GEOLOGICAL PAPERS. 115 



Baer, due to the number of shallow lagoons surrounding the basin, 

 each being a sort of natural salt pan. At Novo Petrovsk , a former 

 bay of the main sea is now divided into a number of basins, showing 

 all degrees of saline concentration. One of these has deposited on 

 its banks only a thin layer of salt, a second is a compact mass of salt, 

 and a third has lost all the water and is a mass of salt covered with 

 sand. 



The concentration is seen on the greatest scale in the Karabogliay 

 (Black Gulf) of the Caspian, where the nearly circular shallow basin 

 is about ninety miles across and almost entirely cut otf from the sea 

 by a long, narrow spit of land, so that the gulf and sea are only con- 

 nected by a channel not over 150 yards broad and five feet deep. 

 Through this channel there passes into the gulf a current with an 

 average velocity of three miles an hour, but which is accelerated by 

 the western winds. 



This current is due to the indraught produced by excessive evapo- 

 ration from the surface of the basin, due to the heat and winds. The 

 shallow depth of the bar prevents a counter-current of highly saline 

 water into the Caspian. This current carries into the Black Gulf, ac- 

 cording to Van Baer, 350,000 tons of salt daily. If this dividing bar 

 of land should be elevated and cut otf the basin from the sea, the 

 gulf would rapidly diminish and become a salt marsh, which, later, 

 drying up, would leave a large salt deposit. 



With a greater depth of water over the dividing ridge, the counter- 

 current would come in as at the straits of Gibraltar, and the evapora- 

 tion could go far enough for the deposition of gypsum, while the 

 concentrated salt brine would pass back into the larger sea. This 

 was more probably the condition in the Michigan gypsum basins, as 

 will now be shown from a study of geological conditions and well 

 records. 



MICHIGAN INTERIOR SALT SEA. 



The Kinderhook sea of the American continent was an interior sea 

 with a bay extending northeast into Michigan. In this bay were de- 

 posited 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 north- 

 ern Indiana. At Lafayette, Ind., 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 sea was sur- 

 rounded by the Marshall series, at this time dry land, 777 (Kalama- 

 zoo), 983 (Coldwater) and 1000 (Hillsdale) feet above sea-level on 

 the south ; 700 ( Huron county) feet on the east ; and 755 (Grayling) 



