104 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V„ A. 1912 



sericitic mica on the bedding planes, through static metamorphism, shows that 

 the rock has suffered some change. It may, for convenience, be called meta- 

 sandstone. From that type all transitions to true metargillite are 'represented 

 in the formation. 



The specific gravity of the metasandstone varies from 2-619 to 2-653, 

 averaging about 2-634; the specific gravity of a typical specimen of metar- 

 gillite is 2-711. The average for the formation as a whole is about 2-65. 



The Wigwam formation is evidently the western equivalent of the Grinnell 

 formation of the Clarke and Lewis ranges. It differs from the Grinnell in the 

 possession of coarser grain, in the greater predominance of sandstone, and in 

 its smaller thickness. 



Siyeh Formation. 



The general equivalence of the Galton and Lewis series is, lithologically, 

 most evident in the thick formations respectively overlying the Wigwam and 

 Grinnell beds. The similarities of age, composition, structure, and origin are 

 so manifest in the field that the same name is here adopted for the formation 

 as in the Clarke-Lewis sections. The principal petrographic difference is found 

 in the greater prominence of argillaceous matter in the Siyeh of the Galton 

 range. The total thickness is estimated, with low limits of error, at 4,000 feet. 

 This corresponds well with the thickness of 4,100 feet yet more closely deter- 

 mined among the better exposures of the eastern ranges. 



More or less complete sections of the Siyeh occur within the Boundary 

 belt, on the eastern slope of the Rocky Mountain Trench. Others were studied 

 along the head-waters of Phillips creek and of Wigwam river. A composite 

 columnar section was constructed, showing the information derived from seven 

 traverses, spaced several miles apart. It was found that, notwithstanding the 

 great thickness of the formation, there are few lithological horizon-markers. 

 The columnar section resolved itself into the following relatively simple 

 scheme : — 



Columnar Section of Siyeh Formation. 



Top, conformable base of Purcell Lava. 



1,200 feet. — Chiefly gray and greenish gray, medium to thin-bedded, silicious, often 

 dolomitic metargillite, weathering light brown and buff. At the top 

 some 250 feet of the beds have a general reddish cast, owing to abun- 

 dant intercalations of red-gray, ripple-marked sandstone. Between 

 400 feet and 700 feet from the top, several beds of light gray lime- 

 stone, weathering gray to whitish, occur; the thickest of these, 25 feet 

 thick and about 600 feet from the top of the formation, was followed 

 for several miles in the Galton range. Sun-cracks are abundant at 

 many horizons in the metargillite. 



2,000 " Dark gray, argillaceous magnesian limestone or dolomite, in massive 

 beds with typical molar-tooth structure. Occasional intercalations! of 

 metargillite. The lower part of the member is more silicious than the 

 average rock. Most of the beds weather brown or buff, a few weather- 

 ing reddish. The individual beds vary in thickness from a fraction 

 of an inch to two feet or more, but generally they are grouped or 

 cemented together in massive plates three to ten feet in thickness. 



