170 



HALL AND SAKDESON — THE MAGNESIAN SERIES. 



sandstone ; 5, the Shakopee dolomite. Of these five formations the first 

 three are considerable in vertical extent, reaching a thickness of 100 to 

 200 feet. Next above them the New Richmond sandstone is inconsid- 

 erable in thickness, seldom appearing more than a few feet and nowhere 

 more than 20 feet. Lastly, the Shakopee, paleontologically important, 

 since it carries a unique and well defined fauna, seldom reaches a thick- 

 ness of 50 feet. 



The following table shows the position of the series, with the maxi- 

 mum thickness of each division as determined for the states under 

 consideration : 



Ordovician (Lower Silurian) 



02 



w. 



(?) 



Upper Cambrian. 



Middle Cambrian. 



Saint Peter sandstone. 



Faunal break. 



Shakopee dolomite, 65 feet. 



Faunal break. 



New Richmond sandstone, 20 feet. 



Oneota dolomite, 175 feet. 



Jordan sandstone, 200 feet. 



Saint Lawrence dolomite and sandy 

 shale, 213 feet. 



Faunal break. 



Dresbach sandstone. 



LITHOLOGIC CHARACTERISTICS. 



The considerations for grouping the five formations named under one 

 term, " the Magnesian series," are partly lithologic and partly paleonto- 

 logic. The lithologic are apparent when we consider the dolomitic com- 

 position of formations one, three and five ; their uniform color, texture 

 and structure; the secondary nature of their crystalline habit; the pres- 

 ence of intercalated oolite in them all ; the regular occurrence of brec- 

 ciated phases in each ; and, finally, the entire absence of any known 

 physical character by which the geologist could distinguish the special 

 traits of each at any of its outcrops throughout the entire field studied. 



The sandstones, formations two and four, are equally identical with 



