Name:  Gypsum

Geologic Name:  Grand Rapids Gypsum

Reference:  Grimsley, G.P., Gypsum deposits of Michigan: U.S. Geol.
Survey Bull. 223, p. 45-47, 1904.

Microscopic characters:  The rock consists of a fine grained aggregate
of gypsum with patches in which the grains are quite large.  There
are also a few sharply outlined crystals of gypsum.