694 SLOARL VIELEN, 
species of the brachiopod genus Wyhitfieldella stamps the fauna as 
of Silurian age. 
In the New York section, which is usually taken as the 
standard for our continent, the Medina formations constitute the 
lowest division of the Silurian system. The Clinton division 
following the Medina consists in New York of a series of strata 
diverse in character. There are beds of shale, sandstone and 
limestone, and one very persistent stratum is the fossil iron ore 
bed. This division was well named ‘‘Protean Group” by the 
early New York geologists. The characters of the strata are 
precisely such as one would expect to find in a series of beds 
deposited during a period of readjustment of local conditions. 
The Niagara division of the Silurian, following the Clinton, 
essentially represents the period when local conditions had 
become readjusted and equilibrium established. It was primarily 
a limestone-forming period, and although the Niagara shales of 
New York are classed with the limestone, it would perhaps be 
more natural from a stratigraphic point of view to place them in 
the preceding division. 
From the point of view here taken, it will be seen that the 
Clinton and Niagara cannot be considered as separate and dis- 
tinct time divisions having the same significance throughout the 
entire area in America which was originally covered by Silurian 
waters. The two divisions, rather, exemplify two sets of con- 
ditions. In the Mississippi valley the Clinton period of read- 
justment was short, and is represented by a very thin sertes of 
sediments. The conditions of equilibrium, with clear limestone- 
depositing seas, very soon became established after the incursion 
of the Silurian waters, but in New York, in the region nearer the 
finally established shore line, this period of readjustment occu- 
pied a much longer time. In the southern Appalachian region 
the Niagara conditions seem never to have been attained. 
Indeed, since it was probably the last region reached by the 
encroaching sea, it is possible that even the Clinton conditions 
did not begin there until long after the Niagara conditions had 
become established in some other parts of the continent. Taken 
