120 The Carboniferous Fauna of America 



extent of surface, to their concordant and uninterrupted 

 superposition, it is possible to arrive at an absolute certainty 

 as to the duration of species, that is to say, the point in the 

 series where they first appear, and where they become extinct. 



In order to compare North America with Europe, it has 

 been necessary for us to give a rapid glance at the grouj)s 

 and stages of which the palaeozoic class is there composed. 

 The differences which are presented to us in the geognostic 

 conditions of the state of New York, and the Western 

 States, such as Ohio and Indiana, have revealed to us the 

 degree of importance which it is necessary to attach to these 

 different groups. We have seen that their number, variable 

 according to their vicinity or distance from lands emerged at 

 the epoch of their formation, had little importance, as regards 

 the establishment of systems founded upon palseontological 

 characters. We have seen also, that in general the lime- 

 stones are more constant than the shaly or arenaceous beds, 

 that they form' more extensive horizons, and furnish a surer 

 guide to the geologist.* 



Passing afterwards to a comparison of the two continents, 

 we have shewn, supporting our vieAvs by geological analysis, 

 how the American sub-stages should be grouped to correspond 

 with the Silurian, Devonian, and Carboniferous systems of 

 Europe. We have not disguised the fact, that the divi- 

 sions introduced upon this principle did not correspond, in 

 certain countries, with the divisions indicated by the miner- 

 alogical character of the rocks ; thus the limit between the 

 two stages of the Silurian system, very well marked in the 

 state of New York, is observed near the Mississippi, in con- 

 sequence of the predominance of magnesian limestone ; it is 

 the same with the Silurian and Devonian systems, the limit 

 between which is found in the upper part of the great cal- 

 careous formation called cliff-limestone ; as well also as with 

 the carboniferous system, in parts of the state of Ohio, where 

 it is in contact with the Devonian psammites of Portage. 



* M. C. Prevost, in his Memoir upon the Synchronism of Formations (ComjHes 

 Rendus, April 1845), has clearly shewn the importance of the pelagic calcareous 

 deposits, as compared with the arenaceous beds formed under the influences of 

 coasts. 



