524 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



If the Hozomeen series is of Carboniferous age, the Custer batholith must 

 have been intruded in late Carboniferous or post-Carboniferous time. The 

 general relations and metamorphosed condition of the batholith point to a pre- 

 Tertiary date of intrusion. The similarity in these respects to the Remmel and 

 Osoyoos batholiths has led the writer to place the date tentatively in the Jurassic, 

 thus making all three batholiths essentially contemporaneous. It is obviotis, 

 however, that such correlations among the older batholiths must be held with a 

 very open mind, for they are founded largely on simple conjectures as to the 

 ages of the metamorphic rocks cut by these batholiths. Until fossils are actually 

 found in the Hozomeen series there is nothing to compel the view that the 

 Custer batholith is of late Paleozoic age; it may, indeed, be an uplifted frag- 

 ment of an old pre-Cambrian terrane. 



In the field the batholith has all the appearance of a typical pre-Cambrian 

 gneiss. It is seldom quite massive, and at no known point has it escaped more 

 or less powerful crushing and shearing. As a rule, the original rock has been 

 converted into a well-banded gneiss, very similar to that produced in the meta- 

 morphosed Cascade and Remmel batholiths farther east. 



Original Rock Type. — The original «rock seems to have been a grano- 

 diorite. Because of the intense metamorphism of the whole body, it is not 

 possible to distinguish the primary phases into which the batholithic magma 

 crystallized. Indeed, there are few places where the crushing and chemical 

 rearrangement of the mass were slight enough to leave remnants of the original 

 granite. One such locality was found at the head of Depot creek and about one 

 mile north of the Boundary line. The rock there is crushed and somewhat 

 gneissic, but it is not banded. It is of a darkish gray colour and of medium 

 grain. The hand-specimen shows the presence of much hornblende, less biotite, 

 and little quartz. Erom the persistently white to gray tint of the dominant 

 feldspar one would suspect, from the macroscopic appearance, that one were 

 dealing with a plagioclase-rock. 



That conclusion is corroborated by the study of thin sections. The essential 

 minerals, named in the order of decreasing abundance, are : plagioclase, varying 

 from basic andesine, near Ab 4 An 3 to basic oligoclase near Ab 2 A^ ; dark green 

 hornblende; orthoclase and microcline; quartz, and biotite. The usual acces- 

 sories, magnetite, apatite, and titanite, are present. The essential minerals all 

 show straining. The plagioclase lamella? are often bent or broken, and some of 

 the orthoclase has been converted, by pressure, into microcline. Nevertheless, 

 there is no doubt that the specimen just described represents a common phase, 

 and probably the dominant phase, of the original batholith. With this grano- 

 diorite type the materials making up the bands which form the staple rock of 

 the batholith at present, are in striking contrast. 



Banded Structure. — As in the case of the Remmel, Osoyoos, and Cascade 

 batholiths, the bands are here often of stratiform regularity. They may be 

 divided into two classes : one acid-aplitic, the other basic in composition. 



The acid bands are light-gray to whitish or very pale pink in colour. The 

 grain varies from rather fine to quite coarsely pegmatitic. In the latter case it 



