26 University Geological Survey of Kansas. 
The geological age of the deposits is Lower Helderberg, a 
formation with maximum thickness of 700 feet, and gypsum 
occurs through it at various places, and the mineral is found 
in most of the deep wells drilled in northern and central Ohio. 
The gypsum beds, according to Orton, are not even and hori- 
zontal, but are found in waves or rolls, whose summits rise 5 
to 8 feet above the general level. The main plaster beds are 
about 12 feet thick, and would yield 50,000 tons to the acre. 
No fossils are found, and Orton regards the origin of the gyp- 
sum as due to a deposit from a shallow, land-locked and contract- 
ing sea during the Lower Helderberg period. ‘The first deposit 
would be the sulphate of lime in form of gypsum or anhydrite. 
The shallowness of sea is established by the sun-cracks and 
wave marks that are well shown in these rocks. 
The gypsum is used as land plaster and calcined into plaster, 
in proportion of about 60 per cent. of the first and 40 per cent. 
of the second, but the total value of the calcined plaster is 
greater than that of the land plaster. In 1895° Ohio produced 
21,662 tons of gypsum ; 6914 tons were sold crude, 3048 tons 
were ground into land plaster, 11,700 tons were calcined into 
plaster of Paris, with a total value of $71,204. 
Gypsum in Pennsylvania.’ 
In Pennsylvania gypsum occurs in the Lower Helderberg in 
thin seams intermingled with mud veins, and the whole series 
lies just below a drab, impure limestone. The origin is re- 
garded as due to an alteration of the limestone to gypsum 
through the agency of sulphur spring water. ‘The deposits are 
not of economic importance. 
Gypsum in Michigan.® 
The gypsum deposits of economic importance in Michigan 
are found in two localities. The most extensive are near Grand 
Rapids, in Kent county, in the western part of the state. The 
gypsum covers an area of about ten square miles, and lies from 
6. U.S. Geol. Surv., 17th Annual, Pt. III, p. 979. 
7. Geol. Surv. of Penn., Summary Final Report, Vol. 2, pp. 913 to 915. 
8. Mineral Statistics of Michigan, pp. 3-20, 1881. 
