THE ARCILEAN ROCKS. 519 



R. 5 E.; and Sec. 2,'T. 11, R. 5 E. The two former are high, rocky points, the latter 

 a low outcrop on the river side. Still another occurs on the S. E. qr. of Sec. 33, T. 12, 

 R. 5 E., near Ableman's. Here a railway cutting passes through the point of a ridge, 

 n^ar the north bank of the Baraboo river. At tha west end of the cutting coarse white 

 sandstone, in horizontal ledges, lies against a craggy cliff of light-colored quartz- schist 

 (1284), resembling that at the south side of the section at the Lower Narrows of the 

 Baraboo (see Fig. 30), but less regularly slaty. At the junction of the two rocks large 

 bowlders of quartzite are included in the sandstone, which itself fills in the cracks be- 

 tween the layers of schist. One hundred and thirty feet from the west end of the cut- 

 ting, the light-colored schist gives place to a gray or greenish clayey rock (1283). Some 

 01 the layers are bright green in color, and marked with very fine lines of lamination. 

 These layers are apparently quite silicious. Seventy feet further, pinkish granular 

 quartzite (1282) is indefinitely exposed. The exposures throughout the cutting, though 

 in places 40 feet high, are very much jointed and confused. The position near the end 

 of the ridge has caused much weathering and alteration. There is evidently a high dip, 

 apparently to the north. 



THE MARCELLON QUAKTZ-PORPHYKY. 



On Sec. 7, in the town of Marcellon, Columbia county, on each side of the road in the 

 south half of the section, are two low rounded hills, 40 to 60 feet in height, of quartz- 

 porphyry (759). The rock exposures are large, and are much rounded and weather- 

 worn, being separated into numerous bowlder-like masses by wide-open, earth-filled 

 joints. The weathered surfaces have a prevailing pinkish tinge, giving the idea that 

 the rock is largely composed of pink felspar. On obtaining a fresh fracture, however, 

 only a very few, sparsely scattered, minute felspar faces are to be seen, the mass of the 

 rock being composed of a brownish to blackish compact matrix. Two general varieties 

 occur, one presenting a light brownish color, showing a tendency to flake off in frag- 

 ments that are translucent on the edges, and containing no distinguishable felspar 

 crystals, the other having a dark-gray to black matrix, in which are to be seen a few 

 distinct crystals of felspar and numerous copper-colored points of iron-sesquioxide. The 

 rock has nearly the hardness of quartz, and fuses only with the greatest difficulty. A 

 more silicious character as compared with other quartz-porphyries of the state is thus in- 

 dicated, and the indication is borne out by the content of silica 76.98 per cent. as 

 shown by analysis. We have evidently, in this case, a porphyry which, in its large 

 content of silica, and in the sparseness of its felspar crystals, approaches the true felsites 

 (petrosilex, halleflinta). Quite a distinct and uniform set of bedding joints occurs, the 

 strike being N. 32 E., the dip 65 to 75 N. W. Numerous cross-joints traverse the 

 rock, and, on weathered portions, cause it to fly into smooth-faced, angular fragments, 

 at the least blow of the hammer. The surrounding country is occupied by the Potsdam 

 sandstone, which is exposed at many points. 



THE OBSERVATORY HILL QUARTZ-PORPHYRY. 



Six miles north of the Marcellon outcrop, in the S. E. qr. of Sec. 7, in the town of 

 Buffalo, Marquette county, a knob of quartz-porphyry rises 250 feet above the general 

 level, and 490 feet above Lake Michigan. On the flanks of the hill and up to a vertical 

 distance above the base of 125 feet, are horizontal sandstone ledges. Above, to the 

 top, are nearly continuous outcrops of porphyry, with a not very plain N. 32 E. strike, 

 and 60 N. W. dip. These bedding directions are the same as on the Marcellon out- 

 crop. 



The porphyry (762) has a dark-grayish to black compact matrix, in which are thickly 

 scattered quite large (one-eighth to one-fourth inch in diameter) brownish to pink facet*. 



