212 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



At the lower contact this lava flow, like the great ba=ie flow, has ruptured, 

 shredded, and hailed up the underlying argillite which was clearly unconsoli- 

 dated at the time of the eruption. 



DIKES AND SILLS IN THE McGILLIVRAY RANGE. 



The fifty-foot dike already noted as cutting the Kitchener beds on the 

 6,583-foot summit merits description, since it is regarded as probably one 

 feeder of the fissure eruption. The dike is vertical and strikes north 10° east. 

 It has a marked zone of chilling on each wall. 



In the chilled zones, acid labradorite, arranged as in diabase, is the only 

 primary essential present. The interspaces are filled with a confused mass of 

 chlorite, calcite, yellow epidote, kaolin, muscovite (the last occasionally in 

 large, distinctive foils), and limonite, with a little secondary quartz. Abundant 

 ilmenite or titaniferous magnetite is the one original accessory. The specific 

 gravity of the freshest-looking specimen, taken three feet from the dike contact, 

 is 2-860. 



From its general habit, mineralogical composition, and mode of alteration, 

 this chilled, fine-grained phase of the dike is almost certainly a much changed 

 diabase. Except for the size of grain and lack of vesicularity it is lithologi- 

 cally identical with the diabase phase of the Purcell Lava. 



The main body of the dike is composed of a medium-grained gabbroid 

 rock which is similar -to the chilled phase in all essential respects except in its 

 coarseness of grain and in the occurrence of chlorite pseudomorphs with the 

 forms of long prisms of amphibole. The latter mineral was an original con- 

 stituent but has been completely altered. Other chlorite has resulted from the 

 likewise complete alteration of interstitial pyroxene which seems to have 

 accompanied the amphibole and labradorite in the list of primary constituents. 

 The main body of this dike had thus originally the composition and structure 

 of a hornblende gabbro, transitional to hornblende diabase. The specific gravity 

 of one specimen is 2-853. 



Two sills, respectively three and four feet thick, cut the sediments imme- 

 diately east of the dike, from which they are probably offshoots. 



At the head of the broad gulch, a mile farther west, the Commission trail 

 crosses a second, thirty-foot, vertical dike, striking N. 30° W. The microscope 

 has corroborated the impression won from the macroscopic appearance that 

 this dike is of essentially the same composition as the first and may also repre- 

 sent the filling of a fissure whence issued part of the Purcell Lava flood. 



PURCELL LAVA IN THE GALTON RANGE. 



The Purcell formation reappears on the eastern side of the Kootenay valley 

 and, as shown in the map sheet, crops out very liberally. Complete sections 

 were made at ten different points, at each of which the thickness was found to 

 be close to 400 feet. The section most favourable for the analysis of the for- 



