62 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 30 SER. 



structure of the serpentine itself. Judging from this, the 

 serpentines may be roughly divided into three groups accord- 

 ing to their probable origin: (i) those derived from pyrox- 

 enites, (2) those from rocks composed largely of olivine, 

 and (3) those from a rock in which both rhombic pyroxene 

 and olivine were among the essential constituents. The 

 first are now characterized by the bastite structure, the 

 second by the mesh-structure. It is probable that the whole 

 of the talc-schist is derived from the first form of serpen- 

 tine. 



6. GARNET-AMPHIBOLITE. 



Along the ridge near the upper limit of the talc-schists, 

 and within that area, are found here and there small, pro- 

 jecting bosses, with occasional larger areas, of garnet-am- 

 phibolite. This rock usually presents a somewhat roughened 

 surface, more or less reddened with iron oxide. It is not 

 compact, and readily crumbles under the hammer. The 

 fresher material is dark or almost black in color, and appears 

 to be composed wholly of a brownish or greenish hornblende, 

 with roughly rounded red garnets in varying size and 

 amount. In some places these garnets attain a diameter of 

 about 3 mm. and form the principal feature of the rock, 

 while in other cases the rock is composed almost entirely of 

 a somewhat fine-grained hornblende, and an occasional 

 minute garnet may be made out only with the aid of a lens. 

 At a few points the rock occurs as a black, rather coarsely 

 granular aggregate, composed entirely of hornblende, so far 

 as can be determined with a lens. 



A slide was made of the facies of the rock containing 

 the largest garnets. There are nine of these garnets in the 

 slide, ranging from 2 to 3 mm. in diameter. Microscopically 

 the rock is composed of scattered, pale pink garnets in a 

 matrix of hornblende. Here and there are small grains of 

 rutile. The hornblende is brownish with a tinge of green, 

 and occurs in allotriomorphic plates, with seldom a hint of 

 crystallographic form. The boundaries are usually well 

 marked by a limonite stain. The sections themselves are 



