416 THE CAMBRIAN. [bull. 81. 



The author agrees with M. de Verueuil that the Potsdam sandstone is 

 represented in Bussia by the beds containing Obolus and Lingula, and 

 in Scandinavia by the sandstones which rest unconformably upon the 

 gneissoid and schistose rocks composing the Azoic system. Of the 

 Chazy, Birdseye and Black River limestones of "New York he says: 



They are not very clearly recognized in Europe. It is probable that some part of 

 the Orthoceratite limestone of Sweden is the equivalent of the Black River mass; 

 but we have no evidence, from fossils or otherwise, that there are any representa- 

 tives of the two lower rocks. * * * 



The Trenton limestone, the Utica slate, and the Hudson River group are repre- 

 sented in northern Europe by the Orthoceratite limestone of Sweden and Russia and 

 by the shales with Graptolites, which succeed to that limestone. In Great Britain 

 the Llandeilo flags and Caradoc sandstone are clearly the equivalent of these groups. 

 It is here that we first find a number of species identical with European ones, and 

 which enable us to institute a comparison. It will be found, however, that the 

 species in the Hudson River group correspond most nearly with the European forms, 

 and that it may be doubtful if the base of the Trenton limestone has been reached in 

 Great Britain. 1 



The Cliuton group, with its beds of Pentamer us oblongus, represents what in Eng- 

 land is regarded as the upper part of the Caradoc saudstone; but we find it not 

 only above this well marked horizon, which we have already indicated, but asso- 

 ciated with numerous species of fossils, some of which are known as upper Silurian 

 forms in Europe, and others pass into forms characteristic of the Niagara group in 

 this country. 



The Niagara group, with its shale and limestone, would seem, at first view, to be 

 the exact equivalent of the ?hale and limestone of Wenlock and Dndley in England, 

 and of Gothland in Sweden, so numerous are the identical species in these groups ; 

 but on examination we find that there are numerous species included in tbese rocks 

 in Europe which occur only in the lower Helderberg limestones, and which are 

 separated from the Niagara by the enormous deposit of the Onondaga salt group. 

 It is clear, therefore, that in the Weulock formation of England and its representa- 

 tives in northern Europe are included parts at least of two distinct groups in the 

 order of time— distinct both in their physical and zoological features; and we can 

 not institute a proper comparison between the rocks of our own country and those 

 of Europe while we regard that as one group. The condition, both in England and 

 on the continent, is doubtless similar to what we find in Tennessee and Virginia, 

 where, from the absence of the intermediate groups not only the Niagara and lower 

 Helderberg are united together, but even the upper Helderberg limestones are 

 superadded. 3 



Barrandc. — A correlation based upon paleontologic data was made by 

 Mons J. Barrande, in 1853, between the formations characterized by the 

 first, second, and third faunas of Europe and those included in the 

 Silurian of America. In commenting upon the identification of the 

 Kussian Obolus apollinis Eichwald, by Owen, with the Potsdam sand- 

 stone species of the Upper Mississippi Valley, he says : 



In fact I am inclined to think that science has passed out of the epoch when it was 

 thought necessary to have identical species in order to recognize equivalence of geo.- 

 logic horizous, even at great distances on the earth's surface. The distribution of our 

 present fauna teaches us that this means can not be altogether relied on to prove 

 strata contemporaneous. 3 



1 Op. cit., pp. 295, 296. * Op. cit, p. 299. 



* Barrande. J: (Silur-fauna in Wisconsin und in New York.) Neues Jahrbucb. fur Miner., 1853, p. 

 338. 



