342 HALL AND SARDESON — PALEOZOIC FORMATIONS OF MINNESOTA. 



From a comparison of McGee's clear statement of the Iowa Oneota 

 ami its continuous rocks above and below with the Minnesota series as it 

 is known to them, the authors feel that the alternation of sands, shales 

 and dolomites winch occurs in the latter state cannot well be considered 

 as identical with the Oneota of Iowa. Paleontologic evidence, so far as 

 it is at hand, bears testimony to the unity of the series between the Pots- 

 dam and the Saint Peter. Again, the structural, lithological and chem- 

 ical identity of the beds is remarkable. An oolitic or a breeciated con- 

 dition is not a marked feature of any one lied, but is found in all three 

 dolomitic layers alike; the rhombohedral shape of the constituent 

 grains is an almost universal character of the dolomites, and the chemical 

 composition of any one layer can be duplicated in either of the others. 

 These facts are equally true of the sandstones, so far as they will apply. 

 The deposition in Minnesota was nearer the shore of the Cambrian sea, 

 and thus exhibits all the phases of sediments from conglomerates through 

 sands and shales to limestones, which in Iowa may not be the case. 

 These different phases, for local purposes, must have different name-. 

 The awkward device "Main body of limestone," first used by Irving* 

 and subsequently adopted by 'Winch el Id" is shown by McGee to be awk- 

 ward simply by the use of it in a geologic discussion. Besides the gen- 

 eral and long-time use of the term Magnesian in Iowa, Wisconsin and 

 Minnesota, a use which has firmly intrenched the word in our geologic 

 literature and speech, with and without the qualifying words Upper and 

 Lower, the dolomitic character of the rocks in question is most perti- 

 nently expressed in that word. Nowhere else on the North American 

 continent have we such a vast extent of rocks carrying so typical a dolo- 

 mitic composition as do the carbonate layers occupying the place between 

 the Potsdam and the Saint Peter in our northwestern states. The terms 

 Shakopee, Jordan and Saint Lawrence have been accepted for some years 

 in Minnesota; their uses have been defined ; the rocks are well known 

 as a single group ; accordingly in the present paper the term " Magnesian 

 series " will comprise the following local members : 



' Shakopee A (upper Shakopee) dolomite. 

 Elevator B (New Richmond) sandstone. 

 Magnesian series. . Shakopee B (lower Shakopee; dolomite. 



Jordan sandstone. 

 Saint Lawrence dolomites and shale-. 



In the following discussion but little attention will be paid to these 

 subdivisions: they are chiefly of local interest, since structural and lith- 

 ologic characters are almost identical in all similar beds. 



* AniiT. Jour. Sei., 3d ser.. vol. ix, 187."'. p. WO. 



fGeology of Minnesota. Final Report, vol. ii, 1888, \>. xxii. 



