468 THE GEOLOGY OF MINNESOTA. 



[Gabbro. Granite. 



Mic. The section shows one of the usual medium-grained red rocks, the only 

 uncommon feature being the presence of practically fresh, green augite. This occurs 

 in small grains of irregular outlines. The feldspar is the usual clouded, but little 

 reddened, variety and is very intimately and abundantly intergrown with quartz to 

 form micropegmatyte. 



Two sections. 



Age. Cabotian. u. s. G. 



No. 644. GABBRO. (Altered.) 



From the top of the hill mentioned under No. 643. Probably from the top of Pine mountain in S. W. J^ 

 see 34, T. 63-1 E. This rock may be a dike protruding through No. 643. 

 Ref. Annual Report, x, pages 66-68; Bulletin ii, pages 79, 80. 



Meg. A rather coarse-grained granitoid rock, pale reddish-brown in color, com- 

 posed of feldspar, a black mineral (augite) and small amounts of quartz. 



Mic. The following is the description of this rock, written by M. E. Wads- 

 worth:* 



"In the section the feldspars are seen to be much altered, showing aggregate 

 polarization, while much of the area is taken up by the graphic arrangement of 

 secondary quartz in a feldspathic groundmass. Considerable augite with some 

 diallage was observed. The latter is largely altered to a greenish-brown fibrous 

 product, which retains the extinction point of the remaining diallage. Part of the 

 augite shows the common prismatic cleavage, but in other portions of the same 

 crystal the fibrous alteration of the diallage is to be seen, thus supporting the views 

 of those authors who hold that diallage is derived, in part at least, from the alter- 

 ation of augite. Apatite and magnetite are common, and some biotite was observed. 



"From this section and others observed, the writer has but little doubt that 

 most, if not all, of Irving's augite syenytes are altered conditions of gabbro and 

 diabase the same as this rock is. This view, Irving's language would indicate, he 

 partially shared."f 



Two poor sections examined. 



Age. Cabotian. u. s. G. 



No. 645. GRANITE. (Red.) 



" ' Red granite,' like No. 642, from this hill range, S. W. % sec. 35, T. 63-1 E., on a little creek." 

 Ref. Annual Report, x, page 67. 



Meg. A brownish-red rock of medium grain composed essentially of red feldspar 

 and a black (magnetite) and a yellowish mineral. 



Mic. The slide shows feldspar, ijuaiis, maynctite, yellowish alteration products 

 and a little greenish material. The feldspar is very highly clouded and reddened, 



* Bulletin ', pp. 79, 80. 



t Copper -Bear ing Rocks, 1883, pp. 



pp. 113-124. 



