Anderson.] 



Point Reyes Peninsula. 



125 



One common type is a moderately coarse-grained rock in which 

 biotite is so conspicuous as to give it a decidedly dark color. 

 Orthoclase is abundant, though not the prevailing feldspar. Pla- 

 gioclase, approaching oligoclase, predominates, twinned commonly 

 on the albite law, while the pericline law is not infrequent. The 

 plagioclase has the most marked idiomorphic tendency of the 

 constituent minerals. Quartz appears in the sections as rather 

 large allotriomorphic plates, very irregular in shape. 



Another type is similar to this in its composition, but the 

 quantity of biotite is reduced to a very subordinate amount. This 

 rock is more compact, finer grained, and lighter in color than the 

 other, and better resists weathering. This is more common at the 

 northern angle of the peninsula. " Mussel Rock," on the beach 

 west of the Pierce Ranch, is of this variety. It stands out in the 

 ocean, almost entirely severed from the mainland of the Point. 

 The rocks in this vicinity show a tendency to segregation into basic 

 and acid zones. Veins of aplite are frequent here, cutting the 

 rocks in various directions. Still, the most common type of granite 

 on Tomales Point is the coarser one, in which biotite forms a con- 

 spicuous proportion. 



The most common accessory mineral here is titanite. It occurs 

 in large idiomorphic crystals, some of which are one-fourth of an 

 inch in length, or even larger. 



Magnetite is common along with chlorite, both of which seem to 

 have resulted from the alteration of brown biotite. This alteration 

 has resulted in the wedging apart of the cleavage plates of the 

 biotite, producing an interlocked structure of the biotite and the 

 chlorite. The chlorite is strongly pleochroic in sections, showing 

 the basal cleavage, but in sections parallel to the cleavage it is 

 almost isotropic. Narrow bands and scattered scales of chlorite are 

 common in some of the sections. The feldspar contains many 

 inclusions of hornblende and biotite. 



The granites of the larger area are more varied in their character 

 than those of the northern area. The rocks are not so well 

 exposed, since they have no ocean cliffs, and are more thickly 

 covered by underbrush, which protects them from erosion. Yet in 



