44 



University of California Publications. 



[Geology 



of movement has taken place is abundantly shown by the sheared 

 and crushed appearance of the rock in many places and numer- 

 ous small faults displayed by the veins of quartz and aplite. So 

 much movement indeed has affected these rocks as to render them 

 utterly unfit for quarrying purposes. In this small area no rem- 

 nant remains of the schists and limestones into which the gran- 

 ites are intruded at Point Reyes and to the South. 



PETROGRAPHY. 



Two varieties of diorite occur on the peninsula — Biotite-dio- 

 rite and Quartz-diorite. The first variety is very abundant, the 

 second is scarce and represented only by a few loose fragments 

 found near the north end of the bay. 



Biotite-diorite. — Microscopically it is a coarse grained non- 

 crystalline rock, made up apparently of only two minerals in 

 about equal proportions : a sugar-white feldspar and a dark green 

 mica. The flakes of mica occur up to 6 mm. in length, and the 

 ordinary length of the feldspar is about 4 mm. 



Microscopically this rock is seen to consist of the essential 

 minerals biotite and a soda-lime feldspar as determined below 

 in about eqiial proportions, together with the accessory minerals 

 apatite, magnetite and titanite, apatite being very abundant, and 

 a large amount of decomposition products in the shape of kaolin 

 and chlorite. 



The twinning lamellae of the feldspars are very unequal in 

 size, very coarse lamellae alternating with very fine ones. Single 

 crystals, however, sometimes show fine, equal sized lamellae, 

 together with a system of alternating coarse and fine ones. Albite 

 twinning is the rule, though an additional pericline twinning 

 was observed in some cases. Carlsbad twinning also occurs. 

 Wavy extinction is very common and shows that the rock has 

 been subjected to great strain. 



By Michel Levy's statistical method a maximum angle of 

 20° was obtained on the albite twinning plane, corresponding to 

 a feldspar of the composition between Ab 3 An 2 and Ab 4 An 3 , or a 

 medium basic andesine. 



One section was found which could be utilized for M. Fouque's 

 method for determining the feldspars on sections cut perpen- 

 dicular to a bisectrix. This section gave an angle of 22° 16' 



