214 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



amygdaloid; in one field section a phase suggesting rolled-in lava-crust forms 

 a local variation. 



m As in the other ranges, care was taken to secure the freshest possible 

 specimens but here also the microscope displayed profound alteration in them 

 all. The dominant constituent is again labradorite (Ah^AnJ, with the usual 

 diabasic arrangement. Abundant chlorite, calcite, kaolin, and limonite, with 

 magnetite in laths and octahedra and many narrow prisms of apatite as the 

 two surviving original accessories, fill the spaces between the idiomorphic 

 feldspars. The rock is almost certainly a greatly altered basalt with diabasic 

 structure. The specific gravities of two typical specimens are 2-828 and 2-846. 

 Lithologically similar lavas have been described in the accounts of the 

 Grinnell, Sheppard, and Kintla formations, in which flows have been locally 

 interbedded. 



DIKES AND SILLS IN THE CLARKE RANGE. 



At the western end of the Sawtooth ridge, north of Lower Kintla lake and 

 at the 7,000-foot contour, the Appekunny and Grinnell beds are respectively 

 cut by two vertical dikes running northwest-southeast. Each dike is about 

 twenty feet wide. Lithologically, even to microscopic details, these intrusives 

 are not to be distinguished from the diabasic phase of the lava just described; 

 the dikes were, most probably, feeders of the extrusive mass. 



To north, south, and east of Upper Kintla lake a persistent intrusive sill, 

 averaging forty feet in thickness, cuts the Siyeh formation at a nearly constant 

 horizon, about 1,200 feet below the base of the Purcell Lava. Macroscopically, 

 the rock of the sill is a deep greenish-gray, fine-grained trap like that of the 

 two dikes. The thin section shows the rock is relatively fresh. Its essential 

 constituents are diopsidic augite, labradorite, and green hornblende. The 

 original accessories are micropegmatite (of quartz and orthoclase) ; much mag- 

 netite in octahedra, laths, and skeleton crystals; apatite, titanite, pyrite, and 

 interstitial quartz. Yellow epidote, chlorite, zoisite. limonite, and a little 

 calcite are secondary products. The feldspar is decidedly subordinate to the 

 bisilicates in amount. Like the hornblende it is idiomorphic. The augite 

 apparently occurs in two generations; a small proportion of it is crystallized 

 in stout idiomorphic prisms up to 0'-6 mm. in length, while most of it is in 

 anhedral grains 0-1 mm. or less in diameter. The feldspars and hornblende 

 prisms average about 0-2 mm. in length or less, and are enclosed in a mesostasis 

 of granular augite, micropegmatite, and quartz. The structure is transitional 

 between that of a diabase and a gabbro with a stronger tendency to the gabbroid. 

 The specific gravity of a type specimen is 3-057. 



In chemical composition, in the dominance of the bisilicates, in structure, 

 in the character of accessories, including the micropegmatite intergrowth, 

 and in specific gravity, this rock closely resembles the staple phase of the much 

 greater sills in the Moyie and Yahk ranges. The principal mineralogical differ- 

 ence consists in the fact that here the bisilicate is mostly augite, while, in the 

 thick western sills, it seems to be entirely amphibole of the same habit as in this 

 sill. 



