3814 '^'^^ American Geologist. December, 1904. 
the sea, more than a hundred times the quantity that probably- 
existed in the basin. 
CASPIAN SEA AS AN ILLUSTRATION OF THE MICHIGAN SEA. 
The gypsum deposits in Michigan do not occur uniformly- 
distributed through all parts of this old sea basin, but they ap- 
pear concentrated in certain areas of comparatively small size. 
For the cause of this localization of the deposits we may look 
for a modern illustration to the conditions in the Caspian sea. 
Into the northern part of the Caspian sea empty the Volga^ 
Ural, and the Terek rivers bringing in a large quantity of fresh 
water so that in this portion of the sea the water is nearly pure 
with a specific gravity of 1,009. This small percentage of salt 
is, according to Van Baer, due to the number of sliailow 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 concen- 
tration. One of these has deposited on its banks only a thin- 
layer of salt, a second has a compact mass of salt on its floor,, 
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 Kara- 
boghay (Black gulf) of the Casoian, where the nearly circular 
shallow basin is about QO miles across and almost entirely cut 
off from the sea by a long narrow spit of land, so tnat the gulf 
and sea are only connected 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 
evaporation from the surface of the basin due to the heat and 
winds. The shallow depth of the bar prevents a counter cur- 
rent of highly saline water into the Caspian. This current car- 
ries into the Black gulf, according to Van Baer, 350,000 tons 
of salt dailv. If this dividing bar of land should be elevated 
and cut oflf the basin from the sea, the gulf would rapidly dim- 
inish 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 evaporation could go far enough for the deposition of gyp- 
