THE ARCH^AN ROCKS. 521 



originally regarded as within the Central Wisconsin district, of which, however, by sub- 

 sequent agreement, the Fox river was made the southern boundary. They will, there- 

 fore, be described by Prof. Chamberlin, in whose district is also the outcrop at the city 

 of Berlin, Green Lake county. As the writer has examined both localities carefully, he 

 may be permitted to allude to the nature of the rock of each, for the sake of comparison 

 In the Marquette outcrops, the prevailing rock (761, 1,426), noticed, has a black, com- 

 pact, flinty matrix, which is streaked with white non-continuous lines. These lines are, 

 for the most part, very prominent, and are frequently much contorted, the whole rock 

 having a very evident parallel grain. The felapar crystals are minute and sparse. The 

 silica content (1,426), is 70.29 per cent, less than obtained from any other of the Wis- 

 consin porphyries. The general course of the contorted laminse points to the same 

 N. E. strike direction, as observed on the Marcellon, Observatory Hill, and Moundville 

 outcrops. 



The Berlin rock has a dark bluish-gray matrix, much streaked with white, and hav- 

 ing a peculiar fine-granular, quartz-like texture, as seen under the lens. The felspar 

 crystals are small, greyish to brownish, and rather numerous. The lamination is very 

 fine and distinct, and often contorted, and the silica content 74.37 per cent. 



A comparison of the rocks of the several porphyry areas shows 

 that though all present the same general kind of rock, no two of the 

 areas are exactly alike in this regard. The porphyry associated with 

 the Baraboo quartzite has a dark brown to black matrix, numerous 

 large, pink, felspar crystals, and 71.24 per cent, of silica. The Mar- 

 cellon porphyry has a brown to black matrix, is almost without felspar 

 facets, and contains 77 per cent, of silica. The Observatory Hill por- 

 phyry has a black, flinty matrix with numerous large, brownish felspar 

 facets, and contains 73.56 per cent, of silica. The Moundville porphyry 

 resembles the Marcellon rock in the color and appearance of the matrix, 

 but contains much brownish felspar, some magnetite, and only 72.76 

 per cent, of silica. The Seneca porphyry is altogether different from 

 the others, in having a light colored, nearly white, somewhat granular, 

 and distinctly quartzose matrix, and in containing much white, glassy 

 felspar, the percentage of silica being 76.39. The Marquette porphyry 

 has a black, flinty matrix, in this regard resembling closely the Obser- 

 vatory Hill rock, from which, however, it differs in being almost with- 

 out felspar facets and in having its matrix streaked with white, and 

 thus presenting a very marked lamination, the silica content being 70.29, 

 or less than that obtained from any other of these rocks. The Berlin 

 porphyry resembles that from the Marquette outcrop in having a 

 marked lamination, but differs in the color of its matrix, in contain- 

 ing plentiful felspar facets, and in having a larger percentage (74.37) 

 of silica. Its peculiar fine granular matrix is also a very distinguish- 

 ing characteristic. 



THE MONTELLO GEANITE. 



In the village of Montello, on the west side of Sec. 9, T. 15, R. 10 E., Marquette 

 county, is an elliptical- shaped rounded mound of pink granite, about a third of a mile 



