706 THE GEOLOGY OP MINNESOTA. 



[Greenwacke. Dioryte. 



tion from the grains of feldspar which now consist essentially of damourite into the 

 nearly opaque substance. It seems as if the change to which the whole rock has 

 been subjected was concentrated here, and that the damourite has lost by it its 

 crystalline structure. There are also, in these semi-opaque spots, signs of the former 

 existence of large mica plates which have now become opaque, except that they 

 show yet a kind of striation of dark and light in the presumed direction of the 

 original cleavage. These mica plates, if they were such, were of the original rock, 

 are much larger than the microscopic damourite scales in the changed feldspars, and 

 were evidently more changeable than they, since they are only found in this altered 

 state, while the damourite, itself a result of a transformation of the same kind, is 

 usually well preserved. 



In short, according to the suggestion of I'rof. Lacroix, this sample, No. 1367 

 (>), is a metamorphosed granite, the original mica having been fused by the heat of 

 the surrounding molten rock, and the molten result having then enclosed some of 

 the smaller and more decomposable feldspars. These feldspars themsel ves, previously- 

 changed to damourite by the same cause, suffered further change, even becoming 

 like the original mica. The only parts of the original rock fragment which were not 

 sufficiently heated for fusion are the quartz, the andesine(?) feldspars and the most 

 of the damouritic feldspars. One section. 



Remark. This being apparently a fragment in the midst of a tnffaceous rock, 

 it must have been thrown out as a granitic mass from the deeper-seated portion of 

 the crust, and the changes above described took place while it was floating loosely in 

 the molten matter within the volcano from which it was finally ejected. It is 

 thus in harmony with the change evinced by specimen (//). What later metamor- 

 phism, if any, this mass has suffered, is not shown here. 



(d) Mic. Is similar to No. 1367 (), with the addition of pyroxene, but the 

 feldspars are not radiated and spindle-shaped. They occur more scantily as in a 

 clastic rock. Much amphibole pervades the slide, usually in fine, granular form, but 

 sometimes spreading and fan-shaped or curved and one-sided. The pyroxene is in 

 large crystals, much shattered and separated along the cleavages. The grain and 

 texture vary from place to place in the slide, with micro-granulitic areas. One 

 section. 



Age. These are all from the Archean (Keewatin). N. H. w. 



No. 1368. DIORYTE. (<ii;Tnsttine.) 



Same place as No. 1367, but intended to show the most evident eruptive characters. Prom near the 

 summit of the hill. 



Ref. Annual Report, xvi, pages 94, 99, 123. 



Mac. A hard, green or gray, compact rock, fine grained. 



