52S THE GEOLOGY OF MINNESOTA. 



[Porphyryte. 



condition, would be called by most lithologists a hornblende andesyte. Plate X, 

 figures 1 and 2, show the general characters of this rock with its porphyritic structure. 

 The hornblende grains on the left in figure 1 have been faulted by the action of the 

 magma and partially corroded. This is shown by the cleavage lines and in the upper 

 form by a twinned band seen in polarized light. In figure 2 the corrosion of the 

 hornblende by the magma is shown, while the line of magnetite grains in the upper 

 hornblende shows the original outline. The lower hornblende has an interior core of 

 the groundmass. This is not owing to the inclusion of the groundmass during the 

 crystallization of the hornblende, but is due to the interpenetration of one of the 

 gnawing, dissolving tongues of groundmass that had penetrated the hornblende, and 

 which has been cut off during the grinding of the section, leaving it as an apparent 



* 



inclusion." 



One section examined. 



Age. Keewatin. 



Remark. We have been unable to find any porphyryte in place on Mallmann's 

 peak, although search was made for it. This specimen (No. 751) was brought to 

 Prof. N. H. Winchell in 1879, by one of his guides, and the former did not visit this 

 place personally. It is, however, quite possible that areas of this porphyryte may be 

 found in the vicinity of Mallmann's peak, as the district has not been examined 

 carefully. Still, this specimen is exactly similar to the porphyryte found near the 

 southwest end of Epsilon lake, and it seems very probable that this came from that 

 locality. This idea is strengthened by the fact that at the Epsilon lake locality a 

 large diabase dike, similar to the next specimen (No. 752), is seen cutting the 

 porphyryte. 



The porphyryte is known definitely from only one locality in this vicinity, and 

 is confined to a belt of not more than a quarter of a mile in width which curves 

 around the southwestern end of Epsilon lake. (See plate II of the Twenty-First 

 Annual Report.) The surface covered by this rock is about one-fourth of a square 

 mile in extent; it is confined, with the exception of a few acres in the S. W. J S. E. 

 J sec. 20, T. 65-6 W., to the N. | sec. 29. This area has been studied somewhat 

 carefully and a number of specimens of the porphyryte have been collected.* These 

 specimens are Nos. 927W, 928W, 929W, 932W and 933W; also Nos. 792G, 793G, 

 793aG, 794G, 794aG, 796G and 797G. A general description of this porphyryte has 

 already been published! and is here given as it bears directly on the rock under 

 consideration (No. 751). 



The single section, which was accessible to Dr. Wadsworth when his description 

 was written, is more or less altered, and does not clearly show the original nature of 



*A. WlNOHKM,: fii.rl,-i;illi A inniul R,-)M,rt, pp. 



^Tiventy-ftrst Annual Report, pp. 55-58. 



