GEOLOGY. 241 



acid, is capable of yielding no less than one hundred and ten 

 to one hundred and twenty parts of crystallized Epsom salts 

 (sulphate of magnesia), and sixty parts of gypsum (anhydrous 

 sulphate of lime), from every hundred parts of the rock. So 

 that if sulphuric acid can be obtained or produced at a suffi- 

 ciently cheap rate in Wisconsin, Epsom salts may there be 

 manufactured profitably, and to an unlimited extent. I have 

 at present, in my laboratory, two hundred and thirty grains 

 of Epsom salts prepared from two hundred grains of the rock. 



*' It is from magnesian limestone that the Epsom salts of 

 commerce are now commonly procured. 



" But though the cliff rock is a magnesian limestone, and 

 though the proportions of carbonate of lime and magnesia, 

 which chiefly compose it, indicate that it is even a chemical 

 compound rather than a mechanical mixture, yet it cannot 

 with propriety, nor without risk of misconception, be called 

 the magnesian limestone, as a late writer on the geology of 

 Upper Illinois has termed the corresponding formation be- 

 tween Chicago and Ottawa ; since it is only a subdivision of 

 the mountain limestone group, always occurring beneath the 

 true coal-measures ; whereas, the magnesian limestone of 

 geologists (the zechstein of the Germans) is one of the lower 

 members of the new red sandstone group, and overlies the 

 bituminous coal formation. 



" Phillips, speaking, as it would seem, of the great scar 

 limestone of the north of England, which he there calls ' the 

 great limestone,' says : ' It is considered to have produced as 

 much lead as all the other sills put together.' This is pre- 

 eminently true of the cliff limestone of Iowa and Wisconsin. 



" The lead region lies, as will be remarked, chiefly in Wis- 

 consin, including, however, a strip of about eight townships 

 of land in Iowa, along the western bank of the Missisippi, 



the greatest width of which strip is on the Little Maquoketa, 



12 



