308 THE GEOLOGY OF MINNESOTA. 



[Dioryte. Granite. 



No. 305. DIORYTE(?) 



North lake, Canadian side, just east of the first narrows in the outlet of North lake. (If the United States 

 survey were extended over this point it would be found in N. W. ]4 sec. 16, T. 65-2 W.) This rock is the same 

 as No. 744W. 



Kef. Annual Report, ix, pages 81, 83, 107; Annual Report, x, page 85; Annual Report, xvi, page 270, 

 Bulletin ii, pages 88, 85, 86, plate VIII, figure 1. 



Meg. Black and white, or pepper-and-salt rock, composed largely of hornblende, 

 quartz and one or more white feldspars, granular, rather coarse, sharply crystalline 

 and fresh. It forms low irregular knolls, veined and blotched with irregularities of 

 composition. 



Mic. This rock has been carefully and fully described by Wadsworth, who 

 shows that the original pyroxene' has undergone great alteration, passing from 

 diallage to green hornblende, thence to brown hornblende, showing its prismatic 

 cleavage, and also into biotite, with which last is associated a little magnetite. The 

 quartz he considers secondary. There is a little titanite (sphene) and epidote, as well 

 as apatite, and doubtfully zircon. He says: 



" The steps in the alteration shown by the different diallage cores irregularly 

 interlocked with the hornblendic substance, and gradually passing into it, are as 

 follows: (1) A palish green substance, not dichroic, and destitute of cleavage. 

 (2) A deeper green substance, having a longitudinal cleavage, but not dichroic, or 

 only slightly so. (3) The same dark green substance (all being connected), but of a 

 somewhat darker green color, and dichroic, varying from a slight yellowish green to 

 a dark green. (4) A well marked light hornblende, with not only the hornblende 

 cleavage in a longitudinal direction, but also across the longer or vertical axis. This 

 is dichroic, varying from a yellowish brown to a dark brown color. These changes 

 resemble those shown by Williams in the Baltimore gabbro. The first three stages 

 are to be seen united about a single diallage core, as are also the third and fourth 

 stages." 



One section. 



Age. Archean. N. H. w. 



No. 306. GRANITE (with biotite). 



Same locality as No. 305, in which it occurs somewhat in the manner of a vein. 

 Ref. Annual Report, ix, page 81. 



Meg. A rather fine-grained, gray granite, composed of quartz, gray to glassy 

 feldspar and biotite. 



Mic. The section shows a granite composed almost entirely of quartz and feld- 

 spar. The feldspar seems to be ortlioclttsc and a jtlagiodase showing fine twinning stria? 

 and a low extinction angle, probably near oligoclase. A little liotite is present, and 

 also some chlorite, and a few small grains of sp/iair. Some of the quartzes and feld- 



