THE LOWER SILURIAN ROCKS. 535 



Ft. In. 



12. Loose brown sand 11 



13. Unexposed 7 .. 



14. Porous and crystalline-textured, yellow limestone, with much coarse and 



bright green glauconite 1 



15. Unexposed 3 5 



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



17. Heavy brown layer of slightly calcareous sandstone 3 



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



with some crystalline calcite 10 



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



20. Greensand layer, like 18, false bedding very marked, cross-laminae very 



plain 13 



21. Porous, yeUow, slightly calcareous sandstone 6 



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



greensand 1 11 



23. Greensand, like 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 limpid 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 silicious white sand, ferruginous brown sand, 



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

 bands increasing in amount of lime 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 formation, 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. No chemical investigation of Central "Wisconsin greensand has 

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



