REPORT OF THE CHIEF ASTRONOMER 365 



SESSIONAL PAPER No. 25a 



According to the older classification the rock is a typical augite-biotite 

 syenite porphyry. 



Nearer the contact of the chonolith the porphyry assumes a much finer 

 grain and a deeper colour, namely, dark greenish-gray. The phenocrysts are 

 soda-orthoclase, plagioclase, and biotite. Augite is absent, both among the 

 phenocrysts and in the ground-mass. The phenocrystic plagioclase regularly 

 affords the extinction-angles of lalbradorite (Ab 3 An, to Ab a AnJ and seems, 

 therefore, to be persistently more basic than the plagioclase of the augite-bear- 

 ing phase. The ground-mass, in everything but size of grain, seems to be like 

 the ground-mass of the analyzed rock. This augite-free phase is an alkaline 

 biotite syenite porphyry. Its specific gravity was measured and found to be 

 precisely the same as that of the augite-biotite syenite porphyry, namely, 2-667. 



Dikes. — Most of the porphyry dikes of the region carry augite among the 

 phenocrysts and, in mineralogical and chemical characters, are practically 

 identical with the analyzed phase of the chonolith. A few dikes have horn- 

 blende in place of augite and a few others carry only biotite as femic pheno- 

 crysts. The dikes are exposed in great size and number on the west slope of 

 Sophie mountain and some of them are clearly connected with the mineraliza- 

 tion of the rocks whence the Velvet and Portland mines have drawn their ore- 

 supplies. (See R. W. Brook, Summary Report, Geol. Survey of Canada for 

 1900, page 75 A). All these injected porphyries are at least as recent as the 

 Coryell batholith intrusion and may be contemporaneous with it. 



A dozen or more syenite porphyry dikes, ranging from 8 to 20 feet 

 in width, cut the Sheppard granite stock south of Lake mountain. The 

 microscopic examination of one specimen has shown a strong resemblance to 

 the porphyry of the dikes and chonolith west of Sophie mountain. So far as 

 such a fact may be used for correlation, it affords evidence that the syenite 

 porphyry displayed in the fringe of dikes south of the Coryell batholith and 

 perhaps the Coryell syenite itself are younger than the Sheppard granite. This 

 view is corroborated by the fact that the Sheppard granite stock on the north 

 side of the Pend D'Oreille river is cut by numerous dikes of a rock which 

 appears to be greatly altered porphyritic olivine-augite-biotite monzonite of 

 gabbroid habit, i.e., dikes which may possibly be correlated with a younger 

 member of the Rossland volcanic group. So far as known, the syenite por- 

 phyry dikes, though abundant and often well exposed, are never cut by gabbroid 

 or monzonitic rocks nor is the Coryell batholith cut by them. 



The evidence is thus fairly good that these syenitic rocks are, next to the 

 granite porphyry, now to be noted, the youngest intrusives of the region. 



A few dikes of gray biotite-granite porphyry cut the Rossland volcanics 

 and at two points, clearly cut dikes of the syenite porphyry. One of these two 

 localities is immediately northeast of the section through the chonolith just 

 described, and it is probable that the syenite porphyry traversed by the granite 

 porphyry forms part of the chonolith itself. This granite porphyry dike is 

 only five feet wide but bears orthoclase phenocrysts up to 2-5 cm. in length, 



