386 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



which it cuts across the bedding of the older rocks and in the contact 

 metamorphism it produced. In places traces of the structure of the original 

 eruptive rock can be made out in the serpentine. In Central camp the 

 serpentine is occasionally somewhat fibrous, approaching asbestos. Near 

 the Koomoos-McCarren Creek divide it seems to pass into a soapstone or 

 talc. Often it is altered to a rusty aggregate of dolomite (and perhaps 

 other carbonates) and white quartz veins. It is doubtful if all the serpen- 

 tine in the district is of one age. Boulders of serpentine are found in the 

 green volcanic conglomerates which would indicate that some of it was 

 older than these pyroclastic rocks. On the other hand, some of it seems 

 to be intrusive in the green porphyrite which is of a little later age than 

 these volcanic conglomerates.' 



No detailed petrography of the serpentine has been published. The fact 

 that the majority of the perfectly analogous serpentine masses of the E-ossland 

 district, and, as we shall see, of the Rock Creek district west of Midway, have 

 been derived from typical dunite, it appears that the ' Boundary Creek district ' 

 serpentines have also originated from intrusions of that rock type. The serpen- 

 tines of all three districts have been correlated and seem to be best regarded as 

 genetically associated with the great andesitic (porphyrite) extrusive masses of 

 the respective districts. 



Granodiorite. 



Finally, it remains to note the many small stocks and other intrusive 

 bodies of granodiorite, which is the youngest rock in the five-mile belt between 

 Grand Forks and Midway except the Midway volcanic formation itself. Mr. 

 Brock's summary account of the rock may be given in full (Report for 1902, 

 pp. 98-99):— 



' At various points throughout the whole district, bosses, irregular 

 masses and dykes of a light gray granitoid rock make their appearance. It 

 is a quartz-bearing biotite-hornblende rock, in places apparently granitic, in 

 others rather dioritic. It is probable that it will prove to be, generally, 

 ' a granodiorite. It sends out numerous dykes throughout the country, 

 especially in the southern portion of the district. These have usually a 

 porphyritic structure with a micro-granitic groundmass. Some are granite 

 porphyries, but a great number are quartz-diorite-porphyrites, as are also 

 some of the smaller bosses. On McCarren creek, north side, are some basic 

 hornblende gabbro-porphyritic dykes which may belong to the same intru- 

 sion. In places these shade off into pure hornblende rocks. 



' This granodiorite is evidently intrusive, cutting all the rocks above 

 mentioned. The mechanism of its intrusion is extremely interesting, for 

 it unquestionably forced its way up through the overlying rocks by digest- 

 ing them and rifting off fragments. This is proved by its contacts, both 

 along the sides and roofs of the masses. These are, except in the case of 

 the dykes, rarely sharply defined, but are irregular and suture-like. The 



