THE ARCH^AN ROCKS. 521 



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

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

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

 of Berlm, 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 prevaHing rock (761, 1,426), noticed, haa a black, com- 

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

 for the most part, very promment, and are frequently much contorted, the whole rook 

 having a verj- evident parallel grain. The felspar crystals ai-e jninute 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 dii-ection, as observed on the MarceUon, Observatory Hill, and MoundviUe 

 outcrops. 



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

 ing a peculiar flne-grannlar, 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 tlie 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 Barahoo quartzite has a dark brown to black matrix, nnmerons. 

 large, pink, felspar crystals, and 71.2i per cent, of silica. The Mar- 

 ceUon 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 Moundvillo 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 niatrix, 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,. 

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

 porphyry resembles that from the Mai-quette 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 Montj:llo Geanite. 



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

 county, is am elUptical-shaped rounded mound of pink granite, about a third of a mile 



