GEOL. VOL. I.] SMITH SANTA C ATA LIN A ISLAND. 59 



ness. It occurs stratiform, with an average dip of from 20 

 to 30 in a northerly direction. The rocks are very hard and 

 compact, and in weathering present an extremely rough 

 surface, with projecting fragments, many of which have 

 sharp, jagged points. It is doubtless owing to this bold, 

 irregular surface that one of these hills has received the 

 name of Granite Peak. The surface of this rock is also 

 irregularly pitted. The occurences are almost wholly of 

 this facies, and little evidence was seen of internal move- 

 ment, causing a slickensided appearance. The general 

 appearance of the rock in the field is in most respects quite 

 unlike that of the serpentine of the Potrero, San Francisco, 

 described by Dr. Palache, 1 which is typical of much of the 

 serpentine of the Coast Range. There are a few small 

 patches of magnesite within the serpentine area. The hand- 

 specimens of the serpentine vary in color from a dirty greenish 

 white to a dark bluish green, more or less mottled with 

 limonite. The compact specimens show an indistinctly 

 banded structure, and have a rather uneven fracture. This 

 surface is entirely different from the smooth and somewhat 

 polished surface of the pale green, slickensided specimens. 

 Traversing the surface in lines approximately parallel to the 

 banding are occasional fine veins and threads of chrysotile, 

 with their fibres at right angles to the enclosing walls, and 

 stained here and there with iron. More numerous and finer 

 threads cross the surface at right angles to the larger veins, 

 and nearly all are stained with limonite. Threads of mag- 

 netite run through the rock, in no fixed direction. In some 

 places the rocks contain many minute veins of secondary 

 silica, running at right angles to the banding. Cross-sections 

 seen on the surface show that they are filled with the silica 

 arranged in concentric rings. No remnants of the minerals 

 from which the serpentine was derived were seen in any of 

 the specimens, but it doubtless consisted in large part of 

 olivine, for the mesh-structure characteristic of the serpen- 



^'The Lherzolite-Serpentine and Associated Rocks of the Potrero, San Francisco." 

 Bull. Dept. Geol., Univ. Cal., Vol. i, No. 5, pp. 161-179. 



