PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 793 



Diabase. Gabbro. Quartz-porphyry.) 



double-pointed needles, which are visible in high power as tiny scales by reason of 

 their brightness when not parallel with either nicol. They cannot be determined 

 specifically, but if they be not impurities in the balsam, they may be some form of 

 mica. The feldspar is reddened by hematite, but is in part a triclinic form. Sphene, 

 hornblende, biotite and pennine are accessory. One section. 



Age. Wauswaugoning quartzyte, of the Animikie. N. H. w. 



No. 1828. DIABASE. 



Concerned in the metamorphism of No. 1827; same place. 

 Ref. Annual Report, xxii, page 8. 



Meg. Dark, basic, intrusive. 



Mic. Augite is ophitic in its relation to the feldspars. The same is true of the 

 olivine, which is abundant. This rare feature has been noted several times before 

 (compare Nos. 258, 512, 560, 1275, 1829, and others). The olivine, however, is some- 

 what altered to antiyorite, while magnetite, quartz, biotite are more or less accessory. 

 One section. 



Age. Cabotian. N. H. w. 



No. 1829. GABBRO. 



"Average rock of the top of mount Josephine, north shore of lake Superior, near the west side of sec. 2, 

 T. 63-6 E." 



Ref. Annual Report, xxii, page 8. 



Meg. A rather fine-grained, gray gabbro, composed of feldspar, augite and 

 magnetite, with perhaps olivine. 



Mic. This rock resembles No. 1828 in having both augite and olivine, ophitic in 

 relation to the feldspars, but the slide contains no quartz. The augite occasionally 

 shows indications of being later than the olivine, which, having n p in the acute 

 optic angle has the diagnostic character which distinguishes fayalite from ordinary 

 olivine. Round the borders of the magnetite is occasionally a brown biotite, and 

 some of the feldspar is penetrated along its planes of albite twinning by a micro- 

 perthitic alteration of another mineral having rather high double refraction and a 

 refractive index higher than the original feldspar, whose nature is undetermined. 

 The relation of the olivine and feldspar is shown by figure 1, plate V. One 

 section. 



Age. Cabotian. N. H. w. 



No. 1830. QUARTZ-PORPHYRY. 



"'Redrock,' kind of quartz-porphyry, from a hill northwestward from mount Josephine, where it forms an 

 irregular patch elongated about east and west ; visible on the southern slope of the gabbro range. 1 ' 

 Ref. Annual Report, xxii, page 8. 



Meg. A gray rock, with a fine-grained, granitic groundmass and porphyritic 

 crystals of quartz and pink feldspar. Intimately associated with the quartz-porphyry 



