2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER XVIII. 



FORMATIONS OF THE SKAGIT MOUNTAIN RANGE. 



General Statement. 



From the Skagit river to the great gravel plain traversed by the lower 

 Fraser river, the Boundary belt crosses a large number of distinct geological , 

 formations which range in age from the Miocene to the Carboniferous, if not 

 to the pre-Cambrian. The oldest fossiliferous sediments so far discovered 

 date from the Upper Carboniferous; these belong to a thick group of rocks 

 (named the Chilliwack series) most of which are believed to be Carboniferous. 

 The as yet unfossiliferous Hozomeen series crops out in the area east of Chilli- 

 wack lake; these rocks are probably contemporaneous with certain phases of 

 the Chilliwack series. A very thick andesitic group forms the upper part of 

 the Chilliwack series as exposed near Tamihy creek and will bear the special 

 name, Chilliwack Volcanic formation. A peculiar intrusive, dike-like mass of 

 highly altered gabbroid rock, forming the western part of Vedder Mountain 

 ridge, may be called the Vedder greenstone. Triassic argillites showing great 

 thickness in the region east of Cultus lake have been grouped under the name, 

 Cultus formation. Southwest of Tamihy creek canyon a group of conglomer- 

 ates and green, massive sandstones, to which the name Tamihy series is given, 

 seems to represent the equivalent of the Pasayten series farther east. On Sum as 

 mountain, north of Huntingdon railway station, fossiliferous sandstones and 

 conglomerates, named the Huntingdon formation, seem to represent the Eocene 

 Puget group. It overlies nnconformably a body of intrusive diorite cut by a 

 biotite granite, which will bear the respective names, Sumas diorite and Sumas 

 granite. These intrusives may be contemporaneous with a b'atholithic mass of 

 greatly sheared granite occurring on and near Custer ridge at the main divide 

 of the range; this body will be referred to as the Custer granite-gneiss. It 

 seems to cut the Hozomeen series and is provisionally assigned to a Jurassic 

 date of intrusion, but this truly old-looking rock may really represent a pre- 

 Cambrian terrane. The eastern slope of the range at the Boundary line forms 

 ar. area where, possibly in early Tertiary time, vigorous volcanic action built 

 a thick local accumulation of andesitic breccias, associated with flows and with 

 more acid lava; to the whole group the name, Skagit Volcanic formation, 

 may be given. The remaining, specially named bodies in the range are the 

 Slesse diorite and the Chilliwack granodiorite, both of which are in batholithic, 

 intrusive relation to the Chilliwack series. The former occurs on Slesse creek; 

 the latter forms the bed of Chilliwack lake and spreads far out on all sides. 

 Both bodies are believed to be of mid-Tertiary age. (See Maps No. 15 and 16.) 



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