520 GEOLOGY OF CENTRAL WISCONSIN. 



r,f felspar, the whole presenting a veiy dark- colored appearance. The silica content is 

 73.56 per cent., and the specific gravity, 2.60. Numerous close joints occur throughout 

 the exposure, causing the rock to split like that of the Marcellon outcrop into small, ir- 

 regularly shaped, smooth-faced, angular fragments. 



The surrounding country shows everywhere the Potsdam sandstone as the surface 

 rock. A high bluff of this sandstone, some 100 feet lower than the top of the Observa- 

 tory, lies on the S. W. qr. of the same section. 



THE MOUND VILLE QUARTZ-POBPHYRY. 



On the edge of the Fox river marsh at the head of Lake Buffalo, on the line between 

 sections 8 and 5, T. 14, R. 9 E., Moundville, Marquette county, are -three low rounded 

 outcrops of quartz-porphyry. These are five miles, in a direction 10 N. of W., from 

 Observatory Hill, which is the nearest Archaean outcrop. No other rock shows in the 

 neighborhood, the country being heavily drift-covered. The largest outcrop is on the 

 east end of a low bluff 35 feet high, and several hundred feet in length. There are 

 quite marked appearances here of the same N. E. strike, and N. 60 dip, as seen at 

 Observatory Hill and in Marcellon. The rock (1430) has a dark brown matrix, re- 

 sembling in this regard the Marcellon porphyry, from which it differs, however, in show- 

 ing throughout traces of crystalline structure, and quite thickly scattered, large, brown- 

 ish felspar surfaces. A few crystals are white and translucent. The weathered sur- 

 face is often of a bnght pink color. Mr. Wright's microscopic examination (Appendix) 

 shows that fine magnetite particles are abundant. Their existence is not rendered evi- 

 dent even by the us"e of the ordinary lens. The silica content is 72.76 per cent. 



THE SENECA (PiNE BLUFF) QUAKTZ-PORPHYEY. 



A rounded elliptical knob of quartz-porphyry, 100 feet high, %ih mile long, and >^th 

 mile wide, lies on the north side of the White river marsh, in Sec. 2,. T. 17, R. 11 E., 

 Seneca, Green Lake county. The greatest extension of the hill is in an east and west 

 direction. It is largely rocky, but there are no abrupt rock ledges, the exposures being 

 almost entirely surfaces conforming to the general contour of the hill, and on a level 

 with the surrounding sod. In places, the slopes of the hill are covered with angular 

 fragments, apparently split off by frost. This is a peculiarity not noticed on any. of the 

 other porphyry outcrops, ar,d appears to be due to the large content of comparatively 

 coarse, cleavable felspar. The hill is only about two miles south from the granite hills 

 of Spring Lake, in T. 18, R. 11 E., Waushara county. The surrounding country is 

 marshy and drift-covered, and shows no outcrop of horizontal rocks. The loose frag- 

 ments are many of them smoothed on one side, and some surfaces are most beautifully 

 striated. Owing to the broken condition of the outcrop, no definite bedding planes were 

 made out, though weathered specimens brought away show distinct traces of lamination. 



This porphyry in its least weathered portions (1410) shows a light-gray to whitish fine- 

 grained matrix, made up largely of fine glassy felspar 'Crystals, and containing numer- 

 ous large surfaces of the same kind. The more weathered specimens (1412) have a pink 

 to white, quite distinctly granular matrix, in which with the lens can be seen what ap- 

 pear to be angular grains of quartz. The glassy felspar crystals are also abundant. 

 The weathered surface is brownish, with a kaolinized undercrust. Nearly all of the rock 

 shows signs of weathering. The silica content is 76.39 per cent. 



THE MAEQUETTE AND BERLIN QUAETZ-PORPHYEIES. 



The large outcrops of quartz-porphyry on sections 34 and 35, T. 15, R. 11 E., and sec- 

 tions 2 and 3, T. 14, R. 11 E., near the village of Marquette, Green Lake county, were 



