526 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



The explanation of the banding is here the same as that offered for the 

 banding of the Osoyoos, Remmel, and other batholiths farther east. The light 

 bands represent the intensely granulated diorite from which the hornblende, 

 biotite, basic plagioclase, and accessories have been slowly leached during the 

 shearing of the batholith. The dark bands represent the shear-zones in which 

 the same basic materials were recrystallized. In many cases there has also been 

 some recrystallization of the light bands with the development of new quartz, 

 biotite, feldspar, and some garnets. 



Sumas Granite and Diorite. 



Rather more than one-half of Sumas mountain is composed of plutonic 

 igneous rock. Nine square miles of the central and northeastern parts of the 

 mountain are underlain by a biotite granite which may be called, for conven- 

 ience, the Sumas granite. An area of about three square miles is underlain 

 by a plutonic breccia. This breccia consists of a vast multitude of blocks of a 

 dioritic rock cemented by the Sumas granite; the whole forms a peripheral 

 intrusion-breccia on a large scale. The diorite is evidently the older of these 

 two rocks and may be called the Sumas diorite. 



On the north and east the plutonic masses disappear beneath the Fraser 

 valley alluvium. On the northwest the granite makes contact with a hard, 

 massive quartzite, into which it is intrusive. On the southwest the granite is 

 unconformably overlain by the nearly fiat Eocene ( ?) beds. 



Granite. — The granite is pre-Eocene in age. The date of intrusion cannot 

 jet be more closely fixed with certainty. The rock is nowhere crushed in any 

 notable way. It seems therefore doubtful that it was intruded before the great 

 orogenic revolution of the Jurassic and the date may be tentatively fixed as 

 later Jurassic, or (less probably) Cretaceous. The diorite of the intrusion- 

 breccia is also massive and unsheared and may belong to the same general 

 period of igneous action, though of course, being older than the granite. 



The granite is a light pinkish-gray, fine, to medium-grained rock, poor in 

 dark constituents. The composition and structure are both those characteristic 

 of mica granites. Quartz, orthoclase (sometimes slightly microperthitic), basic 

 andesine, averaging Ab 4 An 3 , and biotite are the essentials. A pale green horn- 

 blende, magnetite, apatite, titanite, and rare zircons are accessory. In all of 

 the four specimens collected, the rock is seen to be considerably altered. The 

 alteration is so marked even on. well glaciated ledges that one may possibly 

 refer it in largest part to the secular weathering which preceded the deposition 

 of the Eocene beds. The feldspar is often much kaolinized and the biotite is 

 generally chloritized to some extent. 



The freshest specimen (No. 201) has been analyzed by Professor Dittrich 

 with the following result: — 



