THE LOWER SIX.URIAN ROCKS. 577 



possibly exists as a capping oji this lull. From the figures given, it will be seen that 

 tliese layers extend as much as 60 feet lower than the summit of Pilot Knob, which, 

 moreover, is of an entirely different kind of rock. In order that the Pilot Knob layers 

 may pass beneath those across the valley, they must have a descent of at least 50 feet 

 to the mUe in that direction, an amount of descent that would be altogether extraor- 

 dinary in Central Wisconsin. 



One mile eastward from the exposures just described, on the N. W. qr. of Sec. 7, T. 

 17, R. g E., Marquette county, is a large isolated bluft' capped by the Lower Magnesian 

 limestone. Another similar Ijluff lies a mile northeast of this on Sec. 4 of the same 

 tawa. The first named, Imown as "Glover and Merriman's Ume bluff," shows the fol- 

 lowing section : 



Feet. 



1. Lower Magnesian limestone (1409): dose-textured, very finely crystalline, yel- 



lowish-gray to nearly white; holding small cavities lined with brown- tinted 

 dolomite crystals, and others lined with stalactitic hme carbonate; in places 

 marked with fine pencUings of the dendritic oxide of manganese; dolomite — 

 dissolving only in heated acid, with a residue of very fine, white, angular 

 silica, constituting 13 . 96 per cent, of the whole — but containmg no sand 

 whatever; containing Kttle greenish blotches and streaks; weathering with. 

 rough surface; occurring in layers 3 to 4 feet thick, some of which are much 

 displaced 30 



2. Unexposed 10 



3. Madison sandstone: coarse, whitish, moderately firm; forming a prominent 



ledge on the side of the bluff 10 



4. Unexposed 40 



The following occurs on an outlying hiU south of the main bluff: 



5. Mendota limestone: lowest layers only seen; brownish and yellowish. 10 



6. Unexposed 15 



7. Potsdam sandstone: white, crumbling sandstone, with mtercalated yellowish, 



coarse-textured, calcareous layers (773, 1408), exactly like those seen at 



the last locality described 10 



8. Unexposed to base of bluff. 35 



Total height of Isluff. 160 



The base of the bluff is 570 feet, and its summit 730 feet, above Lake Michigan. 

 These figures indicate some descent from the locality on Sec. 12, T. 17, R. 7 E., but ex- 

 actly how much is nOt ascertainable, from the somewhat indefinite position in the series 

 of the hmy layers at the former place. It has been said that Friendship Moimd and tlie 

 'Eoche ^ Cris rise respectively to altitudes of 750 and 665 feet above Lake Michigan. 

 •Neither, however, shows any sign of limestone at top, or any indication of reaching 

 within 100 feet of it« horizon. This might be explained readily enough by supposing a 

 continua.tion westward of the somewhat rapid rise of the strata that is indicated iu the 

 vicinity of the Lime Bluffs. It has been shown, however, that the strata of Pilot Knob 

 'indicate not only no westward rise, but even a slight eastimrd one. 



The occurrence of two limestone outUers as much as 25 miles from the nearest points 

 of the area occupied by the Lower Magnesian is interesting, and of considerable economic 

 importance. That this formation once extended as far north as this is thus rendered 

 certain. Having reached the highest land in the region, it may possibly have had also 

 a still wider spread northward. 

 Wis. Stir. — 37 



