REPORT OF THE CHIEF ASTROXOMER 217 



SESSIONAL PAPER No. 25a 



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

 of the diabase. Five dikes of the same rock, genetically connected with 

 it, were observed on Flattop. They contain inclusions of the argillite, 

 and range from an inch to six 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 

 augite is much more abundant than the feldspar. It is an allotriomorphic 

 mineral, red-brown when fresh, but frequently entirely gone over to chlorite. 

 The small amount of olivine originally present in the rock is now altered 

 to serpentine and chlorite. Besides the chlorite, which is the chief altera- 

 tion product, resulting from the plagioclase as well as from the augite 

 and olivine, much secondary calcite has been derived from the feldspar. 

 Apatite is found and titaniferous magnetite, in grains and definite 

 crystals, is abundant. The medium texture of the diabase is fairly uni- 

 form throughout the flow.' 



The present writer had an opportunity of studying both the intrusive and 

 extrusive types as they occur near the summit on the Swift Current Pass trail. 

 At the upper edge of ' Granite Park,' on the western side of the divide (see 

 Chief Mountain Quadrangle sheet, U. S. Geological Survey), the Purcell 

 formation is represented by two contiguous flows resting on the Siyeh metar- 

 gillite and overlain by typical Sheppard beds. The lower flow is forty feet 

 thick. Its upper surface, as noted by Finlay, is ropy. This structure passes 

 beneath into a pronounced pillowy structure, which, in place-, characterizes 

 most of the thickness of the lava sheet. The pillows are generally quite round 

 and of spheroidal form. They range from a foot or less to two or three feet 

 in greatest diameter. No sign of the variolitic composition so common in 

 pillow-lavas could be detected. The interstices between the pillows are filled 

 either with chert, or, more commonly, with an obscure, breccia-like mass of 

 aphanitic material whose microscopic characters are those of palagonite. This 

 material bears a few minute crystals of feldspar but is chiefly composed of 

 finely divided chlorite, quartz, calcite, and abundant yellowish-brown isotropic 

 substance like sideromelane. The whole seems to form a greatly altered basaltic 

 glass. 



The pillows themselves and the non-pillowy parts of the flow are composed 

 of the same type of vesicular microporphyritic, occasionally diabasic basalt 

 that makes up the upper flow. This is eighteen feet thick and lies immediately 

 upon the ropy surface of the forty-foot flow. The latter is without the pillow 

 structure but is massive like the normal Purcell amygdaloid. Microscopic 

 evidence shows that the rock is of the chemical type recognized in all the 

 occurrences of the lava in the western ranges. 



The writer's examination of the sill (50-70 feet thick) and dike (50 feet 

 thick) noted by Finlay as outcropping to the east of the Swift Current Pass, 



