478 THE GEOLOGY OF MINNESOTA. 



[Andesyte. Porphyryte. 



magnetite and hematite. The rock has been considerably altered and the feldspars are 

 much reddened, the porphyritic ones, however, less than those of the groundmass. 



One section examined. 



Age. Cabotian. u. s. G. 



No. 662. ANDESYTE. 



S. W. y sec. 14, T. 63-3 E. 



Ref. Annual Report, x, page 70. 



Meg. A very dark gray rock with an aphanitic groundmass and a few gray to 

 reddish porphyritic plagioclases. 



Mic. The porphyritic plagioclases are not very abundant and are considerably 

 altered, in part to epidote and chlorite; the species was not determined. The 

 groundmass is composed of very minute, altered, elongated feldspars and a black, 

 opaque material usually in small rounded or irregular grains. Much of this material 

 is magnetite, some of it is hematite and part of it may represent an unindividualized 

 base. In places this black material is collected in irregular areas, almost or entirely 

 free from the minute feldspars, and this gives a blotched appearance to the section. 

 A little epidote also occurs in the groundmass. One area of chlorite was seen sur- 

 rounded by a rim of magnetite; this may represent an original ferro-magnesian por- 

 phyritic constituent such as hornblende. 



One section examined. 



Age. Cabotian or Manitou. u. s. G. 



No. 663. PORPHYRYTE. (Diabase.) 



"S. W. J sec. 19, T. 63-4 E., on McFarland's trail. Here is a short east and west ridge, rising about fifteen 

 feet, in which the rock ia the stellar porphyry seen in pebbles on the lake (Superior) shore. It is fine grained 

 and brown, but is beautifully set with intersecting tabular crystals of flesh-colored feldspar." 



Ref. Annual Report, x, page 70. 



Meg. A rock composed of numerous gray to flesh-colored porphyritic feldspars 

 and a very dark gray, almost aphanitic groundmass. 



Mic. Considerably altered porphyritic plagioclases are seen in an altered 

 groundmass composed largely of feldspars, many of which are lath-shaped in form. 

 Besides the feldspar there are magnetite, hematite, quartz and green alteration 

 products (mostly chlorite). There are also a few opaque grains, flesh-colored in 

 reflected light, which are perhaps the alteration products of ilmenite. Some of the 

 chlorite areas, by their relation to the lath-shaped feldspars, suggest altered ophitic 

 augites. There are also some areas of chlorite which may represent altered 

 olivines. 



One section examined. 



Age. Cabotian or Manitou. u. s. G. 



