14 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



region are covered with rolled pebbles, with several patches 

 of sedimentary deposits. In the eastern portion of the 

 Little Harbor region is a small area of rhyolite, which was 

 not found elsewhere. A narrow strip of quartzite breccia 

 occurs at the southeastern extremity of the island. 



A. ERUPTIVE ROCKS. 

 I. DiORITE. 



There are three observed occurrences of the diorite : one 

 along the cliff bordering the shore just to the north of 

 Avalon, the second near the head of the canon back of 

 Pebbly Beach, and the third in the lower portion of Silver 

 Canon. These are apparently dikes of considerable width. 



Macroscopic Characters. — The diorite is coarse-grained, 

 and of a light grayish color, more or less mottled. The 

 specific gravity of a specimen from Silver Canon was found 

 to be 2.777. In the coarser-grained specimens the separate 

 minerals may easily be seen without recourse to a lens. 

 With the lens the rock is seen to be composed of a ferro- 

 magnesian mineral, feldspar and a varying amount of 

 quartz . 



The feldspars range from a somewhat glassy condition to 

 one in which they are whitish and more or less opaque. 

 They constitute, in general, the principal mineral of the 

 diorite. The ferromagnesian mineral is hornblende, dark 

 green in color, and altered in part to chlorite. This mineral 

 varies in amount from a little less than one-half to perhaps 

 one-fifth of the surface area. Besides the hornblende an 

 ocpasional leaflet of biotite was seen in the specimens from 

 near Avalon, and also in one or two from Silver Canon. 

 The quartz usually occurs in small areas scattered through- 

 out the mass of the rock. In addition to these, magnetite 

 is apparent in nearly all the hand-specimens of the coarser- 

 grained varieties, being rather conspicuous in one specimen 

 from the canon back of Pebbly Beach. 



