430 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



racterized bv the constancy of its fossils. (Ami Boue, Memoires, &c, 

 pp. 140-144.) 



Our American friends have only brought together such contiguous 

 strata as agree in mineral condition, fossil contents, and position, 

 three great tests of synchronism and common origin to which the 

 various sections perfectly respond. These divisions are important 

 as well as distinct, and are applicable satisfactorily over large areas, 

 always occupying the same geological horizon, but perhaps under 

 considerable lithological variations. Nearly every one of these groups 

 has its own mineral character, which usually softens or melts into 

 that of its coterminous stratum, their constituent earths not only 

 alternating in the mass, but in their minuter commixtures also, and 

 becoming more and more seldom repeated as each stratum is pene- 

 trated. In proof of this, reference may be made to the Synoptical 

 View generally. 



Further, each group is a natural and not an artificial division, 

 because it is a new zoological province, a true centre of organic life, 

 with a fauna principally typical, meaning by that term the part 

 which appears with the group and perishes. Examples of this are 

 universal. 



Their claims, therefore, each to its separate place in the great 

 series cannot be doubted ; and they are more readily ascertained in 

 New York than in Wales, from the rarer admixture there of fossils 

 of different epochs. 



On the separate Groups as arranged in the Table. 



Potsdam Sandstone and Calciferous Sandstone, or Group A. — 

 The three lower natural aggregates, marked A, B, C, form, as we 

 shall afterwards see, the lower stage of the Silurian system. 



The first, or lowest, consists of Potsdam Sandstone and Calciferous 

 Sandstone. These strata undergo innumerable slight or limited 

 lithological changes, and graduate one into the other with conform- 

 ableness. All their fossils are typical save one. In general, Potsdam 

 Sandstone contains few animal remains. In New York, Potsdam 

 Sandstone only yields a Scolithus (linearis), two species of Lingula, 

 and a few Trilobites, the latter met with very recently. 



On the Upper Mississipi, Lingulce, Orbiculce, and some highly 

 organized and typical Trilobites have been found by David Owen. 



Calciferous Sandstone possesses a larger fauna, chiefly consisting 

 of Gasteropoda (11 out of its 19 fossils), and three typical Ortho- 

 ceratites. The only fossil connecting these enormous sheets of 

 arenaceous matter with the system to which they belong is the 

 Scalites angulatus of Chazy Limestone. Group A, in its lithology 

 and fossil contents, then, is sufficiently distinct. The second group, 

 B, is established upon equally solid grounds. It is a series of lime- 

 stones with frequent argillaceous impurities and shaly alternations, 

 especially in Trenton Limestone. 



Trenton Limestone is the predominant member. It is three times 

 as thick as Chazy and Birdseye Limestone taken together, and it is 



