THE LOWER SILURIAN ROCKS. 535 



Ft. In. 



12. Loose brown sand \i 



13. Unexposed 7 



14. Porous and orystalKne- textured, yeUow limestone, mth much coarse and 



bright gxeen glauconite 1 



15. Unexposed 3 5 



16. like No. 14, interstratified with bright green seams 2 4 



17. Heavy brown layer of shghtly calcareous sandstone 3 . . 



18. Greensand layer; a mixture of very fine white sand and glaucomte grains, 



_ with some crystalline oalcite 10 



19. Light yellow, friable sandstone; only shghtly calcareous; cross-laminated, 5 4 



20. Greensand layer, hke 18, false bedding very marked, cross-Iamins very 



plain 13 



21. Porous, yellow, slightly calcareous sandstone 6 



22. Fine-grained, cross-laminated, slightly calcareous sandstone, with much 



greensand 1 H 



23. Greensand, hke No. 20 10 



24. Fine-grained, friable, white sandstone, slightly calcareous 1 . . 



25. Unexposed 12 7 



26. Fine-grained, white sandstone, entirely non-calcareous; made up altogether 



of fine rounded grains of Hmpid quartz 22 . . 



Total , 189 3 



The horizon of the base of this section is 146 feet below the top of 

 the rock in the capitol well at Madison, thus covering the gap in the 

 Madison section. Combining the two sections, we obtain for the 

 whole series the following general succession : 



Feet. 



1. Alternations of layers of purely sihcious white sand, ferruginous brown sand, 



yellowish calcareo-arenaceous layers, and layers of greensand; the calcareous 

 bands increasing in amount of Hme and in number towards the top, as is also 

 the case with the greensand layers 165 



2. Entirely non-calcareous, white and yellow, sandstone; friable to indurated; fine 



to coarse-grained 602 



3. Red shale - • 10 



Total 777 



The calcareous layers have never been observed extending more 

 than 150 feet below the Mendota base. The " greensand " layers 

 mentioned are mixtures of green grains of a mineral probably closely 

 allied to the glauconite of the Cretaceous fortnation, rounded grains 

 of quartz, and usually more or less of angular pieces of calcite. These 

 layers are very characteristic of the lower sandstone, occurring, ac- 

 cording to Dr. Owen, at many different horizons throughout the series 

 as developed along the Mississippi. In Central Wisconsin, however, 

 none have been recognized more than 160 feet below the Mendota 

 base. JSTo chemical investigation of Central "Wisconsin greensand has 

 ever been made, but Dr. T. S. Hunt has given an analysis of a green- 



