916 THE GEOLOGY OF MINNESOTA. 



[Quartz-porphyry. Greenstone. 



rock. In this slide also is a larger amount of calcite. Sphene also exists in grains 

 in the groundmass. One section. 



Age, Archean (Keewatin). 



Remark. According to the field description this was an igneous rock. N. H. w. 



No. 10H. QUARTZ-PORPHYRY. 



West side of point in sec. 5, T. 62-16, Vermilion lake. 

 Kef. Annual Report, xv, pages 283, 414. 



Meg. Homogeneous dike-rock cutting graywacke. 



Mic. Coarse, idiomorphic, but somewhat altered, feldspars, with some large 

 quartzes, are in a uniform, fine mosaic of feldspar and quartz. In this fine matrix 

 are many sericites, which frequently pierce the little quartzes-. There are also areas 

 of calcite. One section. 



Age. Archean (Keewatin). 



Remark. The date of the matrix of the large crystals is later than the sericites. 

 The sericites have resulted from an alteration of feldspathic material. They are 

 also distributed through the feldspar crystals, but not so abundantly. The rock 

 shows traces of hornblendic material; therefore, notwithstanding the striking contrast 

 between the crystals (phenocrysts they might be called in an igneous rock), and the 

 fine, uniform grain of the interlocking matrix, it is possible, if not probable, that 

 this rock has resulted from a debris of acid porphyritic rock. Further, the fact that 

 the large feldspars are striated and much twinned, evidently like those of the ester- 

 ellyte of Kekequabic lake, shows that, if from a granite as a debris, the granite was 

 not of the normal type, and that it supplied large, perfect crystals at the same time 

 with a very fine debris, with no grains (or fragments) of intermediate sizes. For 

 these reasons it is quite reasonable to consider this rock as an intrusive, in the 

 manner of the esterellyte of Kekequabic lake, and originating in the same way from 

 some of the neighboring elastics of the Keewatin. N. H. w. 



No. 11H. GREENSTONE. (Altered. ) 



8. E. % sec. 5,T. 62-16, Vermilion lake. 

 Ref. Annual Report, xv, pages 283, 414. 



Meg. From a dike running through graywacke; tough, grayish green, horn- 

 blendic. 



Mic. Roundish, interlocking grains of quartz and of a glassy feldspar (oligoclase 

 or oligoclase-albite) are visible throughout this slide, but they are liberally mingled 

 with hornblende, sericite, calcite and epidote. The minerals are largely or wholly 

 secondary, the hornblendes being the largest. There is no radial or other structure 

 indicating an original igneous origin. The feldspars and quartzes are later in origin 

 than the hornblendes and sericites. One section. 



