488 THE GEOLOGY OP MINNESOTA. 



[Andesyte. Porphyry te. 



Remark. This adds another one to the few occurrences of volcanic tuff in the 

 post-Animikie rocks of northeastern Minnesota. Other tuffs have been reported by 

 the authors from Duluth,* and Dr. A. H. Elftman has collected similar tuffs from the 

 vicinity of Baptism river on the north shore of lake Superior. (See figure 4, plate II.) 



U. S. G. 



No. 681. ANDESYTE. 



" The hill at the right of the portage from Little lake to Little Trout lake is made up of the numbers 681, 

 682 and 683 is [in] downward order, with the thickness stated below (dip 20 8. E.): 



"No. 681 (seen twenty feet; may have more above). A very tine, black rock, approaching aphanitic slate. 



" No. 682 (thirty five feet). Porphyry, both with red and with gray feldspar crystals. This is a part of the 

 great formation lying below the cupriferous, already mentioned, and is in beds of an inch, where weathered, or 

 in heavy layers of five or six feet. The groundmass seems to be the rock No. 678. The red color seems to come 

 from weathering, the original color being gray. This is shown in one of the small samples. It has free quartz. 



"No. 683 (twenty feet; there is a talus below of fifty feet in which the rock is unknown). A fine-grained, 

 heavy, dark rook, apparently consisting of triclinic feldspar and diallage, with a little uralite and magnetite. 



"No. 684. Further north is a layer of four feet of a fine black rock, evidently crystalline, somewhat like 

 Nos. 678 or 683, included between some of the beds of the foregoing porphyry, near the bottom of the same.'' 



These rock samples are from near the centre of S. % sec. 5, T. 63-1 W. 



Ref. Annual Report, x, pages 77, 78. 



Meg. A very fine-grained, compact, brownish-gray rock, evidently breaking 

 like a shale. 



Mic. The section is quite fine grained and is more than half composed of a 

 felted aggregate of somewhat reddened plagioclases, some of which assume a lath- 

 shaped form. In addition to the plagioclase there are magnetite, chlorite, fibrous 

 hornblende and a finely granular strongly doubly refracting mineral, which appears 

 to be epidote. It is possible that some of the latter substance is augite in small 

 grains. It seems quite probable that the rock was originally holocrystalline. 



One section examined. 



Age. Cabotian. u. s. G. 



No. 682. PORPHYRYTE. (Diabase.) 



Same locality as No. 681. 



Ref. Annual Report, x, pages 77, 78. 



Meg. There are two hand samples of this number. One is a dark brown, rather 

 fine-grained rock, with numerous porphyritic gray plagioclases, the largest of which 

 is about an inch in length. On one side of the sample closely welded to it is a small 

 amount of dense black aphanitic rock, looking like the rapidly cooled edge of a 

 diabase dike or flow. The other sample is similar to the first, except that most of 

 the feldspar, both of the phenocrysts and of the groundmass, is brownish red in 

 color, making the sample appear much redder than the first. 



Mic. Sections from the darker hand sample show porphyritic plagioclases (near 

 labradorite) embedded in a considerably altered groundmass, which consists of more 

 or less reddened feldspar, augite, magnetite, chlorite, quartz, apatite and confused alter- 



*American Qeologist, vol. xvili, pp. 211-213, October, 1896. 



