S80 THE GEOLOGY OP MINNESOTA. 



[Dioryte. 



are patches where chlorite and secondary quartz prevail almost to the exclusion of 

 everything else, and others that consist almost exclusively of chlorite and muscovite; 

 (3) There are isolated, sometimes roundish, spots in which there is a coarser granu- 

 litic recrystallization consisting apparently of new feldspar. These fine, new feld- 

 spars embrace no siderite nor muscovite, but the old feldspars are sprinkled with 

 muscovite and sometimes with siderite. Muscovite and siderite seem to have been 

 about cotemporary with the new feldspars. One section. 



Age. Intrusive in Upper Keewatin. 



Remark. It is probable that the mineral identified as calcite in No. 2263 and 

 others may be in part siderite, but no oxidation is apparent in those rocks. This 

 rock is comparable with the porphyrel of Zeta lake and with the porphyry of Keke- 

 quabic lake, standing between them in degree of regeneration. N. H. w. 



No. 2267. DIORYTE. 



On the portage from Moose lake to Flask lake, sec. 28, T. G4-9 W., a little further east than the last. 

 Ref. Annual Report, xxiv, page 81. 



Meg. A rather fine-grained, horublendic rock, "which also has the jointage and 

 aspect of an igneous rock. It is firm, fine grained, weathers coarsely very schistose, 

 and is full of pebbles, rounded and angular, mostly and most evidently those of green- 

 stone. These seem to be of the nature of inclusions in a basic intrusive, owing to 

 the nature of this rock." 



Mic. The rock consists essentially of plagioclase and hornblende, the latter, in 

 its crystal faces, being independent of the former, but never having its faces 110 

 tilled out. On the other hand those faces, as shown in section, are seen to be 

 terminated by vanishing and irregular, ragged ends composed of sharp-pointed fibres 

 that penetrate unequally into the surrounding rock, a feature quite common in 

 regenerated rocks. Most of these hornblendes have borders surrounding darker 

 cores, indicating an original augite, or some earlier condition of the hornblende 

 itself. Besides the larger hornblendes there are a great many isolated spicules of 

 the same mineral (actinolite ?). These in the same manner pierce the plagioclases 

 in all directions. 



The plagioclases are small, and of interlocking, irregular outlines. Some, 

 which appear to be older than others, are pierced by mica scales. 



Accessories are epidote and iron ore. One section. 



Age. Upper Keewatin. 



Ili'inarlc. This is a reconstructed rock, whatever its original condition. It is 

 very difficult to form an opinion as to its origin. Its outward megascopic characters, 

 as above enumerated, appear to indicate a fragmental source, and there is nothing 

 in its microscopic composition, so far as seen in the section examined, that precludes 

 that origin. It would be ordinarily classed, however, as a basic intrusive of the 

 diabase type, uralitized. N. H. w. 



