410 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



tions were not determinable, but the mass appears to form a cross-cutting, 

 injected body and may fall in the chonolith class rather than in that of dikes or 

 sills. 



A third large area of the porphyry crops out on the slope north of Rock 

 creek and west of the large chonolith. It is quite possible that this more westerly 

 mass really forms part of the chonolith and that the two are connected under- 

 ground. They are separated by Tertiary conglomerate overlying pre-Tertiary 

 granodiorite. 



Petrographically, these porphyries are practically indistinguishable from the 

 porphyry of the large Rock Creek chonolith. Both the ' central ' and 'contact- 

 chilled ' phases are represented with typical characters. 



The Tertiary sandstones of the wagon-road section four miles west of 

 Midway and those exposed east of the bridge two miles to the northward, are cut 

 by dikes and sills of the porphyry. In all these thinner bodies the porphyry has 

 the habit of the ' chilled ' phase of the chonolith above described. A small 

 amount of nephelite is almost always present ; it occurs in the ground-mass and 

 often forms small idiomorphic, phenocryst-like crystals. In no case, however, 

 does the nephelite rival the rhomb-feldspar, the augite, or the biotite in abun- 

 dance. As a rule the ground-mass is not vitrophyric and in this respect the dike 

 and sill-rocks are more like the central phase of the Rock Creek chonolith. 



Other dikes or sheets appear to cut the basalts on the heights north of the 

 Kettle river above the bridge. Still others cut the Anarchist schists at various 

 points between the Rock Creek chonolith and Osoyoos lake. The most westerly 

 occurrence discovered is that on the summit four miles east of Osoyoos lake and 

 1-5 miles north of the Boundary line. At that point the Anarchist phyllite- 

 amphibolite complex is cut by several north-south, nearly vertical dikes of the 

 porphyry, several of which approximate a hundred feet in width. These dikes 

 are twelve miles or more west of the Rock Creek chonolith and twenty miles west 

 of the most easterly dikes of the rhomb-porphyry within the Boundary belt. The 

 relatively wide distribution of this peculiar rock-type is paralleled in the classic 

 Norwegian district. Here as there, similar conditions of magmatic differentia- 

 tion and of crystallization seem to have prevailed over a large area. In neither 

 case, however, does it appear necessary to believe that the region was underlain 

 by a continuous, deep couche of the magma respectively represented by the por- 

 phyry actually intruded. It is at least as probable that each of these porphyry 

 types is the product of splitting from one or more deep-seated masses of more 

 usual composition. 



Extrusive Phase of the Rhomb-porphyry. 



Along its northern edge the normal porphyry of the Rock Creek chonolith 

 lies in contact with a highly vesicular and doubtless extrusive phase of the 

 same rock. The relation between the two is very obscure. In the field no 

 sharp line of demarcation, separating the phases, can be drawn. The same is 

 true of the contact between the lava and the intrusive body of porphyry west 

 of the large chonolith. It is possible that the phases merge into each other, 



