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University of California. 



[Vol. «. 



feient degrees of coarseness, and a small amount of gabbro and 

 hypersthene gabbro. Below this the dark serpentine extends until 

 covered by the Miocene. Towards the southeast the belt widens 

 to nearly a quarter of a mile. This portion is comparatively simple; 

 for, although there are deep cuts to expose the rock, the greater 

 portion consists of the compact altered dunite, with a comparatively 

 narrow strip, perhaps less than 200 feet, of fine-grained diabase. 

 But little signs of shearing are seen in this portion. This, the larg- 

 est body of altered peridotite, judging from its massive character, 

 does not bear out the view that is frequently advanced that hydra- 

 tion is the cause of the broken, laminated, or clayey character of 

 many serpentines. Wherever the Point Sal serpentines have lost 

 their massive character, it seems to be due in great measure, if not 

 entirely, to shearing brought about by earth movements. The most 

 disturbed portion is along the coast where it has been intruded by 

 dikes which are generally nearly parallel to the long diameter of 

 the eruptive mass as a whole. This direction corresponds to the 

 lines of movement and faulting in the Coast Ranges. 



Microscopic Petrography. — In the field the gabbros are generally 

 distinctly marked off from the peridotites, although there are facies 

 of each almost identical, besides independent bodies of intermediate 

 composition. The following classification is made purely for the 

 purposes of description, and is not meant to convey the impression 

 that there are any distinct types clearly set off from the rest. In 

 fact, there are all conceivable mineralogical transitions between that 

 consisting almost wholly of feldspar and the magnesian or olivine 

 rock. Three main divisions may be made: (I) The olivine-free feld- 

 spathic rocks, (II) the olivine-bearing feldspathic rocks, (III) peri- 

 dotite and pyroxenite. 



I. 



Feldspar. Anorthosite. 

 Feldspar, hornblende. Hornblende diabase. 



Feldspar, hornblende, quartz. Ophitic quartz diorite. 



Feldspar, hornblende, hypersthene. . Hypersthene diabase. 



Feldspar,augite(with orwithouthornblende). Diabase or gabbro. 

 Feldspar, augite, hypersthene (with or with- 

 out hornblende). Norite-gabbro. 

 Feldspar, hypersthene. Norite. 



