phalen] the rocks of nugsuaks peninsula 207 



advantage could be taken of Michel-Levy's method on sections nor- 

 mal to GIG. Extinctions in one part of the Carlsbad twins range 

 as high as 31°, in the other part 15.5° is the observed angle. Evi- 

 dently the feldspar is entirely different in composition from those 

 already observed and should be classed as a labradorite with com- 

 position very close to AbgAn^. The basic labradorite is not nearly 

 so abundant as the more acid substance. 



Associated with the feldspars are the following minerals : Amphi- 

 bole, pyroxene, zircon, quartz, apatite, and magnetite, besides some 

 hydrous iron ore. Of these minerals amphibole is by far the most 

 important and most interesting, owing to its variety. It occurs in at 

 least three different types, ordinary hornblende of deep chestnut- 

 brown color, a light green phase, while a third variety has been de- 

 rived from the pyroxene with which it is now associated. Of these 

 forms the ordinary variety hornblende is most common. It is 

 strongly pleochroic, the rays vibrating parallel to C being a deep 

 chestnut-brown, those parallel to i? of a similar but lighter shade, 

 while those parallel to A are yellowish-green. The absorption 

 scheme is the usual C "> B y- A, but the difference between B and 

 C is very slight. 



The hornblende is generally lath-shaped with jagged or frayed 

 terminations. Sections parallel to igg and gig are present. No 

 crystallographic terminations were noted, but basal sections (ggi) 

 are frequent with prism and clinopinacoidal faces well developed, 

 the macropinacoidal showing as a rule. The secondary hornblende 

 is light chestnut-brown in color. It is generally associated with the 

 unaltered portions of the original pyroxene, whence it has been 

 derived, and crystals of light-brown pyroxene were noted with ex- 

 tinctions as high as 45°, whose edges and corners had completely 

 gone over to hornblende of a markedly darker hue, with extinctions 

 ranging in the neighborhood of 2G°. 



The third form of amphibole is usually associated with the dark 

 and light brown varieties already described. It occupies in every 

 instance a peripheral position and sometimes has a fibrous radiating 

 structure — though as a rule the structure of the associated horn- 

 blende is repeated in the green mineral. Most commonly, its form 

 is that of the amphibole, i. e., lath-shaped, or rather it forms a pari 

 of the lath-shaped crystals. Cleavage in its prism zone is well 

 developed. Its pleochroism, bluish-green in the prism zone, with 

 yellowish-green hues normal to this direction, together with its posi- 

 tion and other relations, indicate a soda-amphibole of the arfved- 

 sonite group. Evidently we have here a gradual change in com- 



