REPORT OF THE CHIEF ASTRONOMER 525 



SESSIONAL PAPER No. 25a 



is sometimes not easy to distinguish such bands from the younger pegmatitic 

 dikes (off-shoots from the Chilliwack batholith?) cutting the banded granite. 

 In most cases, however, the pegmatitic habit of the light bands is apparently 

 due to some recrystallization of the original rock of the Custer batholith itself. 



In thin section these light bands were seen to consist of dominant quartz, 

 microcline, and orthoclase, with subordinate oligoclase (generally untwinned or 

 poorly twinned), and biotite. A few, small, pink garnets, rare crystals of zircon 

 and apatite, and small anhedra of titanite are accessory. Both hornblende and 

 free iron oxide seem to be entirely absent. These bands have, thus, the composi- 

 tion of many acid, aplitic granites poor in biotite. The component minerals are 

 generally strained and the cataclastic structure is usual. The specific gravities 

 of two fresh specimens of the light bands were, respectively, 2-655 and 2-641. 



The dark bands are of three kinds, according to the character and propor- 

 tions of the constituents. The commonest kind is a dark greenish-gray, foliated, 

 medium-grained, highly biotitic rock, composed of dominant plagioclase (basic 

 andesine), biotite and quartz, with rare orthoclase. A few grains of garnet, 

 some magnetite, and apatite are accessory. One specimen showed a specific 

 gravity of 2-732, but many bands, yet richer" in biotite, would be heavier. Only 

 one thin section of this type — a biotite-diorite gneiss — was studied. It showed 

 neither granulation nor pronounced straining of the component minerals, and it 

 seems necessary to believe that the material of these dark bands was crystallized 

 in its present form during the metamorphism of the batholith and has not since 

 been subjected to extraordinary orogenic stress. 



Dark bands of the second kind differ from those of the first in carrying 

 essential hornblende as well as biotite in large amount. No special study has 

 been made of these, but they doubtless have the same principal features as the 

 biotite-diorite gneiss, excepting for the entrance of essential hornblende. Bands 

 of this class have the composition of basic hornblende-biotite diorite gneiss. 



Basic bands without essential biotite are uncommon but were noted at 

 several points. In these green hornblende is the only important femic mineral. 

 Basic labradorite is the only other essential constituent. Much apatite, very 

 abundant, well crystallized titanite, and some pyrite are the observed accessories. 

 The specific gravity of a somewhat altered specimen is 2-888. Bands of this 

 third class seem to range in composition from amphibolite to hornblende-diorite 

 gneiss. A few of them may possibly be sheared basic dikes cutting the batholith, 

 but the majority, like the other dark bands, must be regarded as forming meta- 

 morphic phases of the sheared batholithic rock. 



The Custer batholith thus includes the following species of rocks :— 



Original type: granodiorite. 

 Secondary, metamorphic types : 



Biotite-aplite gneiss; 



Basic biotite-diorite gneiss; 



Basic hornblende-biotite-diorite gneiss; 



Basic hornblende-diorite gneiss; 



Amphibolite. 



