;W,x Tin American Geologist. May, imu 
Analyses showed this mineral to be essentially a hydrous 
silicate of iron, and selected sections kindly examined by Dr. 
-I. E. Wolff, of Harvard University, were stated by him to 
"resemble in all physical characters glaueonite" In one 
especially fresh specimen there were also rounded grains of 
calcite (probably magnesian), apparently originally detrital, 
and in some limited areas these increased in number till they 
formed nearly the whole bulk of the rock. These glauconitic 
carbonate layers are nearly identical in appearance with the 
glauconitic St. Lawrence (Cambrian) limestone of Minnesota, 
of which sections were cut and compared. Dr. Wolff sug- 
gested tentatively that the rock was an altered greensand. 
This appears to the writer to be the correct view. There is. 
however, a peculiar feature shown by chemical analyses — a 
constantly very small amount of potash. 
Tin-: Processes ok Change. 
It is quite certain that from this rock are derived nearly all 
the other phases in the iron-bearing member. In the altera- 
tions there are various processes, differing by reason of the 
varying forces which have been brought to bear upon the 
rocks. The most common, which may be called the normal 
process, appears to take place under scant access of atmos- 
pheric agents. The decomposition of the green mineral is the 
noteworthy event. It breaks up, forming chiefly silica and 
the iron oxides or carbonate, so that in by far the commoner 
phases of the rocks the rounded bodies or granules are com- 
posed entirely of these decomposition products. 
The various stages in the rearrangement of the silica and 
the iron oxides or carbonate constitute the succeeding phases. 
The chief cause of their separation lies in the different condi- 
tions under which these two chief constituents are taken into 
solution. From very careful study of the separation and con- 
centration of the silica and the iron, there may be deduced 
two rules. First, in regions of comparatively free oxidation, 
chalybeate waters deposit iron and remove silica. Thus 
hands of iron are formed along cracks and fissures, whether 
macroscopic or microscopic, replacing the silica ; and isolated 
spots weakened by weathering become by replacement 
blotches of iron oxide. Conversely, in regions of extremely 
