492 • DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



and smaller ones of green hornblende. The ground-mass is feldspathie and 

 macrocrystalline, bearing many microlites of the green hornblende. The feld- 

 spar is murky with alteration-products but seems to be an acid labradorite. 

 The rock may be classed as a basic hornblende porphyrite. 



Three miles down-stream from the Boundary slash Castle creek crosses a 

 40-foot sill of basic rock clearly intrusive into the green Pasayten sandstone. 

 This sill is lithologically related to the dike (?) just described but is more 

 basic (spec, gravity of a type-specimen, 2-950). The rock is dark-green, coarse- 

 grained and almost peridotitic in appearance. Macroscopically only one con- 

 stituent, hornblende, is clearly visible. Under the microscope this mineral is 

 seen to be extremely abundant, composing more than half of the rock by weight. 

 It occurs in thoroughly idiomorphic prismatic crystals measuring as much as 

 10 mm. or more in length. These phenocrysts are embedded in a fine-grained 

 matrix of soda-lime feldspar (probably labradorite) with which a little magne- 

 tite, apatite, and interstitial quartz are associated. The largest feldspar laths 

 are about 0-5 mm. long, and the average lath is much smaller. 



The rock may be classified as a hornblende porphyrite but it is an anomal- 

 ous member of that species, carrying an extremely high percentage of horn- 

 blende. Chemically this porphyrite must be much like the peculiar gabbro of 

 the Moyie and other sills of the Purcell range. 



Concerning the date or dates of these small porphyrite intrusions no more 

 can now be said than that they are both post-Lower Cretaceous. They may well 

 be specially basic derivatives of the magma represented in the Lightning Creek 

 diorite stocks. 



Castle Peak Stock. 



Its Special Importance. — A brief description of the Castle Peak stock was 

 given in the preliminary paper on the Okanagan composite batholith. The 

 outcrop of the body covers about ten square miles; it is the largest intrusive 

 mass exposed in the Hozomeen range where crossed by the Boundary belt. The 

 stock deserves special study and will be rather fully illustrated, for its contacts 

 are more perfectly displayed than are those of any other stock or of any 

 batholithic mass in the entire Forty-ninth Parallel section. Erosion has bitten 

 deeply into the formations composing this part of the range. The upper slopes 

 of Castle Peak and of the neighbouring mountains are above tree-line. For a 

 double reason, therefore, the geologist can see relatively far down into the 

 depths where this granitic body was intruded. At various points around the 

 periphery of the stock the contact-surfaces can be followed downwards with the 

 eye for a thousand or more feet, measured vertically. It happens also that the 

 folded Cretaceous strata forming the country rock of the intrusive are so 

 arranged that the relations to the stock in plan are as plainly evident as the 

 relations in vertical sections. Since all these structural relations are of 

 primary importance to the theory of the intrusion and since the contact- 

 relations of stocks and batholiths are very seldom seen with equal clearness, the 



