216 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 

 PURCELL LAVA AND ASSOCIATED INTRUSD7ES IN THE LEWIS RANGE. 



The most easterly exposures of the lava, yet described, are those found in 

 the Lewis range by Willis and Fiiilay.* Finlay's account of the formation 

 shows the close parallel between the relations of extrusive and intrusive phases 

 of the rock in this range and their relations in the Clarke and McGillivray 

 ranges. His descriptive note may be quoted in full: — 



' The igneous rocks of the Siyeh limestone are two — an intrusive 

 diorite and an extrusive diabase. 



'Diorite. — On Mount Gould and on Mounts Grinnell, Wilbur, and 

 Robertson there is found a band of diorite 60 to 100 feet thick. Near the- 

 upper . and lower surfaces this intrusive sheet was chilled and is fine- 

 grained. In the center the texture is medium or fine-grained. Several 

 dikes which have acted as conduits for the molten rock are exposed in the 

 region near Swift Current pass. One of these extends across the cirque 

 • occupied by the Siyeh glacier and runs vertically up the amphitheatral 

 walls. It is 150 feet in width. A second dike, vertical and 30 feet wide, 

 comes in beside the Sheppard glacier. Along the trail to the east of Swift- 

 Current pass the diorite sheet breaks across the Siyeh argillite and runs 

 upward as a dike for 500 feet. It then resumes its horizontal position as 

 an intercalated sheet between the beds of argillite. As a dike it skips 

 for 600 feet across the strata on Mount Cleveland. 



' Under the microscope the diorite is found to contain abundant plagio- 

 clase, with small amounts of another feldspar, much weathered, which does 

 not show twinning. This mineral is closely intergrown with quartz. 

 Brown hornblende is the principal dark silicate. The plagioclase has an 

 extinction angle high enough for labradorite, but it gives no definite clue 

 as to its exact basicity. No section of a fresh piece twinned on the albite 

 and Carlsbad laws at the same time could be observed. The quartz is not 

 present in sufficient amounts to make advisable the name quartz-diorite 

 for the rock. The small patches of biotite originally present are entirely 

 altered to chlorite. Pyrrhotite is occasionally met with, apatite occurs 

 in crystals of unusual length, and magnetite in lath-shaped pieces is 

 common. 



• ' Diabase. — In the field this rock is always much weathered, presenting 

 a dull green colour by reason of the secondary chlorite which it contains. 

 It is a typical altered diabase. Exposures are found near the top of Mount 

 Grinnell, where the thickness of the sheet is 42 feet, and on Sheppard 

 mountain opposite Mount Flattop. Here the extrusive character of the 

 flow is well shown, for its upper surface is ropy and vesicular, with amyg- 

 daloidal cavities containing calcite. Its place is at the top of the Siyeh 

 formation, 600 feet above the sheet of diorite, with heavy bedded ferru- 

 ginous sandstone and green argillite immediately below and above it 



•G. I. Finlay, Bull. Geol. Soc. America, Vol. 13, 1902, p. 349. 



