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University of California. 



[Vol. 2. 



the deeper canons and on some of the high ridges, one is able to 

 find firm rock, though not often fresh rock. 



At the two higher and southernmost summits of Vision Hill 

 the granite is coarse-grained and dark, with a large amount of 

 biotite, and occasionally large crystals of feldspar and quartz. At 

 the northern and lower summits the texture of the rock is more 

 uniform and of a finer grain. It has here been pressed and broken 

 into a columnar appearance, consisting of large, roughly cubical 

 blocks. At one of the northern summits the rock is peculiar from 

 the large amount of magnetite which it contains, seemingly as an 

 original mineral. It is a fine-grained rock, containing both ortho- 

 clase and plagioclase, with quartz, hornblende, and biotite. The 

 biotite has been altered to chlorite to a limited extent. The 

 mass may be only a large secretion from the granite, in which there 

 is an unusually large proportion of iron oxide. At the head of 

 Tomales Bay the rock exposed in the road-cuts is a dark gray 

 granite, with zones of basic rock, and with veins of quartz and 

 aplite. One slide of this rock shows a large crystal of titanite, 

 though it does not seem to be common here. All the rock con- 

 tains iron oxide and shows the alteration of biotite to chlorite. 

 At the western base of the ridge occupying this area the rock is 

 compact and resists weathering more perfectly. In other respects 

 it is similar to that just described. The pericline law in the 

 twinning of the feldspars is frequently noticeable. 



A third type of granite is that of the southwestern angle of 

 the field. It is essentially similar in composition though physically 

 different from that of the other areas. This granite is of the 

 porphyritic type, similar to that described by Professor Lawson 1 

 from near Monterey. It is of uniform character throughout, the 

 rather fine-grained ground-mass consisting of a compact aggregate 

 of quartz, orthoclase, and plagioclase with no large amount of 

 biotite. In fresh surfaces it has a characteristic vitreous appear- 

 ance. The phenocrysts are large crystals of pink orthoclase, 

 varying in size from one-fourth of an inch to two inches in 

 length. They are mostly twinned on the carlsbad law, though 



'Bull. Dept. Geol. Univ. Cal., Vol. i, pp. 9-15. 



