240 REPORT OF THE AMERICAN COMMITTEE. 
it necessary to define the subdivisions by means of invertebrate 
faunas. 
From a comparison of the several series of North America it 
is evident that there are several clearly-defined distinct marine 
faunas in the Paleozoic. Without considering the lower Silurian, 
we find three distinct stages of a purely marine fauna : (a) the 
Niagara, (b) the Hamilton, (c) the Sub-carboniferous, the latter 
represented typically in its later members, the St. Louis and 
Chester stages. In the interior basin, where the limestone 
formations are almost continuous, including all three, these three 
faunas appear in sequence and can be compared, showing both 
their diiferences and their likenesses. Where the same genera 
are concerned the species are distinctive, but at the same time are 
so similar as to suggest close relationship. The most distinctive 
differences in the three faunas are in species or genera found at 
only one of the stages, and, when we compare these sections with 
the sections of the east or of the west, we find on the one hand 
that the sharply- defined faunas are marked by specialized and 
modified local stages of one or other of these typical faunas. 
Thus the Corniferous is a coralline modification of the lower 
Hamilton fauna of the interior ; the Chemung is but a specialized 
stage of the upper Hamilton of Iowa, and the Waverly is but an 
eastern modification of the lower stage of the Carboniferous 
fauna of the interior, while in the west there is a blending of the 
Corniferous, Hamilton and Chemung types throughout a series of 
several thousand feet of limestones and shales. In the same way 
the upper Helderberg limestone holds a fauna which appears to 
be a modified or later stage of the Niagara fauna. 
The sharp distinctions are seen in those areas where there was 
a definite interruption of the conditions of the ocean during the 
deposition, and the line between the Silurian and the Devonian 
is sharpest where the Niagara and the Hamilton were neither of 
them strongly calcareous, but are separated by a limestone fol- 
lowed by a marked sandstone and then a limestone again, as in 
the Appalachian area. So also at the top of the Devonian, in the 
same general area, it is the elevation of the bottom expressed by 
the great accumulation of arenaceous shales, sandstones and con- 
glomerates separating the Hamilton marine fauna from the 
marine fauna of the Carboniferous, that makes the transition so 
conspicuous. 
