820 THE GEOLOGY OF MINNESOTA. 



[Zirkelyte. Gneiss. Amphibolyte. 



in a narrow fissure. The angularity of these individual grains is in marked contrast 

 with the roundness of the grains in muscovadyte or noryte, which have been 

 attributed to the peripheral flowage and contact of the gabbro on the older rocks. 



N. H. w. 

 No. 2002. ZIRKELYTE. 



From the contact edge of the same dike as No. 2001. 

 Ref. Annual Report, xxiv, page 8. 



Meg. Fine, dark, somewhat rusty on the contact surface. 



Mic. The grain is so fine that much of it appears to be simply a devitrified 

 glass. The augites are separated into several parts, though lying adjacent, the parts 

 having separate orientations, and similar parts, though smaller, and globular, are 

 thickly disseminated everywhere in the slide, so fine that their united marginal 

 reflections produce the darkness that the slide presents between the nicols. One 

 section. 



Age. Keweenawan(?) N. H. w. 



No. 2005. GNEISS. 



N. E. J4 sec - 13, T. 63-18, opposite the two small islands at seventy-five feet above Vermilion lake, near 

 the shore. 



Ref. Annual Report, xxiv, page 9. 



Meg. Gray gneiss. 



Mic. With a sprinkling of biotite and hornblende, this rock consists of consider- 

 ably altered feldspars and of quartz, the distribution of the alteration being irregular, 

 due in large part to recent weathering. There is no general distinction between 

 old and new feldspars, though some are much more decayed than others. One section. 



Age. Archean. N. H. w. 



No. 2006. AMPHIBOLYTE. 



Associated with No. 2005. 



Ref. Annual Report, xxiv, page 9. 



Meg. Hornblendic, dark. 



Mic. More than one-half of this rock is composed of green hornblende. The 

 rest is composed essentially of a feldspathic element which is so crowded with 

 impurities that its crystalline structure is wholly lost. This substance fills the 

 angles between the hornblendes and occasionally enters them after the manner of 

 corrosion bays in quartz of a quartz-porphyry. There are several pieces of splti'ite 

 and one corroded crystal of a pyroxene. One section. 



Age. Archean. N. H. w. 



No. 2008. AMPHIBOLYTE. 



From large boulders, evidently derived from rock in place not far distant. N. E. 14 sec. 13, T. 63-19, same 

 place as No. 2005. 



Ref. Annual Report, xxiv, page 10. 



