phalen] the rocks of nugsuaks peninsula 1^9 



The upper intercalated dikes (Cat. Nos. 75,482, 75,483, 75,484) 

 are as a rule more coarsely grained than the lowermost, but in most 

 instances the decided schistose structure is still apparent. In addi- 

 tion to the minerals mentioned in the description of the lowermost 

 dike, biotite is developed macroscopically, and is especially prominent 

 in the most coarsely grained phases of the rock. There is present 

 also a silvery, micaceous mineral, evidently a secondary product 

 after amphibole and biotite. When viewed in thin section, the 

 minerals appear pronouncedly coarse in grain and irregularly 

 segregated. At times whole areas of variously oriented amphi- 

 boles occur with no feldspar whatever ; while masses of feldspar 

 with no foreign minerals occur in the same way. The pleochro- 

 ism of this amphibole is similar to that of the hornblende already 

 described, namely. A, yellow with greenish tinge ; B, green with a 

 yellow cast, and C, blue or bluish-green. The absorption scheme is 

 B > C y> A. Generally the mineral appears to be quite fresh ; it, 

 however, shows bright interference tints at its edges, as compared 

 with those of its mass. This is du& to incipient alteration which has 

 produced a light brown micaceous mineral, strongly dichroic, and 

 with bright interference tints, as already noted. In some cases 

 much of the unaltered hornblende still remains. 



The development of the feldspars reaches its .largest scale, of 

 course, in the pegmatitic facies, and here could be observed albite 

 twinning in the macroscopic way. Here, however, the feldspar 

 could be determined only unsatisfactorily in the ground mass of the 

 rock, alteration having proceeded too far to admit of positive meas- 

 urement. A fresh cleavage fragment from one of the pegmatite 

 areas gave extinctions of oligoclase. This is too acid, however, to 

 agree with the main mass of the feldspar, which, as in the case of 

 the lower dike, is made up chiefly of andesite-labradorite or a 

 plagioclase of intermediate composition. Sections which might ordi- 

 narily have served as diagnostic material are completely transformed 

 into brightly polarizing scales, leaving, but not always, a tiny spot, 

 still showing the albite lamellae intact. 



Additional to the minerals already described, quartz and biotite 

 occur in the pegmatitic facies of this rock. The latter mineral is 

 best developed in the coarser portions, but it occurs also in 

 the main ground mass, in the usual lath-shaped forms with fringed 

 terminations, and also in scales froifi the base. In ordinary light 

 it is brown in hue ; in polarized light, strongly dichroic with rays 

 of a brownish tint, vibrating normal to its cleavage, while those 

 parallel to this direction are so strongly absorbed as effectually to 

 obscure all color. 



