5Si GEOLOGY OF CENTRAL WLSCONSIK. 



Ft. In. 



IV. Similar to II ^ • • 



V. Compact, yellowish-brown-and-red-blotclied, calcareo-magnesian rock 



(1210); differing' from I in having a very fine crystalline textiu-e, and 

 cavities lined with dolomitfc crystals; insoluble residue, 41.73 per cent.; 

 layei-s a foot thick; quarry-rock, used in building at Cambria and other 

 places in the vicinity: obtainable in large well-shaped blocks 4 . . 



POTSDAM SANDSTONE. 



VI. Greensand layers; moderately firm, rough-surfaced, brownish, speckled 



with green; calcareous; leaving a residue, on treatment with acid, of 



fine, shai-ply-angular quartz, mingled with dark-green grains 2 . . 



32 6 



The greensand appears without doubt to be that generally recognized as marking 

 the base of the Mondota horizon, which is here more largely silioious than in the typical 

 localities in Dane county, though still sharply contrasting in character with tlie Madisoa 

 and Potsdam sandstone layers. A quarter of a mile up the ravine, on its east wall, 

 another smEtll quarry opening shows the foUoMing: 



MADISON BEDS. 



Fcot. 

 I. Fine-grained, brownish, ferruginous, friable sandstone (1236); non-calcajreous; 



composed of rolled grains of dulled quartz; scolithus -heaxing 4 



II. Very fine-grained, firm, pure white, sihcious rock (1212); composed of exceed- 

 ingly fine, sharply angular quartz; non- calcareous; non-arenaoeous; contain- 

 ing 97.52 per cent, of sUica; close to III of last section; upper layers shaly . . 8 



MENDOTA BEDS. 



1 n. Similar to II, but stained yellow and pink (1213); very hard and firm; con- 

 taining 98.12 per cent, sflica 8 



1 V. Reddish-yellow rock similar to I of last section, and apparently the same horizon. 3 



13 



At P. Scheasman's quarry, on the west line of the S. W. qr. of Sec. 6, in the same 

 tovra, a ten-feet quarry face shows below, in thin and very regular layers, a close- 

 textured, buff-colored, nearly pure dolomite (1205), wluch weathers with a smooth, yol- 

 . lowish surface, is marked finely with dendritic manganese oxide, and is coated in places 

 with white, stalactitic, hme carbonate, and at the top a heavy layer of concretionary, 

 dark-colored dolomite. On the hill above are exposures of the ordinary rougli-.textured 

 Lower Magnesian, near the base of which formation the quarry layers appear to lie. 



In the railroad cutting at Rio, in the northwest corner of the town of Otsego, T. 11, 

 R. 12 E., the Madison and Mendota beds are exposed. The followuig is tlie section :- 



MADISON BEDS. 



Jfeet. 

 I. BroHTiish, friable sandstone 6 



II. Very fine-grained, pure white, firm, silicious rock; non-calcareous; in upper 



portions slightly arenaceous in texture (1219, close to 1212); lower portions 



without ti-ace of gi-anular texture; rough- surfaced and pink-tinted (121G, 



close to 1213); bedding not distinctly seen; composed of exceedingly fine, 



angular quartz 12 



MENDOTA BEDS. 



III. Red-and-yeUow-mottled calcareo-arenaceous rock 4 



22 



