360 THE GEOLOGY OF MINNESOTA. 



[Granite. Quartz-porphyry. Jaspilyte. 



relation to the feldspar, indeed feldspar is not an abundant ingredient. Yet there 

 is a small amount of glassy secondary feldspar. Calcite is an important constituent. 



One section. 



Aye. Archean (probably dike in the Keewatin). N. H. w. 



No. 425. GRANITE. 



Vermilion lake; probably from the island just south of Birch point and crossed by the west line of sec. 15, ' 

 T. 62-16 W. Forms a low island in a large bay west of the government's station near the head of the bay. 

 Ref. Annual Report, is, page 102. 



^f/'|/. The rock is jointed and somewhat schistose coarsely, in about the same 

 direction as the slates. It is light-colored, consisting of quartz and a clouded, or 

 slightly flesh-red crushed feldspar resembling orthoclase. There is also a nearly 

 white feldspar abundant in this rock, which is twinned on the albite plan, and which 

 is probably of later origin. Its form is more evidently that of definite crystals and 

 these lie in a matrix, of the other feldspar and quartz. 



Mic. The feldspars are all much kaolinized. One prism of apatite is visible. 



One section. 



Age. Archean (Keewatin). N. H. w. 



No. 426. QUARTZ-PORPHYRY. (Macerated debris, sheared.) 



The rock of Ely island. 



Ref. Annual Report, ix, page 102; Annual Report, x, page 94. 



Meg. Resembles the rocks Nos. 311 and 375. It is of a light-gray color, often 

 with a' light-green tint, having free quartz in an amorphous siliceous matrix, with 

 sparse pyrite crystals. 



Mic. The section is very fine grained, but there are forms of much frayed and 

 altered feldspars, of the plagioclase type, which are conspicuous although they are so 

 permeated by the fine scales and t other crystalliths resulting from their alteration 

 that they can only be seen in polarized light, otherwise they are lost in the fine 

 matrix in which they lie, and which is composed of the same materials. Sometimes 

 rounded, or sub-rounded quartzes of considerable size are visible, sprinkled sparingly 

 through the hand sample. Crystalline cleavages appearing like those of calcite can 

 occasionally be seen on a broken surface, but this mineral under the microscope in 

 part proves to have the characters of siderite rather than calcite. 



One section. 



Age. Archean (Upper Keewatin). N. H . w. 



No. 427. JASPILYTE. ( Fragment in No. 426. ) 



"Rock of Ely island, containing jaspery pieces." 



" In traveling over the island, where much of the rock is bare, occasionally may be noticed .bright red 

 pieces of jasper superficially embraced in the formation, some of them three or four inches across. The position 

 and structure of these pieces is at variance with the schistose structure of the rock in which they are embraced. 



