174 HALL AND SARDESON — THE MAGNESIAN SERIES. 



clean exposure is here seen for the full extent of these beds. For that 

 reason a somewhat detailed enumeration is here given : 



Coarse sand — base of Jordan. 



r^ f Fine sand, with mingled dolomite 15 feet. 



Dolomite, with mingled green shale 15 " 



CD 

 m 

 O 



o oo 





Hard crj^stalline dolomite, becoming somewhat blue toward the 

 bottom 18J 



Firm dolomite, with silicious grades 6 " 



Buff colored dolomite, with white and yellow sand 7 " 



Fossiliferous sand, with glauconite nodules and green shales 16 '' 



Dolomite, " green sand " and shale 2J " 



02 [ Breccia, changing downward into green shale and sand 8 " 



(?) Covered bank 15 " 



(?) Level of the Mississippi river at low water. 



THE ''MENDOTA LIMESTONES 



In Wisconsin this division of the old lower Magnesian limestone re- 

 ceived from Irving the name Mendota limestone. It was referred, with 

 the underlying and overlying sandstone, to the Potsdam."^ It has been 

 observed at several localities. At McBride's point, Mendota lake, the 

 type locality, it — 



"has a thickness of from 30 to 35 feet, of which the lower 20 feet are of a heavily 

 bedded, dark yellow and brown, jointed, conchoidal fracturing rock, which is 

 stained in seams and patches by the red oxide of iron and leaves on solution three 

 to ten per cent of an aluminous and non-arenaceous residue. This rock quite closely 

 resembles the lower portions of the lower Magnesian proper, having sometimes the 

 concretionary structure cliaracterizing that formation. The upper part of the Men- 

 dota about Madison resembles the lower, except in being in thin rough-surfaced 

 layers and in carrying a somewhat larger percentage of silicious matter." f 



In comparing further two typical samples, Irving points out the close 

 earthy texture of the Mendota and the porous and highly crystalline 

 character of the so-called Magnesian. 



In Iowa the exposures of this formation are not important. At Lan- 

 sing the uppermost 35 feet that lie exposed show a finely textured fos- 

 siliferous dolomite interlaminated with micaceous layers. Immediately 

 below this is a green shaly glauconitic dolomite 15 feet in thickness. It 



♦Geology of Wisconsin, vol. ii, 1877, pp. 260; 535; 544. 

 tibid., p. 543. 



