576 GEOLOGY OF CENTRAL WISCONSIN. 



Nine miles east from Roche a Cris and Friendship Mound, on the N. E. qr. of Sec. 3, 

 T. 17, R. 7 E., is the outlier known as Pilot Knob. This is a narrow jagged crest, 75 

 feet long, 10 to 15 feet wide, and 80 feet high, resting upon a hill with gentler slopes, 

 and about 400 paces in diameter. A section from above downwards is as follows : 



Feet. 



1. Medium to fine-grained, brick-red (1420, 770); composed of rolled grains of 



quartz, coated externally with red and brown iron oxides; containing some 

 hard curving seams, % to ^ inch in thickness, of a dark brown color, made 

 up of glassy quartz grains cemented by much brown iron-oxide, and evi- 

 dently of a concretionary nature; including some lighter colored brown and 

 even white layers, the latter (1419) porous, friable, medium- grained, and 

 weathering with a very hard vitrified crust; containing near the base about 

 a foot of light reddish, veiy friable, fine-grained, fossiliferous rock, con- 

 taining Ptychaspis Miniscaensis and other trilobite impressions 45 



2. White, friable, non-fossiliferous sandstone, to foot of crag 40 



3. Unexposed, on gradual slope -. 30 



4. Fine-grained, non-friable, yellowish sandstone (1421), consisting of fine angu- 



lar quartz, and containing a few scales of mica; thin-bedded and marked 

 by fine lines of lamination, parallel to which it splits with some readiness; 

 fossiliferous, containing Ptychaspis, Conocephalites and disks of crinoidal 

 columns; exactly resembling the fossil rock at the summit of the last sec- 

 tion given, to which horizon it undoubtedly belongs 1 



5. Unexposed, on steep slope , 10 



6. White-and-brown-banded, thin friable layers 20 



7. Unexposed to base 20 



Height of Knob 166 



The two fossil horizons of the above section appear to be the same as reco^ized on 

 Roche a Cris, and the bluffs south of Friendship, though apparently somewhat nearer 

 together. The base of Pilot Knob is 545 feet, the lower fossil horizon 595 feet, the up- 

 per fossil horizon 665 feet, and the summit 705 feet above Lake Michigan. These 

 figures indicate a slight rise, about 4 feet to the mile, of the strata between Roche a 

 Cris and Pilot Knob. It is possible that this rise may be exaggerated by unreliable 

 barometrical observations; there is, however, certainly no rise westward between these 

 points. 



A mile and a half southeastward from Pilot Knob, the intervening ground being low, 

 on the N. W. qr. Sec. 12, T. 17, R. 7 E., is a long ridge facing northwestward, with 

 rock outcrops on the flanks. The highest outcrop seen is some 20 feet below the top of 

 the ridge, and about 25 feet lower than the summit of Pilot Knob. From this point 

 downwards for 40 feet are seen layers of incoherent white sandstone, with intercalated 

 yellowish calcareous bands, 2 to 6 inches in thickness, and 5 to 15 feet apart. The 

 rock of these bands (1405, 1406, 1407) is rough- textured, porous and moderately firm, 

 but crumbling under the hammer. It has the appearance of being coarsegrained, but 

 on close inspection most of the apparent large grains are seen to be due to the aggrega- 

 tion of smaller ones, and the rock is seen to consist of an admixture of fine, yellowish, 

 angular grains, and larger ones of white and much-rolled quartz, with sparsely scattered 

 greensand grains. On solution in acid, the yellowish matter is entirely dissolved, leav- 

 ing a residue of not over 40 per cent., which is made up entirely of the white quartz 

 grams. Cleavable calcite is occasionally to be seen by the naked eye, and from Iho 

 ready effervescence and solution in cold acid, it is judged that the rock is much more 

 largely calcareous than dolomitic. These layers are, beyond doubt, those that are to Lo 

 seen in more southern counties immediately underlying the Mendota limestone, which 



