HO THE GEOLOGY OP MINNESOTA. 



[Basalt. 



None of the minerals, except some grains of epidote, show any crystal outlines. 

 The magnetite and chlorite are in some places more abundant than in others, thus 

 giving an indistinct mottled appearance to the slide. The calcite sometimes spreads 

 widely with the same orientation, embracing numerous grains of the quartz and of the 

 magnetite. 



One section. 



Age. Animikie. 



Remark. It is impossible to state definitely what was the origin of this rock. 

 It is thought to be a slate, or very fine graywacke. The groundmass of crushed and 

 stretched quartz porphyries frequently resembles this section, but in this case there 

 is no evidence of the remains of old phenocrysts, nor of dynamic action in the rock, 

 nor in the associated strata. The overlying rock No. 44 ', occupies the position of a 

 sill similar to those in the Animikie along the boundary. u. s. G. AND N. H. w. 



No. 46. BASALT. (Zirkelyte.) 



Duluth. Brewery creek. 



Ref. Annual Report, ix, page 180. Annual Report, xiii, pages 100 (No. 151), 102. Annual Report, xi, page 39. 



Meg. A black aphanitic rock thickly strewn with porphyritic crystals of dark 

 red feldspar. Epidote has pervaded the rock to some extent, sometimes partly 

 replacing the phenocrysts and sometimes occurring in small areas in the ground- 

 mass. On one corner of the hand specimen is an area of finely crystallized quartz 

 about one half an inch in diameter. This is surrounded by a narrow green rim 

 which contains some epidote, and outside of this is a wider rim of epidote which is 

 not very sharply separated from the mass of the rock. 



Mic. The porphyritic feldspars, which are such a characteristic feature of the 

 hand specimen, and which constitute nearly one-half the rock mass, are imbedded in 

 a groundmass which is composed of minute plagioclase laths and a black opaque 

 substance appearing like glass. The feldspar of the groundmass and also of the 

 porphyritic crystals is highly altered and reddened, and its exact composition cannot 

 be determined; but, judging from the analysis of the whole rock given below, both of 

 these feldspars are quite basic plagioclases. In some cases they still show the 

 remains of polysynthetic twinning, although usually altered too highly to show this. 



The black glass-like material on examination with a high power is seen to be 

 composed almost entirely of black specks (magnetite) and chlorite. It seems most 

 probable that this substance was originally glass. 



Epidote and chlorite have both pervaded the rock in irregular patches. 



One section shows part o'f an area similar to the quartz area described on the 

 hand specimen. This is made up of epidote, quartz, chlorite, calcite and magnetite. 

 In one part of the area the quartz, calcite and magnetite are arranged concentrically. 



Two sections. 



