350 B. WILLIS — LEWIS AND LIVINGSTON RANGES, MONTANA 



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 acro3S 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 pla- 

 gioclase, 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 plagio- 

 clase 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 ad- 

 visable 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 color 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 

 amygdaloidal cavities containing calcite. Its place is at the top of the 

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

 ferruginous sandstone and green argillite immediately below and above 

 it respectively. The argillite has filled in the irregularities of the upper 

 surface of the diabase. Five dikes of the same rock, genetically con- 

 nected with it, were observed on Flattop. They contain inclusions of 

 the argillite, and range from an inch to 6 feet in width. They are nearly 

 vertical. 



Under the microscope the rock is seen to be made up principally of 

 augite and plagioclase, arranged in such a manner as to give the normal 

 diabase structure. The plagioclase is idiomorphic in long, slender laths. 

 It has the habit of labradorite, but no material was studied which offered 

 data for its accurate determination. The extinction angle is high. The 



