522 DEPARTMENT OF TWE INTERIOR 



2 GEORGE V., A. 1912 



At Tamihy creek about five miles from the Chilliwack river, a 100-foot 

 bluff of rhyolite was discovered. The density of the forest-cover in- the vicinity 

 rendered it impossible to determine the relation of this rock to the Paleozoic 

 sediments or to the adjacent Chilliwack andesites. The writer conjectures that 

 the rhyolite is a lava flow occurring at or near the base of the great andesitic 

 series and the rhyolite is tentatively included in the Chilliwack Volcanic for- 

 mation. The rhyolite seems to be about 100 feet thick or even more but it 

 could not be followed far in any direction. ISTo similar acid rock was found in 

 the sections of the formation farther west. 



The rhyolite is peculiar in being coloured almost black by an abundant 

 material rather uniformly distributed through the ground-mass. This substance 

 is quite opaque, dead-black and amorphous and has a dust-like appearance in 

 the thin section. It is certainly not an iron-oxide. The rock decolourizes 

 before the blow-pipe and it seems almost assured that the black dust is carbon. 

 The occurrence of this element in a rhyolitic lava is unusual and the writer 

 can find no record of its having been found in rhyolite or porphyry elsewhere, 

 in anything like the abundance observed in this case at Tamihy creek. 



Vedder Greenstone. 



The northwestern slope of Vedder mountain ridge is underlain by an altered, 

 basic igneous rock which seems to be intrusive into the Paleozoic argillites and 

 sandstones of the ridge. As exposed the body forms a remarkably long and 

 straight band, running from the head of the Chilliwack river alluvial fan to the 

 International line south of Sumas lake. The body was not followed farther to 

 the southward. As shown on the map the known length of the mass is 

 more than ten miles. On the northwest, for most of its length it is covered by 

 alluvium, so that the exact shape and relations of the body cannot be deter- 

 mined. 



At the point nearest to Sumas lake the igneous rock is bounded on the 

 northwest by a narrow belt of dark-gray argillite, cropping out at intervals for 

 about 700 yards along the wagon-road. Here the argillite seems to dip south- 

 eastward and thus under the intrusive rock, at an average angle of 65°, while at 

 the southeastern contact on the summit of the ridge, the dip of the argillite is 

 about 40° to the east-southeast. At this point, therefore, the intrusive appears 

 to have the relation of a great sill, injected into a bedding-plane of the sedi- 

 ments. The width of the outcropping igneous mass is about 1,000 yards. 



Elsewhere in the ridge the dips and strikes of the strata, always highly 

 variable, show no such simple relation to the intrusive. The singular straight- 

 ness of the southeastern contact suggests that the body is a gigantic dike, and 

 this view is tentatively adopted. An intrusive character is inferred more from 

 the petrographic nature of the mass and from its position in the sedimentary 

 terrane than from the usual criteria of apophyses, inclusions, and contact-meta- 

 morphism. Owing to the dense brush and heavy mat of moss and humus, not a 

 single, actual contact was discovered. 



