388 



University of California. 



[Vol. 3. 



where the vein like disposition of this facies is not apparent there 

 may be a larger proportion of feldspar and the structure may 

 have none of the parallelism shown in the veins. A specimen of 

 such a rock was taken for study. Macroscopically it is a coarse 

 aggregate of black hornblende crystals and gray feldspar. The 

 hornblende greatly predominates and the crystals of that mineral 

 are often an inch in length. 



In thin section the hornblende is the same as that found in 

 the normal variety of the gabbro. Associated with it is an 

 occasional allotriomorphic grain of hypersthene. The basic 

 plagioclase is quite fresh. There are enclosed in the latter 

 mineral slender rods of olivine with transverse cracks quite 

 similar to those found in the orbicular structures to be described 

 below. Ilmenite with occasional cleavages and narrow borders 

 of titanite is rather abundant. There is some epidote associated 

 with the hornblende. 



A more careful examination of the mountain would doubtless 

 reveal other variant facies of the normal rock. The writer's 

 attention at the time of his visit was, however, directed more 

 particularly to a search for the orbicular facies of the rock, and 

 little opportunity was afforded for detailed study of the mass. 



Aplitic Dykes. — Near the summit of the mountain the normal 

 hornblende gabbro is cut by a number of small dykes, striking in 

 different directions. These dykes are very light colored and so 

 present a striking contrast to the gabbro. They appear in the 

 field to be of the nature of fine grained granites or aplites which 

 grade on the one hand into facies so siliceous as to be practically 

 quartz veins, and on the other hand into veins of feldspar. In 

 hand specimens one may recognize that the rock is made up 

 chiefly of quartz and feldspar with a sparing amount of biotite 

 in thin scales more or less decomposed. In thin section, allotrio- 

 morphic quartz is the preponderating constituent. The feldspar 

 is for the most part untwinned and is presumably orthoclase, but 

 there are a few grains of an acid plagioclase. The feldspar shows 

 in many cases a well defined idiomorphism and is in nearly all 

 cases murky with kaolinization products. The biotite is decom- 

 posed and stained. The only other mineral is epidote in very 

 small amounts. 



