Michigan Gypsnin Deposits- — Grimslcy. 379 
When such a body of salt water is cut off and evaporated, 
the gypsum is deposited after 37 per cent, of water is removed 
and common salt only after the removal of 93 per cent. The 
normal order of these formations would be a deposit of gyp- 
sum, and then a much heavier deposit of salt. But since 93 
per cent, of the water must be evaporated before the salt would 
be thrown down, the evaporation might go far enough for the 
deposition of gypsum, but not far enough for salt ; or the salt 
might be deposited and subsequently removed by solution. The 
first condition apparently took place in the Kansas gypsum area 
and both conditions probably occur in Michigan. 
In most areas, the amount of gypsum found is far greater 
than the amount that would be found in a body of ocean water 
sufficient to cover the g}'psum area at reasonable depths. This 
has led to a number of modifications of the salt sea theory. In 
Oklahoma and in Canada, the salt sea theory was set aside and 
the origin of gypsum in those areas was given as due to an 
alteration of limestones through action of siwphur vapors or 
waters of springs of volcanic origin. In Iowa, and "\n some of 
the reports of Kansas, and in the older reports on the Michigan 
gypsum, the former salt sea has been compared to the present 
Mediterranean sea with its upper inflowing and lower outflow- 
ing currents over the submarine ridge at the strait of Gibral- 
tar. In this way the waters of the interior sea while evaporat- 
ing would receive an influx of additional salt water adding in 
its evaporation to the total quantity of deposits. The same 
theory is used to explain the great thickness oT salt at Stass- 
furt (1000 feet) and at Sperenberg (3000). 
GEOLOGY OF MICHIG.^N GYPSUM DEPOSITS. 
The geological formations of the Lower Peninsula of Mich- 
igan are represented by an interior Coal Measure basin sur- 
rounded by more or less complete and irregular concentric cir- 
cles of the older formations down to the Lower Helderberg. or 
Monroe dolomyte, the uppermost stratum of the Silurian. 
In Michigan, Winchell in 1862 described a series of sand- 
stones, 296 feet in thickness, whose upper portion was more 
firmly cemented and more homogeneous than the lower, and 
further contained fewer fossil remains, in fact was almost with- 
out organic remains. The upper part was called the Napoleon 
