1L>0 



University of California . 



[Vol. 3. 



Some are very minute, but others of more considerable size are 

 recognizable as magnetite, and these often have the form of 

 prisms with pyramidal terminations. There were observed also 

 a few grains of chromite. 



The enstatite of the Beach Creek specimen has undergone 

 some alteration, resulting in the formation of pale green acti- 

 nolite and a serpentinous substance. The latter occurs in 

 irregular veinlets, usually tilling cracks transverse to the pris- 

 matic axis of the pyroxene. The actinolite, which has an extinc- 

 tion angle of about 17°, was observed in but one section. It 

 apparently arose from a paramorphic alteration, for the actinolite 

 had its vertical axis in common with the enstatite which it partly 

 surrounded. 



Serpentine of Beach Creek. — The mass of this rock consists 

 of dark green serpentine, whose structure is not macroseopically 

 discernable. Distributed rather sparingly in this substance are 

 some lustrous crystals with lamellar cleavage that resemble 

 enstatite. The specimen is also traversed by a few small veins 

 of fibrous chrysotile. 



Examined in thin section, it is seen to be derived from a 

 peridotite, whose essential minerals were olivine and a rhombic 

 pyroxene, but both minerals are now completely replaced by 

 their usual alteration products. The greater portion of the 

 section is made up of serpentine that shows in a typical manner 

 the familiar mesh-structure indicating a derivation from olivine. 

 The pyroxene is represented by pseudomorphs of bastite, 

 identified by its low refractive index and weak double refraction 

 as well as by its fibrous structure. Its outlines are always irreg- 

 ular, and it is often seen to inclose well-defined areas of the 

 serpentine with mesh-structure. These facts mean that the 

 olivine crystallized as usual before the pyroxene. The process of 

 serpentinization has been accompanied by the usual separation 

 of magnetite. Another secondary product is zoisite, small 

 grains of which occur in clusters, most commonly included in 

 the bastite. 



As an accessory, there occurs a little chromite, while some of 

 the magnetite may be primary. 



