88 



University of California. 



[Vol. 2. 



kinds of dikes appear in any one spot, their succession can not always 

 be determined. The main body of the gabbro, and perhaps some 

 of the diabase along the northern side of the complex, are undoubt- 

 edly the oldest of the intrusions. The peridotite at Point Morrito 

 has been intruded through the gabbro and is apparently continuous 

 with the large body of altered dunite toward the east. At Point 

 Morrito there are dikes of greatly varied character later than both 

 the gabbro and peridotite. As a general thing these dikes are 

 poorer in feldspar than the normal gabbro. They are very fine 

 grained, but granular in structure. The dikes of diabasic character 

 occur in many parts of the complex, in both the gabbro and peri- 

 dotite. 



The succession, as far as determined, is as follows: gabbro and 

 some diabase, peridotite, dikes of granular structure and intermedi- 

 ate composition, diabase and ophitic quartz-diorite, and feldspathic 

 veins. A partial analysis was made of the feldspar in the banded 

 troctolite, with the following result: Si0 2 42.6, A1 2 3 38.3, CaO 

 18.65. A' 1 estimation of the silica in the partially altered dunite 

 resulted in 41.3 per cent. This would give an extreme range in 

 silica between the peridotite and the ophitic quartz-diorite of 24.7 

 per cent. If the pegmatitic quartz-feldspar veins were taken into 

 account, the range would be still greater, for in them the silica 

 reaches 73 per cent. The succession in general is basic, more basic, 

 and acid. The occurrences of the acid rocks are, however, compar- 

 atively insignificant in volume. The larger number of the various 

 eruptive bodies do not differ much as far as the silica content is con- 

 cerned. It is, however, when we look away from the silica ratio to 

 that of the alumina, magnesium, and calcium, that the extreme dif- 

 ferences in mineralogical composition are understood. 



In addition to the very common association of diabase and peri- 

 dotite in many parts of the state, there are localities where gabbro is 

 also found with peridotite. Whether this gabbro is of later origin, or 

 fragments of an earlier consolidation, or secretions from the magma, 

 observation as yet seems not to have definitely settled. It is a well- 

 established fact of common occurrence to find inclusions of foreign 

 bodies in the peridotite. This is particularly prominent at Knox- 

 ville, Lake County, and on Angel Island. Although Point Sal is 



