72 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



The Siyeh is the great cliff-maker of the Front ranges. Quite apart from 

 the fact that it is capped by the resistant Purcell Lava, the limestone is itself 

 strong enough to stand up in precipices thousands of feet in height. (Plate 9.) 

 Among the many admirable exposures one of the best within the Boundary belt 

 is that at the head of Starvation creek canyon; this section is typical of the 

 formation as it occurs in both the Clarke and Lewis ranges. 



The formation is notably homogeneous for hundreds of feet together; yet,. 

 as shown in the columnar section, it is divided into five zones of contrasted 

 lithological character. 



Columnar section of Siyeh formation. 



Top, base of the Purcell Lava. 



150 feet. — Medium to thin-bedded, reddish metargillite with subordinate, thin inter- 

 beds of buff-weathering, magnesian metargillites ; sun-cracks, rain-prints 

 and ripple-marks abundant. 



950 " Gray and greenish, thin-bedded, often ealeareo-magnesian metargillites;. 

 weathering buff (dominant), fawn, and gray; sun-cracks, rain-prints and 

 ripple-marks common. 



100 " Gray, concretionary, silicious and non-magnesian limestone weathering 

 light gray, with a five-foot band of buff-weathering dolomite at thirty 

 feet from the top. 

 2,000 " Massive, thick-bedded, dark gray or dark bluish gray, impure magnesian 

 limestone, weathering buff; thin inter beds of dolomitic metargillite,. 

 weathering buff, and a few thin beds of grav sandstone. Molar-tooth 

 structure characteristic of limestone; metargillites bear sun-cracks and, 

 rarely, obscure ripple-marks. 



900 " Thin to thick-bedded, light to dark gray and greenish gray calcareo-mag- 

 nesian metargillites and quartzites, weathering fawn and buff, with 

 a few interbeds of buff-weathering dolomite without molar-tooth 

 structure. 



4,100 feet. 



Base, top of the Grinnell formation. 



Notwithstanding their lithological variations, the strata form a natural 

 unit; the peculiar buff tint of the weathered surface contrasting with the deep 

 browns and blacks of the Purcell Lava and with the strong purplish red of the 

 Grinnell formation below, is a common feature for most of the strata in the 

 Siyeh. The uppermost beds weather reddish but they are so intimately inter- 

 leaved with buff-weathering strata that a clean-cut separation of the red beds 

 is impossible. It has thus appeared best to follow Willis in including all the 

 strata between the Grinnell and the Purcell Lava under the one formation 

 name. It is, on the whole, a single magnesian group. A second general charac- 

 teristic is the massiveness of the beds. This is most prominent in the 2,000 

 feet of dolomite and limestone composing the middle part of the formation. 



The table and Willis' statement afford a sufficient general description of 

 the strata in this section. There are, however, certain structures in the buff 

 magnesian limestone and the thick band of gray limestone which merit special 

 notice. At the outcrop of the buff rock the observer's eye is struck with a 

 repeated colour variation in the rock. The cause is speedily apparent. The 

 calcareous constituent of the rock is seen to be segregated, sometimes irregul- 



