544 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



A mass of composition similar to that of the just mentioned gabbro occurs 

 as a sill or great dike, cutting the Paleozoic strata at the high cliff, facing the 

 mouth of Middle creek on the north side of the Chilliwack river and about 

 2,000 feet above the river. 



In general relations, chemical composition, and degree of metasomatic 

 alteration all of these smaller bodies are much like the Yedder greenstone 

 and they may be tentatively correlated with it both in age and origin. 



STRUCTURAL, RELATIONS. 



In structure and composition the Skagit range is, in many essential respects, 

 analogous to the Columbia mountain system. Here, however, the Paleozoic 

 rocks are less intensely crumpled and metamorphosed. 



The Skagit range is structurally divisible into two parts. Prom the Skagit 

 river to Middle creek it is chiefly composed of intrusive granites or allied rocks, 

 which occur in such large numbers and so differing in age that we may fitly 

 call the whole plutonic group the Skagit composite batholith. The oldest 

 member of the batholith is unconformably overlain by the Skagit volcanic 

 group. A remnant of the Hozomeen formation appears as a second rock-body 

 which is not part of the composite batholith but is a part of its country-rock 

 terrane. -i ; ji 



West of the Slesse diorite the mountains are made of dominant sedimentary 

 rocks. The Paleozoics (Chilliwack series) are very thick, the suggested mini- 

 mum of about 6,800 feet of strata being, perhaps, much below the real thickness 

 of the rocks actually exposed. An unknown additional thickness of conformable 

 strata underlies those beds; thus no base is known to the Paleozoic (largely 

 Upper Carboniferous) sediments of the west slope. The heavy mass of basic 

 (andesitic) lavas and pyroclastics, named the Chilliwack Volcanic formation, is 

 plainly contemporaneous with the fossiliferous uppermost beds of the series. 

 The Triassic argillites and sandstones of the Cultus formation are not well 

 exposed, but they seem also to be of imposing thickness: It is not known whether 

 they are conformable with the Paleozoics, but an unconformity is suspected. 

 Very little of the terrane called the Tamihy series and tentatively equated with 

 the Pasayten series, occurs within the Boundary belt, and it has not been specially 

 studied. It is unconformable upon the Upper Carboniferous and probably upon 

 the Cultus Triassic beds as well. The Eocene ( ?) Huntingdon formation forms 

 only a small patch on Sumas mountain; it is unconformable upon the Paleozoic 

 quartzite and also upon the Sumas granite, provisionally assigned to the Upper 

 Jurassic. 



Throughout the whole width of the range simple folds are extremely rare. 

 A much broken syncline, pitching gently eastward from the summit of McGuire 

 mountain, is one of the very few decipherable structures in the mountains of the 

 Boundary belt. The Chilliwack river, between Slesse and Tamihy creeks, seems 

 to be flowing on the axis of a broken anticline, the east-west axis of which 

 pitches eastward at a low angle. The southern limb of this arch is also the 



