PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 287 



Quartzyte.] 



Mac. Medium-grained basic, uniform in color and composition, rather dark gray. 



Mic. The feldspar is ophitically embraced by the pyroxene. It is not much 

 altered. It constitutes about one-half of the rock. 



The pyroxene is abundant and contains not only the feldspar but also of i vine, 

 which is in granules rounded before the generation of the pyroxene. These grains 

 are sometimes isolated, but frequently are grouped in clusters. The olivine is easily 

 distinguished from the augite by the difference in colors. It is yellowish, while the 

 augite is reddish-brown in common light. 



The wtirjiietite is not secondary, in the sense that it is derived from the altera- 

 tion of the other minerals. It is sometimes closely associated with the augite, but it 

 is also embraced in the feldspars. Still more frequently, it is plainly the latest of 

 the generations of the magma, since it fills the angles between the other elements. 

 There is a brownish-red mineral occasionally associated with the magnetite which 

 has been called biotite by Dr. Wadsworth. It has, however, the appearance of the 

 brown mineral mentioned already as possibly boiclingite. 



One section. 



Aye. Cabotian(?) 



This rock differs from the rock of mount Josephine in having an ophitic struc- 

 ture in place of a granitic one. N. H. w. 



No. 262. QUARTZYTE. (Gray.) 



" Below the outcrop of No. 261 can be seen slaty red quartzyte beds with slate, in the southern slope 

 of the hill, dipping toward the north, or into the hill, at a low angle. The hill [in N. E. # sec. 25, T. 64-6 E.] is 

 largely made up of this kind of rock". 



Ref. Annual Report, ix, page 63; Annual Report, xiii, pages 100 (No. 163), 103; Bulletin viii, page xxxiii. 



Meg. A fine-grained, gray quartzyte. It contains numerous red grains (feldspar) 

 and small black grains. 



Mic. The section shows rounded and subangular grains of quartz and feldspar. 

 The larger of these grains appear rounded or water-worn in ordinary light, but in 

 polarized light their peripheries are seen to be jagged, as if the original rounded 

 grains had been enlarged and filled up the spaces between the grains. These larger 

 grains are surrounded by a very fine-grained aggregate of quartz and feldspar, but 

 there is no sharp distinction between large and small grains, as all intermediate sizes 

 can be seen. The feldspar, except for a very few plagioclase grains, is much clouded 

 and reddened and often shows very little effect on polarized light; it is probably 

 orthoclase and anorthoclase. This cloudy feldspar is quite abundant and even seems 

 to pervade the rock in the nature of a cement, although this feature is not so marked 

 as in some of the other quartzytes from this immediate vicincity (especially No. 264). 

 There are a few small flakes of greenish biotite in the section, also minute flakes of 

 chlorite, and small opaque gray spots. 



