568 H. W. FAIRBANKS 
greater number of the dikes are fine grained-and amygdaloidal 
on the edges. Dikes of diabase are also numerous. The rock 
having the most limited distribution is the peridotite. This is 
not as much decomposed as might be expected, considering its 
Morro Rock (dacite granophyre) on the coast at mouth of Morro Bay. 
age, and consists of olivine, a rhombic pyroxene, and sometimes 
augite and a little feldspar. The dikes follow the strike of the 
rocks, and are more abundant in the jasper horizons. Contem- 
poraneous flows were not recognized with certainty, and it is the 
writer’s opinion that they are more rare upon the San Francisco 
peninsula than Lawson has supposed. They are not all of the 
same age. The surface flows did not take place until after the 
Golden Gate series had been upturned and planed off. This is 
shown in the hills east of Los Osos Valley. 
The dacite and andesite granophyres are strikingly interest- 
ing rocks. The occur as roundish or lenticular plugs forming 
the large buttes, besides many small ones, between San Luis 
Obispo and Morro Bay. The dacite variety is confined to sev- 
