of the Older Formations of the United States. 13 
panied by changes in the organic remains. We sometimes find a 
series of strata where the Delthyris, Atrypa, and Orthis abound ; 
and again, above or below this point may be a series where few : 
of these forms are seen, while the strata are crowded with 
Avicula, Cypricardia, and Modiola, with perhaps a few Stro- 
phomena. Large numbers of Fucoides appear in this group, 
which continue for a long distance east and west, and are of 
essential service in recognizing certain portions. 
As we pass into Ohio, this group, which in its greatest exten- 
sion in New York is scarcely less than two thousand feet thick, 
has diminished to four hundred or five hundred feet. Here we 
find a few of the prevailing forms of Delthyris, Strophomena 
and Atrypa, while others are exceedingly rare. Still farther 
west, in Indiana, the mass may be said to be almost non-fossil- 
iferous, scarcely any fossils being noticed throughout its whole 
thickness. From its constant decrease m thickness in a west- 
erly direction, it doubtless entirely or very nearly disappears 
from thinning out, in the vicinity of the Mississippi River. 
This group must be considered the termination of the rocks 
of the New York system, which attain a greater development 
in this part of our country than perhaps in any other part of 
the world. 
The facts show that in all the calcareous formations where 
there is uniformity of composition, there is little change in 
organic remains over wide areas. Again when we find a change 
in the lithological character of strata, there is a corresponding 
change in organic contents. This is fully illustrated in the 
2 Hudson River Group and some of the higher rocks. The gra- 
dual change in mechanical deposits at different distances from 
heir source is also attended by a change in the nature of the 
ssils, as exhibited in the Hamilton and Chemung Groups. 
L ing a view of the great area between metamorphic 
ranges on the east and the Mississippi River, we find that dur- 
ing the period elapsing from. the commencement of fossil- 
iferous deposits to th Hudson River Group, 
5 which may be termed. the am era in organization, 
E 
