36 
PALAEONTOLOGY OF NEW-YORK. 
Thus the strong line of demarcation between the Silurian and 
Devonian which exists where the Lower Helderberg group is absent, is 
softened to a gentle gradation through the intervention of these strata 
and the Oriskany sandstone. Where these are present in all their 
members the line of separation becomes less sharply defined, and we 
have some evidence that there may exist other intermediate members, 
or a more full development of those now known between the two 
formations. 
However true therefore it may be that both in Great Britain and in 
parts of this country the Eurypterus beds represent the Upper Silurian 
horizon, we must carry the line of that horizon upwards and include in 
the Silurian series the fauna of the Lower Helderberg period. At the 
same time we are ready to admit that there are many points of argu¬ 
ment for classing the Oriskany sandstone as an intermediate formation, 
terminating the Silurian and forming the base of the Devonian system, 
as now most generally recognized in Europe. In this view of the sub¬ 
ject, the evidence of the relations of these faunae will be given in the 
following pages and illustrations. 
Having now carried on our observations through the successive 
formations as far as the line where vertebrate remains are known to 
appear, and which has generally been acknowledged as the commence¬ 
ment of the Devonian system, we may review the same in their physical 
conditions, and the circumstances attending their distribution. 
In the earlier groups of strata, and more particularly in those which 
are highly calcareous in composition, we find the trend of the outcrop 
to have a generally east and west direction, following the northerly 
curve which has been described. Thus in all the lower Silurian lime¬ 
stones we trace the outcrop to the west and northwest, from the base 
of the Appalachians in New-York or in Canada, to the Mississippi 
river, and thence still in the same northwesterly direction. 
The Hudson-river group, in its finer sedimentary portions and in its 
calcareous shales, trends away to the northwest beyond the Mississippi 
river, The same is true of the Clinton group, though this is scarcely 
