PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 447 



Quartzytr.] 



Age. Anirnikie (metamorphosed). 



Remark. This is one of the "intermediate rocks" mentioned by Bayley. 

 (Bulletin cix, U. S. Geol. Survey.) 



The section examined by Wadsworth, contains, in addition to the minerals 

 mentioned above, hypersthene, which is of dark yellow color and characterized by 

 parallel extinction and pleochroism. This mineral can hardly be recognized in the 

 section made later and above described, but certain much altered, evidently pyrox- 

 enic, grains, now consist of biotite (showing sometimes a greenish or chloritic 

 tendency), and of hornblende, may have resulted from that mineral. The mineral 

 hypersthene was apparently among the earlier minerals of the rock. It does not 

 hold an ophitic relation to the plagioclases, and therefore preceded them in generation. 

 It seems, therefore, that the magma that cooled to form this rock was derived 

 essentially from the elastics, probably with only partial fusion, and perhaps with 

 basic intermixture from the gabbro, forming an intermediate magma in respect of 

 acidity, before the generation of this hypersthene. 



On the other hand, this rock, which in color and general appearance approxi- 

 mates the dark gabbro of Pigeon point, may be looked upon as an altered condition 

 of that rock. Its environment is such that it must have been permeated, on that 

 supposition, very readily by the acid elements of the adjoining country rocks. It is 

 also a common- observation, under such conditions, to see the basic rocks thus affected, 

 as at Duluth, at Wauswaugoning bay, and at Silver Islet. N. H. w. 



No. 607. QUARTZYTE. (Altered.) 



Pigeon point. Next rock north of No. 606, and underlying it. 



-Re/. Annual Report, is, pages 69, 70; Annual Report, x, pages 57, 58. 



Met-/. The hand sample shows one of the phases of the altered quartzyte of 

 Pigeon point. The sample varies some in grain and color, and is composed of quartz, 

 red to gray feldspar, hornblende and chlorite. 



Mic. The rock is composed of quartz and feldspar, which is clouded and more 

 or less red; these two minerals are sometimes intergrown to form micropegmatyte; 

 yellowish-green hortih/onJc, chlorite and biotite are also common, some magnetite and 

 a few almost opaque rough rods, whose exact nature is not clear. Two sections. 



Age. Animikie. . u. s. G. 



No. 608. QUARTZ YTE. (Metamorphosed.) 



Pigeon point. Apparently a modification of the quartzytes. Lies below No. 606. 

 Ref. Annual Report, ix, pages 69, 70; Annual Report, x, pages 57, 58. 



Mr;/. Fine grained, reddish, quartzose. (See figure 30.) 



Mic. The abundant quartz grains are wholly the result of regeneration and 

 rearrangement of the quartz of the original clastic rock. About one-third of the 



