678 THE GEOLOGY OP MINNESOTA. 



[Quart 7,- porphyry. 



* 



grains. They are usually not in contact, but occasionally they are. It is to be 

 inferred, hence, that if the fresh grains are of later date they are of a different 

 species from the clouded grains. The fresh grains are themselves frequently in 

 contact, but their orientations are different. 



There is no quartz, but there are a few fine, black, opaque particles which are 

 indeterminable, but which are not magnetite. One section. 



Age. Archean (Keewatin). 



Remark. This schist may consist of one feldspar and a lot of micro-granulated 

 debris. This debris may have been originally largely feldspathic, or it may have 

 been a volcanic glass. N. H. w. 



No. 1279. QUARTZ-PORPHYRY. (Sheared debris.) 



Associated with No. 1278. Compare No. 311. 

 Ref. Annual Report, xvi, pages 67, 119. 



Meg. Coarser grained, sub-porphyritic, with pinkish feldspars. 



J//V. In a fine, granular matrix of fresh feldspar and of quartz, lie many large 

 crystals of /r/V/.v/w and grains of quartz. This matrix is more angular than in the 

 last, and, while belts of chloritic matter (pennine) cross it, yet the grains are not 

 uniformly elongated. The quartzes have occasionally a hooked or serrated border 

 and the hooks and teeth extinguish with the grain, but the common orientation does 

 not extend beyond the ends of the hooks. Occasionally, but not commonly, these 

 quartzes are broken, the parts still lying adjacent, with slightly differing orientations, 

 and sometimes many small fragments are strung off one side of the pi'incipal mass. 

 These large quartzes are not of vein formation, but have come from a quartz- 

 porphyry, if not now in their native places, for they occasionally show the marginal 







embayments peculiar to such quartz. The feldspars are much twinned and much 

 altered, being charged with micaceous scales, ailcite, epidotc, and with kaolinic 

 impurities. They occasionally are also broken and deranged in orientation in a 

 manner similar to that of the quartzes. They are also occasionally bordered by 

 narrow rims of secondary growths, which are not clouded by the impurities seen in 

 the body of the crystal. Extinction on p indicates oligoclase for the body of the 

 crystal,* and there is no observable difference in extinction between it and that of 

 the rims. 



In the slide are several phenocrysts of apatite and spots that appear to be 

 leucoxene. A considerable amount of the coloring matter, scant as it is, is of biotite, 

 and a few small sphenes are scattered in the fine matrix. There is one grain in the 

 slide which appears to have the form of a basal section of augite, and a portion of the 

 grain polarizes like augite, the rest of the area being dark. One section. 



* The actual extinction angle read on an albite twinning line on HP is 4, but as Fouqu docs not give so small an an^lo 

 on |. in any feldspar, this is taken to be the supplement of the true angle given by him. which would thcrc-forc be 8fl. 



