THE LOWER SILURIAN ROCKS. 57J 



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 mea.sured section was taken, showivg in all 

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

 beginning at the top of the bluff:— 



Ft. In. Ft. 



1 . Unexposed 20 



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



posed of subangular grains of glassy quartz; showing numerous 

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

 ous sand; fossihferous, containing ScoUthus, numerous small indefi- 

 nite trilobite fragments, and the pygidium of a large tiilobite, ?i^- 

 Tpaxentlj Dicellocephalus Minnesotemis ; resembling exactly the fossil- 

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

 at Mauston; elevation of the Mauston rock, 47(>, 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 ScoKrtws 2 .. .. 



5 . Unexposed 1,5 



6 . Same as No. 4 3 



7 . Unexposed 5 g 



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



posed of much roUed grains of dulled quartz; tliickly coated on exte- 

 rior by hydrous iron oxide; containing ScoUthus, and numerous 

 iron-stained cavities; irregularly bedded , 2 6 . .' 



9. Unexposed to foot of steep ascent '. 27 .. .. 



Total height of steep ascent 88 



10 Unexposed; on flat bench 150 paces -svide 18 



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



roek, at top of vertical cliff 6 . . . . 



12. Heavily-bedded, white^and-brovm-laanded, 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; aU 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 thiclaiess, and has an ill-defined inner edge 16 . . . . 



Total height of chflf ■ 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 thick, of light-colored, brownish- 



tinted, medium -grained, sugary, friable sandstone (1437, 14373^, 

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

 jecting in shelves 14 . . . . 



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



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



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



18. Thin layers lilce 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- 



