386 The American Geologist. December, i904. 
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 separated by ridges of land. 
The sea would gradually become like the Caspian with smaller 
basins around it, in which all degrees of concentration would 
be found. 
In the deep basin near Saginaw the dividing ridge would 
be exposed 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 re- 
newed, resulting in the 20 to 25 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 g}^psum rests on a limestone 
floor, but with no traces of salt over it. The salt deposits are 
below the gypsum series in the underlying porous Marshall 
sandstone. 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 present Atlantic, 17.9 times as much salt as gypsum, and 
the salt over the gypsum or in the lower part of the basins 
toward the interior where the waters deprived of their gypsum 
content had retreated. 
If these conditions were true, the salt might later have been 
removed by solution in downward percolating ' waters which 
dissolved the more soluble sodium chloride. The g}rpsum now 
remaining does show marked effects of solution agents, the 
surface being rounded and furrowed by solution, and in places 
it is entirely removed. These efifects would have been far 
greater in common salt. The salt-laden waters or brines 
would flow downward along the slope of the rocks and through 
them, finally remaining at rest in the porous sandstones of the 
Marshall where it is now found. Further, the salt seems to be 
found in greater amounts toward the interior of the basin than 
near the edges; more at Saginaw, Ann Arbor, Lansing, etc., 
