In Eastern Tennessee, Mr. Safford describes, (1st.) on the con- 

 fines of North Carolina, a great volume of ^neissoid and micace- 

 ous rocks similar to those of Pennsylvania, succeeded to the 



