Anderson.] 



Point Reyes Peninsula. 



127 



this is not always to be seen. As far as was observed no parallel 

 orientation could be noticed in these crystals, as is sometimes the 

 case in the Monterey granites. Orthoclase is much more abundant 

 in these rocks, as well as quartz, though plagioclase is the domi- 

 nant feldspar. As a rule, the rocks of this area are less broken 

 and more resisting to the weather than those of the other areas. 

 Aplites and basic segregations are not common. Veins of calcite 

 were seen at several points penetrating the rocks. Usually they 

 were of only an inch or more in thickness. 



There is yet one other occurrence of granitic rock so peculiar 

 in its character that it deserves mention. It belongs to the larger 

 area, and occurs near its southeastern boundary in a canon west of 

 Olema. It resembles, at first glance, a partially decomposed aplite, 

 and was at first taken to be such. Further study of it showed, 

 however, that its peculiar appearance was due to a complete and 

 very thorough crushing it had undergone. 



It occupies a zone of more than half a mile along the canon, 

 with a width of at least one or two hundred yards. It is perhaps 

 a fault breccia. Under the microscope it shows angular fragments 

 of quartz and feldspar cemented together in a matrix of pulverulent 

 and partially kaolinized material containing some calcium carbon- 

 ate. Both orthoclase and plagioclase fragments are plentifully 

 distributed through the mass along with quartz, and all of them 

 are very much cracked and shattered. Few traces of biotite or 

 any dark mineral are to be found in it. It lies in the midst of a 

 system of intersecting faults, and serves to indicate how great the 

 disturbances of this district have been. 



In most of the granites of the peninsula the more transparent 

 minerals show an undulous extinction when turned between crossed 

 nicols. This is very noticeable in the rocks of Tomales Head, 

 both in the quartz and in the orthoclase and even in the plagioclase. 

 In the rocks of Vision Hill it is even more conspicuously seen, 

 while at the southwest angle of the field it is apparent only in 

 the quartz and orthoclase. All the larger crystals show a large 

 amount of fracturing, and irregular cracks are very abundant. 

 This still farther shows the great stress to which the rocks have 

 been subjected. 



