626 THE GEOLOGY OF MINNESOTA. 



[Greenstone. Quartz schist. 



Meg. A medium-grained, dark-gray, massive rock, with much hornblende. 

 Compare No. 356. 



Mic. Dr. H. Hensoldt's description of this section is as follows:* 

 " The principal constituents of this rock are plagiodase, hornblende and a greenish 

 alteration product, which may be termed viridite. If we examine a section under a 

 magnification of, say, sixty diameters, we observe a grayish feldspathic matrix or 

 groundmass, in which numerous crystals of hornblende of a dark-green color lie 

 embedded. These crystals vary considerably in size; some exceed one millimeter 

 in actual diameter while others are barely distinguishable on account of their 

 minuteness; their shape is mostly that of elongated scales or prisms, though rhom- 

 bohedral outlines are by no means uncommon. These hornblende crystals have 

 been rendered more or less turbid by decomposition and have become clouded by 

 ferric hydroxide or dust-like magnetite, as alteration products, but in the main their 

 characteristics are preserved. 



"In addition to the hornblende crystals, large, irregular, colorless patches, 

 surrounded by a pale, greenish fringe, are noticeable in the feldspathic matrix, and 

 in many instances these colorless masses present distinct hexagonal outlines. It is 

 extremely probable that these are pseudomorphs after biotite, the latter having been 

 completely changed into calcite and greenish fibrous epidote.f 



" In the original condition of the rock, the grayish matrix was doubtless a 

 colorless triclinic feldspar (possibly labradorite) which has been rendered turbid by 

 partial kaolinization. Its optical properties are now almost completely destroyed, 

 no twin lamellation and only a very faint chromatic display being noticeable. A 



< 



few small colorless hexagonal sections indicate the presence of apatite. 



" Dust-like grains of magnetite are scattered all over the field and a number of 

 larger crystals with distinct cubical and octahedral outlines are likewise observable." 



One section. 



Age. Lower Keewatin. u. s. G. 



No. 1000. QUARTZ SCHIST. (Magnetic.) 



Prom the upper end of the rapids from Garden lake to Pall lake. 

 Ref. Annual Report, xv, pages 310, 320, 393. 



Meg. Very fine-grained, dark, and heavy with magnetite. 



Mic. In the midst of the interlocking fine quartzes and the grains of magnetite, 

 are acicular sections of a rather highly polarizing mineral with no apparent 

 pleochroism, and constant multiple twinning, which is prob&hlygrunerite. One section. 



Age. Lower Keewatin. N. H. w. 



^Bulletin vi, pp. 39, 40. 



t All the smaller biotite scales have been transformed into chlorite or epidote, without a trace of calcite. 



