872 THE GEOLOGY OF MINNESOTA. 



[Granite. Felsyte. 



Aye. Intrusive in the Lower Keewatin. 



Remark. This rock has the appearance of several others which have been 

 supposed to be derived from the re-crystallization of a fine acid, feldspathic debris. 

 It is allied to the quartz-porphyry of the Lower Keewatin, from which it is perhaps 

 an apophysis. N. H. w. 



No. 2250. GRANITE. (Fine.} 



Same rock as Nos. 2248 and 2249, at its northern contact on the greenstone. 

 Finer grained. No section. 



Remark. This rock has much pyrite in scattered cubes, and is scatteringly 

 "porphyritic" with a feldspar. Its southern line of contact on the greenstone is 

 curious, for it is mixed with the greenstone very confusedly. There are many 

 angular pieces of the porphyry in the schistose greenstone through an interval of six 

 or eight feet, and in many places these two rocks both appear to share in that confu- 

 sion, there being many pieces of greenstone mingled with the porphyry. It is diffi- 

 cult or impossible to decide whether the porphyry, as an intrusive, has spread itself 

 amongst the greenstone, involving and surrounding many pieces, and itself losing 

 many, or the greenstone as a fragmental has formed a basal sedimentary contact on 

 the porphyry or whether, again, this confusion is due to friction along a plane of 

 contact between the two rocks. Whatever the cause, it is apparently at the same 

 horizon as seen near the section line between sections 5 and 8, three-fourths of a 

 mile further south. This is probably the rock which forms fine-grained, red-weath- 

 ering dikes in the upper greenstone that holds the jaspilyte along the south side of 

 this lake, and which appears amongst the graywackes near Nelson's cabins, as 

 described under Nos. 2244, 2245 and 2246. These are, hence, probably all later than 

 the great quartz-porphyry, but belong above the agglomeratic greenstone. As 

 granites or porphyries they may appear as apophyses in later formations if by meta- 

 morphism that great quartz-porphyry should become plastic. N. H. w. 



No. 2251. FELSYTE. 



Near the same place as No. 2250, but a little to the west. 

 Ref. Annual Report, xxiv, page 78. 



Mey. Nearly as fine and siliceous as flint, the only mineral visible, besides the 

 fine-grained quartz, or mesh of quartz, which can be identified, being pyrite, which 

 is sprinkled sparsely through the rock. From a narrow red dike, three inches wide, 

 cutting the greenstone No. 2252 in a winding zigzag course along the side of a vertical 

 "cliff which looks northeast. No section. 



Remark. It is reasonable to infer that this little dike is an offshoot from the 

 mass represented by Nos. 2248-2250. Its densely fine grain is somewhat like that at 

 the contact on the greenstone (No. 2250) and proves that it entered the greenstone 

 when the latter was much cooler than it was. N. H. w. 



