INTRODUCTION. 4^ 



On a careful comparison between the Chemung fossils of New- York 

 and those of the Mississippi valley, we find scarcely a single species 

 that can be considered positively identical. Many of the species are 

 representative forms, and so closely allied to those of New- York, that 

 heretofore I had regarded them as the same ; and every one acquainted 

 with the eastern species must perceive the close similarity, not only 

 in the general features of each one, but in the grouping of the species 

 together in the beds. 



These examinations in the west have furnished some important terms 

 in our problem, not only for comparison with the physical conditions 

 prevailing and the direction of the distribution of sediments, but like- 

 wise in reference to the fauna and its persistence over wide areas. It 

 is unfortunate that our comparisons of the faunae of the east and west 

 must end here ; for there are no other formations in the palaeozoic se- 

 quence, below the Coal measures, that afford an opportunity for com- 

 parison over so great an area as from the IJudsor^ to the Mississippi 

 river. 



In considering the distribution of the masses qf the formations which 

 we have here described, we find that the greatest {iccumulations have 

 been along the direction of the Appalachian chain. The original current, 

 transporting the material, has been in the same direction, and con- 

 sequently a greater deposition of the coarser sediment has marked 

 the lines of the transporting force, which, necessarily diminishing on 

 either side of the centre of this great current, the fine calcareous mud 

 would be gradually conveyed to greater distances and slowly deposited. 

 The material thus transported would be distributed, precisely as in an 

 ocean traversed by a current, like our present Gulf stream ; and in 

 the gradual motion of the waters during that period, to the west and 

 southwest, the finer materials would be spread out in gradually dimi- 

 nishing quantities, till, finally, the deposit from that source must cease 

 altogether. 



We see this illustrated in the minor members of a group ; and the 

 same must be true in the larger ones, and of the whole together, since 

 [ Paljsontoloqy III.] 7 



