412 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



is composed of that lava. At the col just north of that summit and for a half 

 mile farther north, the ridge is capped by a great mass of lava of a type related 

 to the one described but distinguished from it both chemically and mineralogi- 

 cally. 



This second kind of lava is often highly vesicular, very massive, and of 

 the same range of dark colours and general habit as the first type. Though 

 very compact, it always bears small glassy phenocrysts of rhomb-feldspar and 

 a few augites visible to the unaided eye. In the field it was not suspected that 

 the second lava was to have any special interest not shared by the type above 

 described. As a consequence of this view, only two hand-specimens of this 

 lava were collected. The desired facts of field occurrence were likewise not 

 obtained in the measure in which they might have been if the writer had been 

 conscious of the unusual character of the rock. It is known only that it occurs 

 in one or more very thick flows and that the lava appears, from its topographic 

 position, to overlie the first, the more normal rhomb-porphyry lava, though the 

 two almost certainly belong to the same epoch of extrusion. Whether the two 

 types are separated by a sharply defined surface of contact is not known. This 

 rarer type seems to cover about a third of a square mile within the five-mile 

 Boundary belt and at least as much more beyond its northern limit. How 

 much additional area is covered by it is also unknown. It is thus clear that 

 a second visit to the mountain and a careful field-study are required before 

 a satisfactory account of this occurrence of lava can be given. All that is now 

 possible is to furnish a brief note on the character of the lava and thus suggest 

 to some future geologist one more point for study in this complicated and 

 interesting region. 



The real character of this lava, unique among all the formations occurring 

 in the whole transmontane section, was revealed only after its optical and 

 chemical analysis had been performed. Macroscopically, as already noted, 

 it is much like the rhomb-bearing lava to the south. The rock is usually very 

 fresh and breaks sonorously under the hammer. The colour is usually a very 

 dark slate-gray, tending to dark brown on weathered surfaces. The rhomb- 

 feldspar phenocrysts vary from 1 mm. to 3 mm. in length while the few augite 

 prisms are even shorter. Under the microscope a few small olivines, altered 

 to brownish-yellow serpentine or to carbonate, and exceedingly rare foils of 

 deep brown biotite are seen to form phenocrysts, but both of them are only 

 accessory constituents. The leading peculiarity of the rock is found in the 

 ground-mass, which is largely composed of minute but perfectly formed analcite 

 crystals in dodecahedral development. With these are associated many feldspar 

 microlites and the accessories, apatite, magnetite, a few grains of pyrite, and 

 probably titanite. All of these minerals are embedded in an abundant, trans- 

 parent, pale brownish glass which contains a few grains of secondary carbonate, 

 and may be somewhat zeolitized. With the exception of the alteration of the 

 olivine and glass, the rock seems to be practically as fresh as the day it first 

 solidified from fusion. 



