REPORT OF THE CHIEF ASTRONOMER 285 



SESSIONAL PAPER No. 25a 



in contact with the Kitchener ( ?) quartzite which have been faulted down 

 against it. This faulting is believed to be of later date than the intrusion of 

 the granite; no apophyses were found in the quartzite. 



Lithologically and structurally the batholith is unique in the whole 

 Boundary belt, although in both respects this granite is paralleled by many intru- 

 sive bodies both in Idaho and in British Columbia. The rock is distinguished 

 by a very coarse grain and commonly by an unusually perfect gneissic struc- 

 ture due to erush-metamorphism. (Plate 30.) The colour is a light gray to a 

 light pinkish-gray. In the ledge and hand specimen the most conspicuous ele- 

 ments are large phenocrysts of alkaline feldspar, and, less commonly, of acid 

 plagioclase; these are embedded in a coarse matrix of quartz, feldspar, and 

 biotite. The phenocrysts range from 2 cm. to 3 cm. in length. In the less 

 crushed rock they are subidiomorphic and lie with their longer axes parallel, 

 recalling a true fluidal structure. Such phenocrysts lie sensibly parallel to the 

 planes of crush-schistosity. Generally, however, the crushing has been so intense 

 that the phenocrysts are now lenticular and more or less rounded. In this case 

 they stand out as ' eyes ' and, while the core of each crystal still holds its glassy 

 lustre and recognizable cleavages, the outer shell of the crystal, for a depth of 

 one to two millimetres, is opaque-white and lustreless, owing to the peri- 

 pheral granulation of the phenocrysts. A third and very common phase consists 

 of zones from a few inches to fifty feet or more wide, in which the crushing has 

 developed a medium to coarse grained, equigranular biotite-gneiss or muscovite- 

 biotite gneiss. This gneiss is devoid of phenocrysts, probably for the reason that 

 these have been completely merged with their ground-mass through excessive 

 granulation in zones of maximum shear. Of the three phases the augengneiss is 

 the most abundant. 



The planes of schistosity of the granite have a fairly constant attitude with 

 a strike varying from N. 30° W. to N. 10° W., and dips varying from, 60° W. to 

 75° E. The average attitude is about: strike, N. 15° W., and dip, 80° W. The 

 gneissic bands are very seldom, if ever, crumpled, but continue nearly vertical 

 through thousands of feet of depth in the mass. 



The apophyses of the batholith are often coarsely pegmatitic. They are often 

 greatly faulted, distorted or pulled out into discontinuous pods, showing that the 

 country-rock about the intrusive has shared in the energetic deformation of the 

 batholithic body. It is possible that the deforming stresses were at work before 

 the granite had thoroughly solidified, thus explaining the apparent flow-structure 

 in certain phases of the batholith; but most of the deformation must have 

 followed the crystallization of the ground-mass, the minerals of which are so 

 greatly strained or granulated. 



Under the microscope the phenocrysts are seen to be chiefly orthoclase or 

 microcline, more rarely acid oligoclase, near Ab 8 An r The ground-mass is com- 

 posed of quartz, orjthoelase, oligoclase, microcline, microperthite, biotite, and 

 muscovite with a little accessory magnetite, apatite and titanite. All of 

 these minerals are more or less bent or fractured. The crushing has been 

 so intense that it is now impossible to state the original diameters of the 

 ground-mass essentials, though the average for the quartz and feldspars 



