48 The American Geologist. Jan. i89i 
shale immediately above the lower limestone. In just what sense he 
intended the term -replacing" to be understood, I am in doubt. 
How did this sandstone replace the shale ? They both lie con- 
formably upon the limestone and apparently pass into one another. 
The line of junction is obscured by debris and overgrowing vege- 
tation. The sandstone is overlaid by blue and green shales — No. (> 
of the section. 
At first glance it might be supposed that a fault had caused 
one of the lower sandstones of the Chester group to be brought 
up beside the shale. But faults, while common enough in the 
Chester group in western Kentucky, are rare in Illinois. Besides 
the underlying limestones can be traced continuously. The sand- 
stones could not have been a slip on an extensive scale of the 
upper sandstone, as the green and blue shale overlying it is part of 
the thick bed of shale (No. 6 of section ), which, from the abun- 
dance in it of the bryozoan Lyropora, may be termed the Lyro- 
pora beds. The latter fact also forbids the view that the sand- 
stone is of more recent date, Tertiary or later. 
In view of the facts given above, the following explanation is 
suggested. Where the sandstone now is, shale was deposited just 
as in the territory joining. After some dozen or fifteen feet had 
been deposited, some change occurred in the Chester seas by 
which a swift current scooped out the soft, shah* material at this 
point. Presently the strength of the current diminished and an eddy 
was formed in which line sand was deposited rapidly. After about 
twelve feet of this had been deposited, there was a return of the 
former conditions and shale was again laid down as before. To 
substantiate this explanation further observations are necessary. 
THE GEOLOGICAL WORK OF MOSSES AND ALG^E. 
By WaIiTBB Harvey Weed, Washington, D. C. 
The part that low vegetable organisms play in providing 
chemical change is of interest to geologists since it often results 
in the formation of deposits that have considerable geological 
importance. 
The secretion of carbon and of carbonate of lime by plants and 
by animals is a familiar phenomenon, and constitutes the subject 
of very important chapters in chemical geology ; but while much 
