868 THE GEOLOGY OF MINNESOTA. 



[Quartz-porphyry. Granite. 



these structures grade into each other, and while it is certain that some portions of 

 the slide consist of a fine reconstructed original clastic debris, other portions are 

 simply granulitized feldspars. 



A few spicules of bluish-gray tourmaline are in the granular matrix. One section. 



Age, Quartz-porphyry of the Lower Keewatin. 



Remark. It was the examination of this rock, and generally of this belt of 

 quartz-porphyry, with its great alteration and its occasional clastic characters, that 

 suggested the idea that this belt of quartz-porphyry is the result of Archean sedi- 

 mentation combined and cotemporary with oceanic precipitation of silica and 

 potassa. There must, if such be true, have been a thick akaline mud in the bottom 

 of the ocean, and these crystals must have formed in it as in a saturated solution, 

 many becoming broken by transportation, and all of them much decayed prior to 

 the consolidation of the rock. The reader is referred to Part III. N. H. w. 



No. 2238. QUARTZ-PORPHYRY. 



Near the same place as No. 2237. 

 Ref. Annual Report, xxiv, page 70. 



Meg. A form of the porphyry having a dense, fine, siliceous matrix which sur- 

 rounds distinct quartzes and indistinct feldspars. The matrix appears like the fine 

 silica of the great jaspilyte lenses. 



Mic. Microscopically the matrix of the section examined does not appear so 

 much like the jaspilyte, but is characteristically like that of quartz-porphyry. It is 

 manifestly composed of very fine quartz and feldspar, with some muscovite, some 

 epidote and a little pyrite. The phenocrysts, whether of quartz or feldspar, are much 

 broken and disturbed, the latter having given rise to calcite, chlorite and muscori/c. 

 Two sections. 



Age. Quartz-porphyry of the Lower Keewatin. N. H. w. 



No. 2239. GRANITE. (PorphyriticJ 



Twenty rods west of the last ; in the descent to a swamp. 

 Ref. Annual Report, xxiv, page 71. 



Meg. Reddish gray, rather fine grained, but holding feldspar pheuocrysts about 

 a quarter of an inch in diameter. 



Mic. The matrix is siliceous with fresh secondary quartz and enlargements of 

 feldspars. It also contains calcite, hornblende, muscovite, biotite, chlorite, spliene and 

 epidote. The feldspar phenocrysts, although for the most part entirely lost as to 

 their optic characters by a kaolinization (accompanied by much epidote) that has 

 permeated them, yet about their borders are fresh and glassy by reason of a recrys- 

 . tallization which has strengthened the whole rock. Throughout the phenocrysts, 



