Michigan Gypsum Deposits- — Grimsley. 385 
sum while the concentrated salt brine would pass back into the 
larger sea. This was more prohahly the condition in the Michi- 
gan gypsum basins as will now be shown from a study of geo- 
logical conditions and well records. 
MICIIICAX IXTFRIOR SALT SEA. 
The Kindcrhook sea of the American continent was an in- 
terior sea with a bay extending northeast into Michigan. In 
this bay were deposited the Marshall sandstones. The close of 
the period was marked by uplift in this area causing a retreat of 
the sea southwestward finally exposing a wide area of land in 
southern Michigan and northern Indiana. At Lafayette, Indi- 
ana, the floor of this sea was at least 563 feet above sea level. 
North of this barrier was a large interior sea with its floor 375 
feet above sea level near Grand Rapids, lower by nearly 200 
feet than the ocean to the southwest. This area was sur- 
rounded by the Marshall series, at this time dry land, yyy 
(Kalamazoo), 983 (Coldwater), and 1000 (Hillsdale) feet 
above sea level on the south; 700 (Huron county) feet on the 
east; and 755 (Grayling) feet at the north. A sea, like the 
Caspian, with a depth at first of probably 700 feet or more and 
an area of 22,686 square miles. 
In this sea were elevations and depressions, a ridge at 
Lansing 500 feet above sea level and a depression east of Sag- 
inaw 380 feet below sea level separated from the main basin by 
a ridge 187 feet above the sea floor. 
This sea probably had its tributary streams coming from 
the highland at the north and northeast flowing down across 
the recently emerged flats of the Waverly and Marshall land, 
bringing a supply of sediment and doubtless salt from the Sali- 
na beds at the north. The lake basins of Michigan and Huron 
were not in existence at this time but belong to a much later 
chapter in the geological history of our continent. The irreg- 
ular clay seams and the clay dividing planes in the gypsum rep- 
resent an influx of sediment, wind blown material, or tidal 
currents. 
As the evaporation of these waters went on, the first de- 
posit would be carbonate of lime thrown down when the specific 
gravity was raised to between 1.0506 and 1.1304; bv further 
concentration the gravity would reach 1.22 and in this interval 
gypsum would be deposited. At this period 37 per cent, of the 
