544 THE GEOLOGY OP MINNESOTA. 



[Quart/-porphyry. Diabase. 



Mir. The section is composed of a very fine-grained granitic mosaic of quartz, 

 feldspar, Motite and magnetite. The feldspar is much clouded and often considerably 

 reddened; its species was not determined carefully, but it appears to be orthoclase 

 largely. Scattered through the section are a few considerably larger grains of quartz 

 and of highly altered feldspar. These grains, which have the appearance of porphy- 

 ritic crystals, do not have crystal outlines, but have serrate borders into which fit 

 the smaller grains of the groundmass. The section resembles some of the metamor- 

 phosed quartzytes of Pigeon point, but has no distinctly f ragmen tal- characters. It 

 also resembles a quartz-porphyry with a micro-granitic groundmass. No definite 

 statement can be made from the section alone as to whether this is an acid igneous 

 rock or a recrystallized sedimentary. One section examined. 



Age. Cabotian. u. s. G. 



No. 784. QUARTZ-PORPHYRY. 



"At the place where the portage trail leaves Wind lake is a bluff which, in form, is a duplicate of the red 

 ijuartzyte bluffs at New Ulm, and in Rock and Pipestone counties. 



" No. 784 is from this bluff, and constitutes the most of it. It is u hard, red, fine, sub-granitized quartzyte, 

 somewhat sprinkled with darker specks, that may be chloritic or micaceous, or graphite, and having rarely a 

 distinct crystalline grain of orthoclase. It is in distinct sedimentary layers that dip south at an angle of 20 to 

 25." Perhaps near the centre of the S. E. % sec. 15, T. ffi}-5 W. 



Ref. Annual Report, x, pages 101, 102. 



Meg. A very fine-grained, granular rock, of a flesh color, and resembling No. 

 783. Small porphyritic crystals of quartz and feldspar are present. 



Mic. The section shows a quartz-porphyry with a microgranitic groundmass, 

 and is quite similar to No. 783. The porphyritic crystals are more marked than in 

 No. 783, and some of the quartzes show bipyramidal outlines. 



Two sections examined. 



Age. Cabotian. u. s. o. 



Remark. The feldspar "pheuocrysts " embrace the fine quartzes of the ground- 

 mass poikilitically. This is an instructive section, which, taken with No. 783 and 

 similar changed quartzytes of Pigeon point, illustrates the manner of genesis of this 

 rock. N. H. w. 



No. 785. DIABASE. 



This rock occurs in irregular patches and areas, apparently in No. 784; showing on the face of the bare 

 bluff rounded off by glaciation. 



Ref. Annual Report, x, pages 101, 121. 



Meg. A very dark, greenish-gray, fine-grained diabase. 



Mic. The section shows rather fresh, small, lath-shaped />/<i</i<>cl(/xcx in a consid- 

 erably altered fine-grained background, which consists of a little unaltered aityifr, 

 lionililendc, r/tlorite, magnetite, hlolHe and a little />i/i-/fr. 



One section examined. 



Age. Cabotian. u. s. G. 



