198 HALL AND SARDESON — THE MAGNESIAN SERIES. 



The paleontologic evidence bearing on the classification arrived at by 

 the authors may be stated. categorically as follows : 



1. Between the Saint Lawrence and the underlying Potsdam sand- 

 stone (Dresbach sandstone, page 170), there is a faunal break which, so 

 far as known to the writers, is complete. 



2. The Saint Lawrence, Jordan and Oneota have a large proportion 

 of their known species in common, as, for example, Orthis pepina, Hall; 

 Raphistoma minnesotensis, Owen. 



3. Between the Oneota and Shakopee there is a faunal break over 

 which no species passes. 



4. Between the Shakopee and the overlying Saint Peter sandstone 

 there is another faunal break over which no species passes. 



There are, therefore, at least three faunas in the northwestern states 

 between the Algonkian and the Saint Peter subdivision of the Ordovician. 



The geographic distribution of the Magnesian series is then outlined 

 from the field-notes of the authors, and reasons for the nomenclature of 

 the several subdivisions are to some extent discussed. The characters 

 which led to the classification of former writers are pointed out and the 

 faunal types of the several formations are named. 



The lithology of the series is next discussed and the clean, pure con- 

 dition of the sandstone formations shown, together with local cementa- 

 tion of the grains with calcite, which on fracture discloses the crystallo- 

 graphic continuity under which it was deposited. Enlargement of quartz 

 grains is shown to be frequent. The dolomites also assume interesting 

 lithologic characters. The development of rhombohedral grains is a 

 general tendency. Among the impurities present are silicious oolite and 

 glauconite. The latter has every evidence of secondary origin rather than 

 primary, as is sometimes thought. 



In the closing pages of the paper the genesis of the series is considered. 

 The sandstone is assumed to have originated in essentially the same way 

 as that of more recent geologic times and of less chemically pure compo- 

 sition. In discussing the genesis of the dolomites the writers seek to 

 find in natural and every-day processes familiar to geologists the mode 

 of formation of these extensive beds. They do not attempt to explain it 

 through the addition of vast amounts of magnesium carbonate, because 

 no source can be thought of as conforming to all the physical and 

 paleontologic facts ; but noting the great reduction in the thickness ; the 

 successive yet distinct faunas which the Magnesian series carries, and, 

 finally, the present rapid removal of calcium carbonate as compared 

 with magnesium carbonate through the agency of percolating waters, 

 they conclude that it has been chiefly through the removal of calcium 

 carbonate that the present dolomitic condition has been reached. 



