PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 475 



Andesyte. Apotrachyte.] 



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No. 657- ANDESYTE (with quartz). 



South line of sec. 16, T. 63-2 E., just east of a creek. 

 Sef. Annual Report, x, pages 69, 70. 



Meg. A fine-grained, dark-brown rock, reddish brown on weathered surfaces, 

 containing a few small scattered porphyritic plagioclases. 



.V/V. The section consists of a few much altered porphyritic feldspars in a 

 groundmass composed of a more or less confused aggregate of feldspar, quartz, epidote, 

 magnetite, hematite, indistinct green alteration products, and a few apatite needles. 

 Two of the porphyritic crystals are not particularly reddened, but are partly altered 

 to a gray cloudy material and to epidote; these two feldspars are evidently plagio- 

 clases, but the species could not be determined. 



The feldspar of the groundmass occurs in the form of small more or less lath- 

 shaped crystals and in grains of irregular outline; these two forms, however, are not 

 sharply separated from each other. In reflected light, the lath-shaped feldspars in 

 general are seen to have altered to a gray cloudy material, while the rest of the 

 feldspar is somewhat reddened. These laths are quite commonly polysynthetically 

 twinned, and extinction angles in sections approximately normal to 010 do not 

 indicate a plagioclase more basic than andesine. Much of the other feldspar shows 

 no polysynthetic twinning and is perhaps orthoclase, but owing to the alteration of 

 both forms of the feldspar, and also to the small size of the grains, satisfactory deter- 

 minations of the species cannot be made. 



The green alteration product and the epidote of the groundmass gave no certain 

 evidence as to the original material now represented by them, but it seems probable 

 that this was a mineral (either augite or hornblende) rather than a glassy residue. 

 The green alteration product is one of the chlorites, and from its double refraction 

 is thought to be most probably clinochlore. 



One section examined. 



Age. Cabotian or Manitou. 



Remark. In this, as in many of the altered post-Animikie rocks of the north 

 shore of lake Superior, it is quite probable that the quartz of the groundmass is 

 secondary. If this be the case, the original rock was an andesyte. u. s. G. 



No. 658. APOTRACHYTE. ( Spheruiit-ic. ) 



Near, probably a short distance east of, No. 657. 

 Ref. Annual Report, x, pages 69, 70. 



Meg. A fine-grained rock, porous in texture and of a reddish-gray color. A 

 few small red porphyritic feldspars occur. The groundmass is indistinctly mottled 

 with small reddish and greenish-gray areas. This rock resembles macroscopically 

 Nos. 655 and 656. 



