REPORT OF THE CHIEF ASTRONOMER 381 



SESSIONAL PAPER No. 25a 



Quartz 43-2 



Orthoclase 37-8 



Andesine-labradorite • 14-7 



Biotite 3-8 



Apatite, etc "5 



100-0 

 The specific gravity of this band is 2-636. 



A similar estimate of the weight percentages of the minerals in a typical 



dark band gave a strongly contrasted result : — 



Quartz 17-7 



Orthoclase 9-5 



Acid labradorite, Ab 4 An 3 44-1 



Biotite 22-5 



Garnet 3-3 



Magnetite 1'9 



Titanite -6 



Apatite *4 



100-0 

 The specific gravity of this dark band is 2-980. 



In the development of the banded structure there has evidently been an 

 advanced segregation of the basic minerals, including the accessories in the 

 dark bands, with a corresponding concentration of the quartz and orthoclase 

 in the light bands. The specific gravities are directly related to these concen- 

 trations and differ, respectively, from the specific gravity of the more massive, 

 less altered phase of the batholith (2-6S9). 



The dark bands are, on the average, much narrower than the light ones, 

 thicknesses of more than one or two inches being quite uncommon. Often 

 they are separated by rock which, although it is gneissic, has nearly the com- 

 position of the original unsheared granite. The distribution of the dark bands 

 is that which would characterize zones of shearing in such a batholithic mass, 

 and it is probable that this highly micaceous phase of the gneiss has been 

 produced through a leaching of the more basic material from the original 

 granite, followed by the recrvstallization of that material in the zones or planes 

 cf shearing. This hypothesis will be more fully presented in connection with 

 the precisely similar phenomenon of banding in the sheared batholiths of the 

 Cascade range. The hypothesis merits attention since it implies the idea of 

 the efficiency of lateral secretion on a colossal scale. 



Over large areas the batholith is free from intrusive dikes. A few narrow 

 basic dikes were observed on the Canadian Pacific railway track. A thin section 

 of a specimen from one dike occurring near the western contact of the batholith, 

 showed evidence of profound alteration. The dike-rock is now a mass of chlorite, 

 pyrite, and plagioclase, and originally was probably of diabasic composition. 



Smelter Granite Stock. 



Immediately northwest of Grand Forks the Cascade batholith and the 

 Grand Forks schists are cut by a boss or small roundish stock of a quite different 



