INTRODUCTION. 
49 
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 Hudson to the Mississippi 
river. 
In considering the distribution of the masses of the formations which 
we have here described, we find that the greatest accumulations 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 
[ Palaeontology III.] 7 
