100 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



was seen in the Boundary belt. From its occurrence at Mount Hefty, this 

 assemblage of beds has been named the Hefty formation. The full thickness, 

 estimated at 775 feet, was measured at both localities. In both cases the 

 exposures are good. On the whole the formation is homogeneous and, as with 

 the associated Altyn, useful subdivision did not seem possible in the field. 



The staple rock of the formation is a heavily bedded, red or reddish gray, 

 fine-grained sandstone. As a rule, it is not metamorphosed to the condition of 

 true quartzite. Its mass is interrupted by thin beds of red shale and by rarer, 

 light greenish-gray, brown-weathering quartzites. Occasionally the sandstone 

 is somewhat calcareous. While the group of beds is generally red, this colour, 

 being mixed with gray, is not so striking as in the case of the overlying 

 Wigwam formation. Sun-cracks and ripple-marks are common at various hori- 

 zons. A further general characteristic of ledges and hand-specimen is the 

 relatively abundant development of metamorphic sericitic mica in the bedding 

 planes. 



The formation passes upward with some abruptness, into the MacDonald 

 sandstone. As already noted, there is some dovetailing with the underlying 

 Altyn. 



Under the microscope the dominant sandstone is seen to be composed of 

 rounded grains of quartz and less abundant feldspar, with a compound cement 

 of angular quartz and feldspar fragments, carbonate (probably dolomite or 

 magnesian ferrocalcite), and a small amount of iron ore. As a rule the quartz 

 is glass-clear and uncrushed; occasionally the grains show enlargement by the 

 familiar addition of silica, crystallographically orientated. The feldspars are 

 again microperthite, microcline, orthoclase, and plagioelase (probably andesine) 

 named approximately in the order of their relative abundance. As a rule 

 the sections show the occurrence of well-rounded grains of cherty silica. 

 The clastic grains vary from 0-1 mm. to 0-6 mm. in diameter, averaging abot.it 

 0-2 mm. The rock is thus a typical fine-grained feldspathic sandstone, some- 

 times calcareous and always more or less ferruginous. 



The subordinate, argillaceous interbeds of this formation are usually in a 

 more visibly metamorphosed condition, and recall the type phases of the 

 Appekunny formation. The alteration (again by deep burial, not by moun- 

 tain-building thrust) approaches the degree of true metargillite but it cannot 

 be said that amorphous matter is entirely replaced. Grains of carbonate (pro- 

 bably dolomitic) are practically constant accessories, as they are in the more 

 sandy phases. 



From its position and its petrographic nature we may conclude that the 

 Hefty formation is the coarser-grained equivalent of the lower Appekunny in 

 the Lewis series, especially of its variegated basal beds as exposed at the eastern 

 end of South Kootenay Pass. 



The average specific gravity of seven hand-specimens of the sandstone is 

 2-646, and of three hand-specimens of the more argillaceous phase is 2-743. 

 The average specific gravity for the whole formation is about 2-695. 



