L. #. Hieks— Waveriy Group in Central Ohio. 223 
reasons are: Ist. A physical break at the close of the Hamilton, 
the previous movement of elevation being then reversed and a 
new cycle of deposition begun which culminated in the deposi- 
tion of the Subcarboniferous Limestone. 2d. “ A change of 
fauna ; the fossils of the Chemung and Upper Portage, that is 
Erie, show great development of the Productus family, and 
other fossils of Carboniferous type.”* 
A physical break of some magnitude and a marked change 
of fauna are certainly the two things, and the only two, upon 
which to base the boundaries of formations. But the break 
here appealed to is only one of a number of such changes, and 
of no greater magnitude than some others in the same series. 
For instance, at the close of the Chemung a large area in central 
and western New York was raised above the sea-level, 
Crinoids an forming limestones Manual of Geology, p. 
281.) In this passage a contrast of physical conditions is indi- 
cated which is certainly equal, if not superior in importance, to 
the physical break at the close of the Hamilton period. 
As regards the change of fauna it was not general enough to 
be of commanding importance. The Carboniferous aspect of 
Chemung fossils is confined to those from western New York 
never be done, for it would open the door to endless contro- 
Versies respecting the new boundary. When we remember 
that all classification must be somewhat arbitrary ; that the 
logical rhythm of any system often demands the accentuation of 
* From a letter to the writer. 
