126 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



almost invariably lies in the plane of bedding. The heart of each segregation 

 is especially rich in biotite and sericitic muscovite. Their main mass is com- 

 posed of a grayish-white, granular base of interlocking grains of quartz and 

 subordinate feldspar, in which are embedded abundant, conspicuous foils of 

 black biotite 5 mm. or more in diameter, and many highly poikilitic red 

 garnets, with large anhedra of titanite. The material of the dark-coloured 

 minerals has plainly migrated inward from the surrounding rock-mass, for 

 each segregation is enclosed in a white, decolourized shell of quartzite, consist- 

 ing of nearly pure quartz and feldspar. In each of the larger segregations 

 the mica and garnet are not regularly distributed with reference to the 

 periphery but occur in numerotxs small clumpy aggregates within the main 

 body of the segregation. In some of the smaller segregations the micas are 

 more evenly distributed, in a manner similar to that observed in concretions 

 in the Kitchener quartzite. 



At a few other horizons the feldspathic quartzite is spangled with large 

 biotite foils up to 1 cm. in diameter. These cut across the bedding plane at 

 all angles. The cause of their growth and of their restriction to a very limited 

 number of strata in the great, apparently homogeneous series is not under- 

 stood. Neither special dynamic metamorphism nor the thermal metamorphism 

 of igneous intrusives were feasible explanations for the spangled quartzite at 

 the localities where it was actually discovered. As a rule the Creston quartzite 

 is not cleaved, but in the fault-block at the Moyie river there is a distinct 

 cleavage crossing the bedding planes at relatively low angles. In this case 

 sericite is developed in the secondary planes as well as along the bedding. 



Eastern Phase. — The western phase just described characterizes the forma- 

 tion as it crops out in the Boundary belt between Porthill and the Moyie river. 

 Eastward of the river the Creston gradually assumes the features which are 

 normal to the eastern phase. The latter is typically developed at the Yahk 

 river, where the formation finally disappears beneath younger rocks. 



The most important lithological contrasts with the western phase consist 

 in:— »first, a decided decrease in the average thickness of the beds, often leading 

 to a fine lamination at many horizons; secondly, a pronounced increase in the 

 amount of argillaceous matter which here forms many distinct beds and also 

 occurs ^s a notable impurity in the still dominant quartzite; and thirdly, the 

 appearance of calcium and magnesium carbonates as subordinate elements in 

 both the quartzite and the more argillaceous strata. The increase of the 

 carbonate manifests itself in the rock-ledges, which, on account of the special 

 solubility of the carbonates, present, to sight and touch, a characteristic rough- 

 ness on weathered surfaces. In general, the calcium carbonate seems to be in 

 some excess over the magnesian carbonate, as shown by a certain amount of 

 effervescence with cold dilute acid. 



In order to obtain a definite idea as to the composition of the eastern 

 phase, type . specimens were collected at the Yahk river section and have been 

 studied microscopically. One of these specimens, taken from a large outcrop 



