THE LOWER SILURIAN ROCKS. 571 



Back " bluff, an isolated sandstone outlier, rises from the general level. From the sum- 

 mit of this bluff to the water in the river a measured section was taken, showivg in all 

 a thickness of 310 feet of the sandstone. The following are the details of the section, 

 beginning at the top of the bluff: 



Ft. In. Ft. 



1. Unexposed 20 .. .. 



2. Fine-grained, porous, friable, light-brownish sandstone (1431); com- 



posed of subangular grains of glassy quartz; showing numerous 

 small iron-stained cavities, and larger ones filled with loose ferrugin- 

 ous sand; fossiliferous, containing Scolithus, numerous small indefi- 

 nite trilobite fragments, and the pygidium of a large trilobite, ap- 

 parently Dicellocephalus Minnesotensis ; resembling exactly the fossil- 

 iferous rock on top of the bluff, and just below the greensand layer, 

 at Mauston; elevation of the Mauston rock, 470, of this rock, 530. . . 6 . . 



3. Unexposed 12 6 .. 



4. Yellowish sandstone (1432), resembling No. 2; in upper part with a 



vitrified crust; below, very loose; carrying Scolithus 2 



5. Unexposed 15 .. 



6 . Same as No. 4 3 . . 



7 . Unexposed 5 6 



8. Rather coarse-grained, dark-brownish, friable sandstone (1433); com- 



posed of much rolled grains of dulled quartz; thickly coated on exte- 

 rior by hydrous iron oxide; containing Scolithus, and numerous 

 iron-stained cavities; irregularly bedded 2 6 



9 . Unexposed to foot of steep ascent 27 



Total height of steep ascent 88 



1 Unexposed; on flat bench 150 paces wide 18 



11. Heavily-bedded, coarse- grained, friable, brown, ferruginous sand- 



rock, at top of vertical cliff 6 . 



12. Heavily-bedded, white^and-brown-ljanded, coarse sandstone; almost 



without coherence; having in places an exterior hardened crust 22 .. 



13. Alternating layers of pink, brown and white sandstone; medium to 



fine-grained, saccharoidal; thin pink layers stand out in knife edges 

 from the body of the rock; all affected by a vitrified crust composed 

 of glassy, closely adherent, quartz grains, on removing which the 

 rock within falls to loose sand; the crust is one-thirty-second to one- 

 half inch in thickness, and has an ill-defined inner edge 16 .. .. 



Total height of cliff 44 



14. Unexposed; a long slope one-half mile to top of the cliff at the river 



bank (Rood's Glen) 75 



15. Thin layers, one-eighth to two inches tluck, of light-colored, brownish- 



tinted, medium -grained, sugary, friable sandstone (1437, 1437^, 

 1438); composed of much rolled grains of dulled quartz; layers pro- 

 jecting in shelves 14 



16 . Heavy, coarse-grained, firm, ferruginous layer 1 . . 



17 . Heavy, projecting layer, with under surface ripple-marked, of medium- 



grained, light- brownish sandstone (1439); grains much rolled 1 7 .. 



18. Thin layers like No, 17 4 5 



19. Thin-bedded, coarse, sugary, very friable sandstone (1440); in alter- 



nate pink and brownish seams; cross-laminated; the transverse lam- 

 inae thin, warped, and abruptly terminated above and below 6 



20. Heavy layers, resembling No. 19; not cross-laminated, grains some- 



