GRIMSLEY. | Origin of Gypsum. 83 
tances. Some sand, clay, lime carbonate, and organic material 
are shown by chemical analyses and by the microscope, and 
these may be due to surface agencies. The water circulating 
through or near the underlying gypsum rock dissolved a por- 
tion of the rock and carried it upward in the springs to the sur- 
face of the swamp, where the mineral was precipitated through 
evaporation aided by the action of organic matter of decaying 
vegetation. 
A crust of gypsum would thus be formed and would increase 
in thickness until all the underlying rock was removed. Now, 
in some of these deposits borings detect no gypsum below the 
deposits, but it is found in wells outside at a level below the 
earth. In such places probably all the gypsum rock adjacent 
to the gypsum earth area has been removed by solution. Again 
by building up the swamp floor to a certain hight the rise of 
gypsum water springs may have been so checked as to hinder 
the earth formation. Whatever the cause, the gypsum earth 
deposit is not now forming over the entire area in any appreci- 
able amount. 
The uneven thickness of the deposits, some varying from 3 to 
8 feet within the main part of the deposit, shows that the con- 
ditions were more favorable at certain points than others. 
Probably these thicker portions were nearer the outlet of 
stronger springs. 
The deposits were formed in a comparatively short period of 
time. The presence of modern fresh water shells shows that 
the deposit is a recent one, formed long after the rock gypsum 
of the same region. 
