Michigan Gypsum Deposits- — Grimsley. 387 
than at Tawas and Grand Rapids, though it is found in all these 
places, , . i 
Another possible explanation of the final history of this sea 
is to be found in the great' extension of the sea in the next epoch 
when the St. Louis limestone was formed. The sea in the St. 
Louis epoch extended its borders north and south and passed 
across the interior basin of Michigan to Grand Rapids on the 
west and to Huron county on the east. Possibly this renewal 
of the waters took place before the Michigan sea had disap- 
peared by evaporation or before it had evaporated enough to 
deposit a large quantity of salt except in certain smaller basins 
separated by the dividing ridges. 
From the evidence of sandstones and shales of the Michigan 
series found in the well borings of the interior it would seem 
that the ocean flowed over the southern barrier into the interior 
basin a number of times before the greater St, Louis inundation, 
and at these times deposited the sediments which are lacking in 
gypsum and salt contents. At these times the water would be 
diluted, its specific gravity lowered so that precipitation of the 
salts would not take place. These overflowing waters, local in 
their occurrence, cannot be correlated with other sections unless 
with those of the Logan series of Ohio, whose origin may be 
similar. 
In the deeper Michigan borings, gypsum appears to be re- 
placed by anhydrite ; but where the depth of concentrated wa- 
ters is 325 feet, giving a pressure of ten atmospheres, anhydrite 
is formed instead of gypsum. 
This theory as outlined for the Michigan gypsum deposits is 
based on the study of a few well borings and a comparative 
study of the conditions in the Caspian sea of today and those of 
the Michigan area as far as they can be determined. There is 
a wide range of probability involved and while the theory is 
advanced as a theory resting on limited data, it may be taken 
as representing approximately the conditions of origin of these 
deposits. 
