516 THE GEOLOGY OP MINNESOTA. 



[Amphibolyte. 



Mic. The section shows a mass of numerous, small, green hornblende flakes, 

 which interlace with each other. They run in every direction through the section, 

 but there is still a roughly parallel arrangement of a number of them, giving an 

 indistinct schistose structure to the section. Under a higher power some of these 

 hornblendes are seen to have fine twin lamellae, which run parallel with the elonga- 

 tion and cleavage of the hornblende flakes. With the hornblende is some chlorite, 

 and both of these minerals are set in a background of interlocking, completely 

 allotriomorphic grains of feldspar, and possibly a few grains of <jnai1z. The last 

 named mineral was not determined with certainty, and perhaps does not exist in the 

 slide. The hornblendes are included in the feldspars frequently and also pass 

 through one feldspar grain into another. The feldspar is rarely twinned, shows 

 almost no cleavage and is somewhat cloudy; its species was not determined. A few 

 grains show indistinct fine albite twinning lamellae, which are usually not visible 

 except near the point of extinction, and which have a closely parallel extinction. 



Running through the section, in general parallel to the more usual elongation of 

 the hornblende, are some irregular, discontinuous, roughly lens-shaped areas which 

 contain a little etilcite and much of another mineral. These areas are evidently 

 smaller forms of the veinlets seen in the hand specimens. The mineral which makes 

 up most of these areas was not determined. It is gray or colorless and has a finely 

 granular appearance, although under crossed nicols it breaks up into irregular areas 

 of considerable size. The granular appearance reminds one of granular epidote, but 

 this mineral has not the high index of refraction nor the strong double refraction of 

 epidote. Its index of refraction is higher than the feldspar of the rock and its 

 double refraction is more marked; the feldspar polarizes in grays of the first order, 

 while this mineral is usually yellow or red and sometimes reaches blue of the second 

 order. This mineral is clearly biaxial. 



One section examined. 



Age. Archean, Lower Keewatin or older. 



Remark. This rock, and also Nos. 733, 734, 735 and 736, are parts of the 

 " greenstone " terrane which extends along the south side of the Saganaga granite 

 mass from Frog Rock lake eastward to within nearly two miles of Gunflint lake, 

 where it disappears under the Animikie strata. (Compare the map and report on 

 the Akeley Lake plate.) This terrane is of earlier date than the Saganaga granite, 

 which cuts it, and is thus as old as the Lower Keewatin. There are also some facts 

 which tend to show that this "greenstone" terrane along the southern side of this 

 granite mass may be older than the Lower Keewatin, and thus belong to a pre- 

 clastic series, /. e., to the " Archean " or Basement Complex, as these terms are used 

 by the U. S. Geol. Survey. u. s. G. 



