542 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



may believe that the porphyrite is an off-shoot of the same magma as the diorite. 

 That relation would be parallel to the one just postulated for the neighbouring 

 rnonzonite dikes and the Chilliwack granodiorite. In fact, it seems simplest 

 to suppose, first, that all four rock-types belong to one eruptive period, the 

 more basic intrusions antedating the acid intrusions by only a short interval 

 of time; and, secondly, that all four rocks were differentiates from one great 

 m agma-chamber. 



Dikes cutting the Chilliwack Batholith. 



Two different kinds of acid dikes cut the Chilliwack granodiorite. One of 

 these kinds seems to be merely a later expression of the same magma from which 

 most of the batholith itself was crystallized. Such dikes are not common and 

 were never found far from the main batholithic contacts. This fact suggests 

 that the batholithic magma first solidified along the contacts and that this early 

 formed shell was injected by dikes from the still molten interior of the mass. 

 Four of these dikes were observed on the ridge north of Depot creek. They are 

 all composed of light gray granodiorite porphyry, somewhat more acid than the 

 staple quartz-diorite of the contact-shell into which they have been intruded. 



Acid dikes of the second kind also specially affect the borders of the batho- 

 lith but occur in the interior as well. They are not numerous and rarely attain 

 widths greater than four feet. They are light pinkish-gray to whitish, fine- 

 grained granites of aplitic habit. The essential constituents are quartz, micro- 

 perthite, orthoclase, andesine (Ab 5 An 3 ), and biotite: titanite, magnetite and 

 apatite are accessory. The structure is the hypidiomorphic-granular. The rock 

 is an alkaline biotite granite, verging on biotite aplite. Its relation to the 

 granodiorite recalls the similar succession of granites — acid, alkaline and micro- 

 perthite-bearing biotite granite succeeding granodiorites — in the Okanagan and 

 Selkirk ranges, as so often in other granitic provinces of the Cordillera. 



Two classes of basic, dikes cut the granodiorite. So far as known, the one 

 class is represented only in one 10-foot, nearly vertical dike at about the 5,000- 

 foot contour on the southern slope of Pyramid mountain (the high conical peak 

 northwest of the outlet of Chilliwack lake). This rock is fine-grained, dark 

 greenish-gray, and of lamprophyric habit. Under the microscope it is seen to 

 have the composition and structure of an acid camptonite. 



The other kind of basic dikes has been recognized at several points, but 

 always in bodies of small size; no one of them is known to be wider than two 

 feet. Tour of these dikes were found at a point on the same southern slope of 

 Pyramid mountain at about the 4,100-foot contour. A fifth was encountered in 

 the gulch running eastward from the southern end of Chilliwack lake and at a 

 point about 2,200 feet above the lake. All of the dikes are greatly altered and 

 their diagnosis is not easy. One of the thin sections showed, however, some 

 residual augite intersertally placed in a web of basic plagioclase, the only other 

 primary essential. The quantities and relations of the minerals as well as their 

 alteration phenomena show pretty clearly that these dikes are normal diabase. 



