THE ORIGIN OF THE INCLUSIONS IN DIKES 



175 





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bearing dike was observed by the writer at Rossland, British 

 Columbia. The dike, which is known locally as the ''White Dike," 

 is 6 feet in width and consists of a mass of large bowlders imbedded 

 in an augite kersantite matrix. Two types of bowlders were 

 observed in the outcrop; one a white, fine-grained rock which was 

 mistaken in the field for a quartzite, the other a bluish-grey rather 

 coarse-grained rock with many small dark-brown patches. The 

 texture of both is gneissic. A microscopic examination has shown 

 that both types are anorthosite, the former consisting almost 

 entirely of labradorite (Ab4o,An6o), the latter of similar labradorite 

 with small amounts of pyroxene, probably an orthorhombic variety 

 with positive optical character. 

 The pyroxene has in part altered 

 to biotite and chlorite. Magnet- 

 ite occurs in small amounts in 

 both specimens. They both show 

 the effects of some dynamic move- 

 ment. 



The inclusions vary in size, as 

 shown in Fig. 2, the larger being 

 I to 2 feet long. In shape they 

 are partly angular, but in one of 

 the surface exposures they are 

 subangular or well rounded and 

 do not have sharp edges, nor do 

 the edges show any effect of corrosion. The inclusions weather out 

 of the matrix, but slightly fused contacts are shown in the weathered 

 material. The dike-rock consists of basic andesine or labradorite 

 and augite phenocrysts in a matrix of labradorite, augite, biotite, 

 and some magnetite. 



The dike is in the extreme western portion of the Rossland dis- 

 trict, about 5 miles north of the international boundary. It is 

 shown on the Candian Geological Survey map of Rossland, at the 

 western edge of the map, outcropping in a cut of the Great Northern 

 Railway and at a flume 1,000 feet to the northwest. It cuts por- 

 phyritic monzonite in the first outcrop and augite porphyrite in the 

 second. It has also been found in the 900-foot level of the Josie 



Fig. 2. — ^An exposure of the "White 

 Dike" at Rossland, British Columbia, 

 showing the abundance of inclusions. 

 The sketch has been traced from a 

 photograph. The scale is given by the 

 width of the dike, which is 6 feet. 



