GABBEO AND DIORITE-POKPHYEIES. 243 



lar feldspars 5 mm. long and large augites. In thin section it is holocrystal- 

 line, the groundmass consisting of short tabular and indistinctly outlined 

 plagioclase, with low double refraction and moderately low extinction angles, 

 besides considerable magnetite and minute grains of epidote and chlorite, 

 which appear to replace pyroxene. The degree of crystallization, compared 

 with the rocks of Electric Peak, varies from grade 8 to 16. Throughout 

 the description of the more crystalline rocks of this district it has been 

 found convenient to refer to the grades of crystallization established for the 

 series of rocks at Electric Peak and Sepulchre Mountain (Table XVII, 

 Chapter III). This serves to correlate the various phases of crystalliza- 

 tion of the rocks of Crandall Basin, not only with one another, but also 

 with those of the district just named. 



In the more crystalline forms of this rock the feldspars are more lath- 

 shaped, magnetite is abundant, and the ferromagnesian minerals are pale- 

 green augite, some hypersthene, a little biotite, pale-green amphibole, and 

 a little quartz. Apatite occurs in stout crystals. The phenocrysts are 

 large, and are mostly labradorite in idiomorphic crystals and augite in less 

 regularly defined ones. The labradorite is especially noticeable on account 

 of clouds of dust-like and rod-shaped inclusions, which give it a brown- 

 ish tint, besides clusters and rows of rounded grains of magnetite, augite, 

 and biotite. Immediately surrounding these larger inclusions is a zone of 

 feldspar substance, free from the cloud of minute inclusions, indicating- that 

 the material composing the minute inclusions is the same as that forming 

 the larger ones, which has been concentrated in certain spots into the 

 minerals mentioned. THis phenomenon appears in perfectly fresh feldspars 

 which exhibit no cracks or signs of alteration, and in those which are 

 greatly cracked there is no relation between the position of the cracks and 

 the distribution of the inclusions, which frequently lie in crystallographic 

 zones. They are unquestionably primary microlitic bodies, inclosed at the 

 time of the crystallization of the feldspar, and are not of secondary origin, 

 like those described by Judd in the minerals of certain peridotites. 1 In the 

 unaltered rock the augite is fresh, and biotite appears as a primary crystal- 

 lization, but not in phenocrysts. The amphibole is secondary and 

 accompanies the uralitization of the augite. There are patches of irregularly 



'J. W. Judd, On the Tertiary and older peridotites of Scotland: Quart. Jour. Geol. Soc., Aug. 

 1885, pp. 374-389 ; also, On the relations between the solution planes of crystals and those of secondary 

 twinning, etc. : Min. Mag., Vol. VII, pp. 81-92. 



