876 THE GEOLOGY OP MINNESOTA. 



[Porphyrel. Kersantyte. 



MCIJ. Gray or greenish-gray, granular, clastic; the matrix of a coarse con- 

 glomerate. 



Mic. For such a rock this contains comparatively little quartz, many of the 

 few translucent grains being biaxial and hence probably feldspar. None of the 

 grains are friction-rounded. Other feldspars, which are now micro-granulitized, 

 must have been much longer subjected to disintegration, and are rounded, but these 

 are few. Some are plainly striated in the manner of the soda-lime feldspars. The 

 most of the rock is of the darker minerals, hornblende and leucoxene predominating, 

 while chloritic substance gives darkness to the slide between crossed nicols. One 

 section. 



Aye. Upper Keewatin (Ogishke conglomerate). 



Remark. The fragmental fresh feldspars are apparently only a finer dissemi- 

 nation of the clastic ("porphyritic") feldspars seen in No. 2260. N. H. w. 



No. 2260. PORPHYUEL. 



Prom the same rock as No. 2209, but from the northern part of the ridge. 

 Ref. Annual Report, xxiv, page 79. Compare the porphyrel of Zeta lake. 



Meg. The specimen collected has a porphyritic aspect, due to the dissemination 

 of white feldspars, the matrix being grayish green. 



Mir. This rock, in its general composition and structure, is quite similar to 

 No. 2259, but it contains larger feldspars. These are conspicuously banded on the 

 pericline and albite plans of twinning, resembling those of the porphyry of Keke- 

 quabic lake, and of the porphyrel of Zeta lake. This rock also embraces notable 

 amounts of calcite. One section. 



Age. Upper Keewatin (Ogishke conglomerate). N. H. w. 



No. 2261. KERSANTYTE. (Dyke.) 



4 



Intrusive in No. 2260. 



Ref. Annual Report, xxiv, page 80. (Compare No. 2178.) 



Meg. Greenstone, porphyritic with mica, the groundmass having a flesh- 

 color tint. 



Mic. The porphyritic scales are biotite. About one-half of the area of the 

 section is occupied by a very low refractive mineral which is probably apophyl/id; 

 but that determination is not certain. It but rarely transmits sufficient light, 

 between crossed nicols, to become visible. The rock contains much calcite, as 

 well as innumerable small flakes of mica. One section. 



Age. Dike cutting No. 2260. 



Remark. This rock is unquestionably identical with rock No. 2178, which 

 shows evident apophyllite. Compare, also, No. 2258. N. H. w. 



