40 University of California. [Vol. 2. 



in fresh almost colorless idiomorphic crystals with no pleochroism. 

 The feldspars of the first generation occur more rarely. They are 

 so completely reduced to a green feebly polarizing mass as to be 

 hardly distinguishable. The groundmass consists of a minutely 

 granular substance of brown color, polarizing like augite, small feld- 

 spar laths, often jagged and irregular, abundant magnetite grains, 

 quartz, and calcite. The rock has so changed that it is not certain 

 if glass is present, but the numerous greenish non-polarizing areas 

 may be referable to that substance. Chlorite is abundant in aggre- 

 gates of small needles. Quartz is present in all the slides, thickly 

 dotted through the groundmass, and is in all cases probably second- 

 ary. The slender feldspar laths possess low extinction angles as a 

 rule. 



SPHEROIDAL BASALT AND DIABASE-GABBRO COMPLEX. 



Field Relations. — Under this head is embraced a series of erup- 

 tive rocks forming the main portion of the Point Sal ridge for a dis- 

 tance of nearly three miles. As far as noted, the basalt forms the 

 summit, northern slope, and a part of the southern slope of the 

 ridge. It is almost invariably so decomposed that it was 

 with difficulty that specimens fresh enough for study could be 

 obtained. It is amygdaloidal almost everywhere, and in places the 

 amygdules form the greater part of the rock. The spheroidal 

 basalt with its characteristic features is best exposed in the cliffs 

 north of Point Sal. For half a mile there are clean-cut sections of 

 a dark aphanitic rock which is in part at least andesitic and gener- 

 ally more or less amygdaloidal. Near the northern edge there 

 is a cliff with a triangular-shaped face composed of sheeted lavas 

 dipping northwesterly at an angle of 40 . The different layers 

 have weathered out very distinctly. They are quite irregular in 

 thickness, ranging from six to eighteen inches, so that a pro- 

 nounced wavy structure is to be seen. Between this cliff and the 

 point the spheroidal character is the most striking feature. The 

 term spheroid is somewhat of a misnomer; for the forms assumed 

 by the lava are rather elongated ropy masses packed over each 

 other in all conceivable positions (Fig. 3). At one spot there is a 

 rude parallelism in the arrangement, and occasionally a fairly spher- 



