REPORT OF TEE CHIEF ASTRONOMER 521 



SESSIONAL PAPER No. 25a 



igneous rock formations. 



Chilliwack Volcanic Formation. 



On Tamihy creek about three miles below the Boundary-line crossing, a 

 large body of altered basic lava was discovered during tbe season of 1901. It 

 was followed soutbwestwardly to tbe top of a very rugged ridge where the lava 

 was found in close association with strong beds of obscurely fossiliferous lime- 

 stone dipping under the lava. It was, however, not until the Commission trail 

 west of the creek was opened up that the writer was (in 1906) able to secure 

 definite evidence as to the age of the lava and as to its relation to the sedimen- 

 taries. A few hundred yards south of Monument 48, the southern limit of the 

 lava was found. It there makes direct contact with a fifty-foot bed of fossili- 

 ferous limestone similar in habit to that at the lower contact of the lava. From 

 the fossils collected in the upper limestone Dr. Girty has concluded that this 

 limestone is certainly Paleozoic and in all probability upper Carboniferous in 

 age. The limestone at the base of the lava formation is likewise apparently 

 upper Carboniferous. Since the lava is conformably intercalated between the 

 two limestones, it must also be referred to the upper Carboniferous. From its 

 position in the limestone one may fairly conclude that the eruptions were 

 wholly or in part submarine. 



Westward from Tamihy creek the band of old lavas was followed for a 

 distance of some ten miles. Throughout most of that stretch the northern con- 

 tact of the lava lies only a few hundred yards south of the Boundary slash. The 

 best exposures are on the ridge southwest of Tamihy creek. The formation may 

 be here called the ' Chilliwack volcanics.' The total thickness can be only roughly 

 estimated but it must be at least 2,000 feet. 



The formation consists mainly of thick, massive flows, which are so welded 

 into one another and so altered, as to make the individual flows very hard to 

 distinguish in the field. Among the flows a notable, though subordinate amount 

 of ash-bed material is intercalated. At no point, however, were the conditions 

 favourable for working out a detailed columnar section of the formation. 



Although every effort was made to secure the freshest material for study, 

 it was found that all of the twenty type-specimens collected were greatly altered. 

 The exact petrographic nature of the different flows is therefore obscure. The 

 net result of the microscopic . study of nearly all of the twenty specimens went 

 to show that two rock-species are represented — augite andesite and hornblende 

 andesite. The former is probably the more abundant. 



The lavas are often amygdaloidal, with calcite generally filling the pores. 

 Very often the andesites have been altered into typical greenstones, or, where 

 the shearing has been particularly intense, into green schists. In a few speci- 

 mens, the augite has the relations and abundance observed in olivine-free basalts. 

 Olivine was not found in any thin section, but its absence may be due to the 

 profound alteration of the lavas. Some of the specimens carry quartz in the 

 ground-mass but it may all be of secondary origin. 



