168 HALL AND SARDESON — THE MAGNESIAN SERIES. 



Page 



The genesis of the series 187 



The sandstones 187 



The shales 188 



The dolomites „ 189 



Historic outline 189 



Sediment-building 191 



Contents of seawater 191 



Composition of modern sea-deposits 192 



Composition of spring and other waters 194 



Analyses of travertine and impure coral 194 



Geographic considerations 195 



Continental movements 195 



Removal of calcium carbonate and its effects 196 



Basis of the discussion 197 



Summary , 197 



Geographic Position of the Series. 



The Magnesian series, which it is proposed to discuss in this paper, 

 consists of several alternating beds of dolomite, dolomitic shale and sand- 

 stone. The series lies between the so-called Potsdam ♦sandstone below 

 and the Saint Peter sandstone above. It occurs in widely distributed 

 localities throughout southern Wisconsin, southeastern Minnesota and 

 northeastern Iowa. Nowhere in the states named has either one of the 

 delimiting formations been found absent, save in eastern Wisconsin, 

 where, at a single locality, Chamberlin found an arch of the billowy 

 lower Magnesian rising into actual contact with the Trenton limestone 

 and thinning out the Saint Peter sandstone to zero.* 



Historic Resume. 



A resume of earlier investigations of the extent and contents of the 

 Magnesian series of Minnesota has already been given by the writers.f 

 To that resume these notes can be added : 



In Wisconsin the geologists of the last geological survey very carefully 

 explored the lov/er Magnesian beds as they occur in that state. J They, 

 however, relegated the Mendota limestone (Saint Lawrence) and the 

 Madison sandstone (Jordan sandstone) to the underlying Potsdam. The 

 lithologic and structural characters of the series are very fully discussed 



* Geology of Wisconsin, vol. ii, 1877, p. 272. 



t Paleozoic Formations of Southeastern Minnesota: C. W. Hall and F. W. Sardeson, Bui!. Geol. 

 Soc. Am., vol. 3, pp. 331-368 and plates 10-12. 



J See the Geology of Wisconsin, vol. i, 1883, pp. 138-144; vol. li, 1877, pp. 268-285,547-555, 577-607, 

 671-675 ; vol. iii, 1880, pp. 397, 398 ; vol. iv, 1882, pp. 64-81, 123-129, 194-204, 248, 249, 511-518. 



