SALT ENTEEPRISE IN MICHIGAN". 261 



Series"; but a little later, the "Michigan Salt Group/' 

 It has a thickness of 180 feet, and consists of argillaceous 

 shales, clays, magnesian limestones and beds of gypsum.* 

 Here is the origin of the brine which escapes in a circle 

 of springs marking the contour of the formation. This 

 group of strata underlies 17,000 square miles in the cen- 

 tral portion of the state. It is dish- shaped, and consti- 

 tutes an immense reservoir or saliferous basin. The 

 edges are sufficiently elevated to prevent the efflux of 

 water which finds its way into it; and hence the saline 

 particles have never been washed away, as would have 

 been the case if the formation possessed any continuous 

 dip from border to border, or even had a depression on 

 one side sufficiently deep to drain its contents. Beneath 

 this series of shales is a porous sandstone the Napoleon 

 sandstone of the Marshall Group which, within the cir- 

 cumference of the basin which it forms, becomes saturated 

 with brine from above. From the nature of the case, it 

 is evident that the strongest brine must accumulate in 

 the deepest part of this basin. 



In April, 1859, an artesian boring for salt was begun 

 at East Saginaw, under the direction of Dr. G. A. Lathrop. 

 I am not informed of the creoloofical reasoninc^ which led 

 to the selection of that location. There were no brine 

 springs in that vicinity, nor were there any outcrops of 

 the underlying rocks. The result showed that the rocks 

 were buried a hundred feet deep. Dr. Lathrop had trav- 

 eled extensively in the regions about Saginaw Bay, and 

 possessed a large amount of exact information on the 

 geology of that part of the state. At Bay City, moreover, 



* W^ithout doubt these strata correspond to rocks which, in other states, con- 

 stitute some of the lower part of the great Carboniferous Limestone series. 



