SALT ENTEEPEISE IN MICHIGAN. 269 



to be a secondary product, resulting from chemical reac- 

 tions in the rocks, and especially from the action of sul- 

 phuric acid on limestones or dolomites.* The conception 

 of a continuous gypsum formation having a sedimentary 

 origin had probably seldom if ever been entertained. 

 That view resulted from the present writer's researches, 

 and at this day scarcely any other finds defenders. Salt 

 basins, therefore, are the sites of ancient areas of salt water 

 which have gradually dried up.f 



Before these investigations the gypsum of Mackinac 

 and Grand Rapids had been regarded as of the same 

 geological age. It was now shown that the Mackinac 

 gypsum is of the age of that in central New York and 

 on Sandusky Bay. It followed that the whole peninsula 

 is underlaid by a second and deeper salt basin, the 

 Salina formation, and that quite probably this basin 

 would also be found a source of brine supplies. The re- 

 port showed that the salt springs of the peninsula follow 

 especially the lines of outcrop of the principal salt basin 

 and mark the geographical boundaiy of the formation. It 

 showed that the area of the peninsula had never been 

 subjected to disturbing agencies ; hence the strata were 

 but little fissured, and few opportunities existed for the 



* See Reports on the Geology of New York. This improbable view is even 

 still held, in reference to the gypsum of central New York, by James D. Dana 

 (System of Mineralogy, 614, 639; Manual of Geo'ogy, 3d. ed., 234.) Without doubt, 

 sulphuric acid may result from the action of oxygen on sulphuretted hydrogen. 

 Without doubt the reaction of sulphuric acid and limestone produces gypsum. 

 But sulphuric acid may also result from the decomposition of gypsum, and, as 

 a fact, native sulphuric acid is not found in any connection with sulphuretted 

 hydrogen except where evidences of the presence of gypsum also exist More- 

 over, the lenticular masses of gypsum, inclosed in regular beds in the clays of 

 the Salina group, present vastly more the appearance of the relics of a once con- 

 tinuous formation dissolved away than of the products of chemical action in 

 place. 



t See the writer's Sketclies of Creation, ch. xxvi. 



