THE ARCHAEAN ROCKS. 5I9 



E. 5 E.; and Sec. 2,'T. 11, R. 5 E> The two former are high, rooky 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 Ablemau's. Here a railway cutting passes through the point of a ridge, 

 D^ai the north bank of the Baxaboo river. At the west end of the cutting coarse wliite 

 sandstone, in horizontal ledges, Ues against a craggy cUfF of light-colored quartz-soliist 

 (l'-<i34), 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 rooks large 

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

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

 ting, the Hght-colored schist gives place to a gray or greenish clayey rock (1283). Som? 

 oi 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 

 quaa-tzite (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 tlie ridge has caused much weathering and alteration. There is evidently a high dip, 

 apparently to the north. 



The Maecellon Quaetz-Poephyey. 



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

 south half of the section, are two low rounded hiUs, 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-Hke masses by wide-open, earth-filled 

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

 the rook 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 browm'sh to blackish compact matrix. Two general varieties 

 occur, one presenting a light brownish color, showing a tendency to flake ofl' 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-sesqnioxide. The 

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

 more sUicious 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, haUeflinta). 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 ha,mmer. The surroundmg country is occupied by the Potsdam 

 sandstone, which is exposed at many points. 



The Obseevatoey Hill Quaetz-Poephyey. 



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

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

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

 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 plam N. .82° E. strike, 

 and 60° N. W. dip. These beddmg directions are the same as on the MarceUon out- 

 crop. 



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

 acatteied quite large (one-eighth to one-fourth mch m diameter) brownish to, pink facets 



