684 THE GEOLOGY OF MINNESOTA. 



[Porphyrel. Hornblende schist . 



and more tortuous boundaries with respect to each other, and enclose partially some 

 of the areas of kaolin. These larger areas in part break up into grains having 

 separate orientation. 



There are many roundish areas, occupied chiefly by the fine kaolinic substance, 

 which extinguish as individuals, but which are filled with secondary substances, 

 amongst which can be distinguished ca/citc, fjiittrtz of secondary origin, muscovite and 

 zoisite. 



ZotNiti' is also scattered throughout the slide. It is distinguished by its clear 

 transparency and its high refraction, the latter property being brought out on lower- 

 ing the condenser. Between crossed nicols also it is often blue (or gray). It is in 

 the midst of the actinolite and sometimes occupies the central portion of the actin- 

 olite groups, but in general it is in fine isolated grains scattered through the kaolin, 

 as well as in large cleaved but loosely aggregated masses. 



Apatite is found in a few quite sizeable grains. 



fiiotitc is rare, in very fine brown leaves, but more frequent in certain areas 

 than in others. 



Spl/etK- is seen in a few isolated grains, some of them being quite large. Two 

 sections examined. 



Age. Archean (Keewatin). 



Revuirlc. This rock is one of those regenerated, sub-porphyritic elastics which, 

 while revealing its originally fragmental condition, is yet a perfectly crystalline rock. 

 It has the outward megascopic aspect of a fine-grained granite, and is light colored. 

 The old feldspars are the most decayed portion of the rock, but they are now firm 

 with the formation of innumerable grains of other feldspar, of quartz, of rauscovite 

 and of zoisite. The smaller clastic feldspars are micro-granulated and lost, but the 

 largest are evident by the four extinctions that occur on rotation. Their borders are 

 interhooked with the matrix by new feldspathic growths. The quartz is wholly 

 secondary. N. H. w. 



No. 1300. HORNBLENDE SCHIST. 



About three-fourths Of a mile north of Gunflint lake ; from the " black belt" in the Keewatin schists. 

 Ref. Annual Report, xvi, pages 69, 120. 



Meg. Coarse hornblendic rock, associated with micaceous schist, lying in a 

 rudely stratified belt in the green schists and other rocks that constitute the Kee- 

 watin north of Gunflint lake. 



Mic. Green hornblende abundant. Two forms of feldspar, one in large grains, 

 much altered, and the other fresh and glassy, evidently of later date. Quartz not 

 abundant, Dinj^ide common, but generally not in large masses. Sphene, epidote, 

 ralcite. apatite. One section. 



Aye. Archean (Keewatin). 



