416 WEED AND PIRSSON 
feldspar phenocrysts in a feldspathic finely granular groundmass. 
Only occasional ferruginous products represent some former 
ferro-magnesian mineral. The large feldspar phenocrysts are 
chiefly orthoclase, but there is also a plagioclase present whose 
optical properties indicate an acid oligoclase. The groundmass 
in which these lie is composed of unstriated alkali feldspar with 
quartz practically absent. It is micro-granitoid in structure, with 
tendency to a broad trachytoid type like those seen in the 
orthophyres. The rock is considerably altered, the groundmass 
quite turbid from kaolinization, and the feldspars are also 
changed though interior cores are still fresh and limpid. 
Mission Butte is the sharp, somewhat isolated mountain that 
is the most prominent point of the western part of the moun- 
tains. It is composed of gvanite-porphyry weathering in crags 
‘that form a sharp summit that is in strong contrast to the smooth 
and purple débris slopes and pine covered flanks of the adjacent 
mountains. The character of this porphyry is somewhat unlike 
the type prevailing in the mountains. It weathers in rough 
crags, and breaks in irregular surfaced blocks, in sharp distinc- 
tion to the fine débris which generally prevails throughout the 
range. At the west base of the butte where the porphyry is in 
contact with the sedimentary series, the contact form of the 
rock resists weathering more effectively than the main body of 
the porphyry, and forms a wall projecting above the general 
surface of the ground. In this outcrop the rock is platy, and 
the lamination is parallel to the contact and to the bedding of 
the limestone. 
The rock, although it differs in weathering and in the appear- 
ance of its craggy outcrops from that forming the main mass of 
the mountains, upon microscopical examination is found to bea 
facies of the same rock. It is a granite-porphyry with somewhat 
open structure and miarolitic cavities; has a generally light 
rusty-gray color, and contains large phenocrysts of glassy ortho- 
clase 10™™ to 15™" in diameter, with abundant and less promi- 
nent square feldspar crystals of somewhat uniform size, which are 
2™™ or 3™™ across; occasional corroded quartz grains are also 
