534 GEOLOGY OF CENTRAL WISCONSIN. 



son, the summit of this well being at a lower level, and the drift 70 

 feet in thickness. In the former well, the top of the rock is 63 feet, 

 in the latter, 100 feet below the base of the Mendota limestone, as ex- 

 posed in the neighborhood. On the banks of Lake Mendota, near 

 Madison, we find exposed, beneath this limestone and above the lake 

 level, one foot of greensand and 31 feet of fine-grained, light green- 

 ish, very friable, sandstone, including very thin dolomitic seams, and 

 carrying throughout some dolomitic and calcareous matter, the con- 

 tent of purely silicious sand being 84.45 per cent. Altogether, then, 

 we find in the Madison region, the following succession of layers, 

 between the Mendota base and the Archaean: 



Feet. 



1. Greensand layer 1 



2. Calcareous and dolomitic, friable, fine-grained, greenish, sandstone ... 31 



3. Not known 31 



4. Light colored sandstone, for tlie most part j)urely silicious, being made 



of rolled quartz grains; but no specimens obtained from the upper- 

 most layers • 704 



5. Red sliale 10 



Total 777 



Along the Wisconsin river bluff's in Columbia, Sauk and Dane 

 counties, the Mendota horizon is very prominent, the sandstone 

 showing below it for a thickness of 150 to 200 feet. The very bold 

 bluif that rises from the north bank of the Wisconsin river at the 

 mouth of Honey Creek, in the town of Prairie du Sac, Sauk county, 

 shows the following section, from the Mendota base downwards: 



Ft. In. 

 1. Greensand with thin, brown, calcareous layers; the greensand layers made 



up of fine grains of glauoonite and wliite sand, mingled witli crystalline 



calcite 3 ' . . 



'2. No exposure 7 . . 



3. Fine-grained green and brown sand, calcareous 2 2' 



4. Loose white and brown sand, some layers partly calcareous 10 . . 



5. Firm and heavy layers of yellowish, porous, calcareous, sandstone, inter- 



stratified with layers of white sand 6 3 



C. Alternations of pure white, non-calcareous, ftne-grained sand, with nodular- 

 weathering, yellow, calcareous layers, and layers of dark greensand, the 

 whole showing fine cross-lamination; the wliite sand layers predominating, 45 . . 



7. Fine-grained light-colored sandstone; often pure white, and loose; in parts 



cross-laminated, the lines of cross -lamination being marked by rows of 



glauconite grains 9 6 



8. Porous, yeHowish-brown, slightly calcareous la.yers 4 11 



9. No exposure 11 6 



10. Fine-grained, friable, gxeyish sandstone, only slightly calcareous, carrying 



a few yellow calcareous layers as above 11 5 



11. Firm layers of ferruginous sandstone, more calcareous tl\an the last 5 6 



