864 THE GEOLOGY OF MINNESOTA. 



[Greenstone. Quartz-porphyry. 



enlarged, and also that numerous hornblendic spicules have been developed, the latter 

 piercing the fresh feldspars which are also secondary. Quartz, in fine isolated and 

 sometimes spreading forms, is also secondary. A dark iron mineral which appears 

 to be magnetite or ilmenite, is in close association with a highly refractive and trans- 

 parent mineral, which in some of its elongated sections has parallel extinction and 

 is probably rutile. This supposed rutile is not in needles, but usually in globular 

 scattered individual grains resembling sphene which are occasionally grouped. Two 

 sections. 



Remark. This rock may have been originally a diabase. It is a firm rock and 

 appears fresh, but it is plain that its present condition is wholly due to metasoniatic 

 alteration of earlier minerals. N. H. w. 



No. 2227. GREENSTONE. 



From the prominent ridge at the corners of sees. 8, 9, 16 and 17, T. 63-9 W., north of the Kawishiwi river. 

 Bef. Annual Report, xxiv, page 69. 



Mey. Fresh, green, diabase-looking rock. Sample represents the non-agglom- 

 eratic portion. 



Mic. The trachytic feldspars have a radial manner of piercing the hornblendes, 

 suggesting that the hornblende is an alteration product from ophitic augite. Two 

 sections. 



Age. Lower Keewatin. N. H. w. 



NO. 2229. QUARTZ-PORPHYRY. 



Two hundred paces south of the quarter-post, east side of sec. 8, T. 63-9, southwest of Snowbank lake. 

 Ref. Annual Report, xxiv, page 69. 



Mey. Light-weathering, massive, porphyroidal rock, containing phenocrysts of 

 quartz and feldspar in a fine siliceous matrix; also showing a little pyrite. 



Mic. The quartz is bipyramidal and somewhat resorbed, having embayments 

 filled with the fine matrix. It is not abundant. The feldspar is much twinned and 

 somewhat resembles that of the porphyry of Kekequabic lake. They are much 

 altered and in some places wholly disintegrated. The resulting secondary minerals 

 are chiefly calcite and muscovite, the former sometimes constituting areas of consid- 

 erable (microscopic) size, but the latter occurring in myriads of isolated minute 

 scales. The same minute muscovite scales are also quite abundant throughout the 

 matrix. Biotite in sizable groups is distributed through the rock, but sparsely. 

 Its scales are from twenty-five to fifty times larger than those of the muscovite. 



A secondary feldspar, in fine isolated grains, has been generated not only 

 throughout the matrix, but not infrequently in the bodies of the old feldspar pheno- 

 crysts. These are glassy and cannot be specifically determined because they are 

 destitute of visible cleavage and other criteria for measurements. While the old feld- 



