116 KANSAS ACADEMY OF SCIENCE. 



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 Saginaw 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 high 

 land 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 Salina 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 irregular clay seams and the clay-dividing planes 

 in the gypsum represent an influx of sediment, wind-blown material, 

 or tidal currents. 



As the evaporation of these waters went on, the first deposit would 

 be carbonate of lime, thrown down when the specific gravity was 

 raised to between 1.0506 and 1.1301. By further concentration the 

 gravity would reach 1.22, and in this interval the gypsum would be 

 deposited. At this period 37 per cent, of the water must have 

 been evaporated. If the sea was 700 feet in depth, it would now be 

 440 feet, still covering the Saginaw ridge but exposing the Lansing 

 ridge. Further well records might give a clue to other basins sepa- 

 rated by ridges of land. The sea would gradually become like the 

 Caspian, with smaller basins around it, in which all degrees of con- 

 centration would be found. 



In the deep basin near Saginaw the dividing ridge would be ex- 

 posed before salt was deposited. In such an evaporating basin the 

 deposit of salts would occur around the borders of the basin first, and 

 by the influx of water across the Saginaw ridge the water in the 

 concentrating basin was probably renewed, resulting in the twenty to 

 twenty-five feet of gypsum now found in that area. 



The normal order of deposits should be lime carbonate, on which 

 would be a deposit of gypsum, covered by layers of salt. In the 

 present developed areas the gypsum rests on a limestone floor, but 

 with no traces of salt over it. The rock-salt deposits are below the 

 gypsum series, in the Monroe or Salina series. Further, in the 

 salt series of Saginaw, Grand Rapids and other places there are no 

 traces of rock salt, but the salt-wells secure the salt from natural 

 brines. 



If the Michigan interior sea evaporated completely, there would 

 have been, on the assumption its waters were like those of the presen 

 Atlantic, 17.9 times as much salt as gypsum, and the salt over the 



