904 THE GEOLOGY OP MINNESOTA. 



[Muscovadytr. Porphyry. 



No. 740G. MUSCOVAD YTE. ( Matrix of altered conglomerate. ) 



N. W. J N. E. % sec. 12, T. 64-7, south of Kekequabic lake, in the vicinity of the gabbro border, No. 741G. 

 Ref. Annual Report, xxi, page 59 ; Annual Report, xxiv, page 88. 



Meg. Gray, firm, showing in the field a few pebbly forms. 



Mic. This is a rather uniform muscovadyte. The minerals tend to have a 

 roundish form, and embrace, poikilitically, many small, round grains, each of the others. 

 Feldspar, occasionally albite-twinned, is the most abundant. Augite and biotite come 

 next in amount, the former sometimes surrounding small globular feldspars and 

 sometimes piercing them, and frequently containing small granules of augite, and 

 most frequently having the form of isolated roundish grains scattered promiscuously 

 and apparently of about the same date of generation as the feldspars. Biotite 

 acts poikilitically toward the other minerals. A little pleochroic hornblende serves 

 the same purpose, but is usually in larger crystals. Quartz is not abundant, but 

 some large secondary grains are distinctly composed of quartz. A little fine magne- 

 tite is scattered in the feldspars, in the hornblende, the augite and the biotite. One 

 section. 



Age. Upper Keewatin. N. H. w. 



No. 766G. PORPHYRY. 



Kekequabic lake, E. J^ S. W. % sec. 34, T. 65-7, point on the north side of the lake. 

 Ref. Annual Report, xxi, page 60. 



Meg. Conspicuously porphyritic with a white feldspar in a fine matrix ; contains 

 considerable pyrite. The specimen shows the intrusive contact of the porphyry on a 

 black slate, the former not showing, however, a finer grain at the exact contact, some 

 of the large feldspars being jn absolute contact on the slate. 



Mic. The feldspars, which are twinned, both by pericline and by albite, the 

 former sometimes coarsely, are much altered by the usual microscopic growths, such as 

 sericite, calcite and perhaps epidote, and sometimes appear to be distorted or crushed, 

 evinced by the interruption of the albite twins. There are several conspicuous 

 original quartzes and much quartz that is finer and evidently of later origin. The 

 later quartz forms a coarsely micro-gran ulitic interlocking matrix, and is mingled 

 with some feldspar grains of about the same size, and also with much calcite. The 

 earlier quartzes are somewhat fissured or granulated, and occasionally give an undu- 

 latory extinction. The hornblendes, which are not common nor conspicuous, are 

 fragmentary and feebly pleochroic, usually much chloritized indeed, it cannot be 

 affirmed safely that a trace of hornblende remains in the slide examined. One 

 section. 



Age. Intrusive in the Upper Keewatin. 



* 



Remark. It is evident that since the formation of this rock it has undergone a 

 rough and precarious history. It is also evident that the phenocrysts did not form 



