PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 539 



Granite.] 



" In one portion a pyroxene, in a triangular mass, is seen remaining as pyroxene 

 in one part and altered to a greenish-brown biotite carrying magnetite in another 

 portion of the same mass. 



" A radiated group of epidote crystals was observed in the section. 



" Plate IV, figure 2, shows the relation of the biotite formed from the alteration 

 of the diallage. It extends along the cleavage and fissure lines with portions of the 

 unchanged pyroxene between. The whole is surrounded by somewhat kaolinized 

 feldspars bearing biotite." 



The radiating group of crystals mentioned above at least only one such group 

 was seen in the section lias the characters of tounnnlinc rather than of epidote. 

 ( 'dlfilc also occurs. ' One section examined. 



Age. Cabotian. u. s. G. 



No. 774. GRANITE (with augite). 



At or near Duck lake. This lake is in sec. 3, T. 63-5 W. 



" This comes in apparently through No. 773 at first, but afterward it lies on No. 773, and in rude, lenticular 

 and broken bedding dips to the south. It forms a range of hills about 100 to 150 feet high next west and south 

 of Mesabi and Duck lakes. It has been molten, as evidently as No. 773, and in that sense it is an igneous rock, 

 but it differs from it in its mineral composition. It is a reddish syenyte, but seems to contain mica, as well as 

 some plagioclase. In passing along Duck lake considerable confusion prevails in the relative positions of Nos. 

 77.'! and 771. Along the south side of the point that separates the lake into northeast and southwest parts, No. 

 774 plainly lies on parts of No. 773; but on the east side of the lake, there is considerable mixing, the two changing 

 places suddenly, and either one being in patches in the other particularly No. 774 embracing masses of No. 

 773, and suddenly cutting No. 773 like dike-rock." 



Ref. Annual Report, x, pages 99-101; Bulletin ii, page 103. 



Mt'ff. A rather fine-grained, granitoid rock, pinkish gray in color, composed of 

 feldspar, quartz and black minerals. A little pyrite is seen both in the hand samples 

 and in the section. 



Mic. M. E. Wadsworth's description of this section is as follows:* 



" The section is largely composed of feldspar and <nutrtz arranged in graphic or 

 eozoon form. Besides these minerals much ntagnetite, augite, a little diallage, Itorn- 

 hlemle, some biotite, yellowish pseudomonphs after olivine or augite, and microliths 

 occur. The augite in places is altered into urulite and at others into broirit hornblende, 

 which exhibits the characteristic cleavage. The brown hornblende occupies part of 

 the original augite areas, is contiguous with the augite mass and was formed from 

 its alteration. The biotite is mainly associated with the magnetite. Although the 

 pyroxene is abundant here, this rock answers to the augite syenyte of Irving, but 

 has been formed from the further alteration of a basaltic diabase." 



Two sections examined. 



Age. Cabotiau. 



Remarks. This rock is similar to others of the augite granites and its composition 

 and field relations place it with the granites or " red rocks " of the Cabotian rather 

 than with the gabbro. u. s. G. 



* Bulletin ii, p. 103. 



