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



it. In the first type the finer portion of the ground-mass has 

 a microcrystalline structure. In the other case the matrix 

 is largely isotropic, and is principally of secondary silica in 

 the form of opal. These latter areas have usually a small 

 irregular central cavity somewhat rounded or oblong in 

 shape. In some cases these centers have been filled with 

 secondary quartz. These darker areas of secondary silica 

 are usually separated from the lighter ones by an irregular 

 and generally narrow band of a yellowish green, showing 

 high polarization colors. Under crossed nicols and with 

 higher powers, this is seen to be composed of finely polar- 

 izing aggregates, doubtless of some secondary ferromagne- 

 sian mineral. Similar minute, radial aggregates occur 

 scattered through the lighter areas of the slide. 



The slide from the central portion of the dike does not 

 show the division into light and dark areas, though it con- 

 tains a small amount of opaline silica and quartz. There is 

 a larger amount of the greenish yellow ferromagnesian 

 mineral, which is more evenly distributed through the rock, 

 a part in the form of radial aggregates and a part as minute 

 flakes. The matrix is cryptocrystalline, showing a feeble 

 polarization. The ground-mass is filled with opaque, dust- 

 like, microscopic particles. 



3. Rhyolite. 



Occurrence. — The rhyolite occurs in a single area, to 

 the west of the main area of the andesite, in the Little 

 Harbor region. It caps the summit of the ridge at this 

 point, and extends as a light covering to the basement rocks 

 for several hundred feet down the southern and western 

 slopes, the underl3dng formations appearing here and there. 

 At a point about midway down the western slope the rock 

 has a roughly bedded appearance, dipping toward the west 

 at a rather high angle. The relation of these rocks to the 

 other igneous rocks of the island was not learned. 



Macrosco-pic Characters. — The rhyolite varies from com- 

 pact to very vesicular, and is of a light color, nearly white 



