THE LOWER SILURIAN ROCKS. 577 



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

 these 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 mile 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. 8 E., Marquette county, is a large isolated bluff' capped by the Lower Magnesian 

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

 town. The first named, known as "Glover and Merriman's lime bluff." shows the fol- 

 lowing section : 



Feet. 



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



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

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

 marked with fine pencilings 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 containing no sand 

 whatever; containing little 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 hill 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 intercalated 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 bluff. 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 limy layers at the former place. It has been said that Friendship Mound and the 

 Roche a 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 its horizon. This might be explained readily enough by supposing a 

 continuation westward of the somewhat rapid rise of the strata that is indicated in 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 eastward one. 



The occurrence of two limestone outliers 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. SUR. 37 



