72 



University of Califowiia. 



[Vol. 2. 



The Iherzolite- type with enstatite as the rhombic pyroxene is 

 rather sparingly represented. A thin section from the dark serpen- 

 tine east of the Chute contained a serpentinized olivine in excess, 

 with here and there irregular grains of a colorless rhombic pyroxene, 

 and still more rarely those of a monoclinic pyroxene. Another spec- 

 imen obtained near the Lion's Head contained, in addition to the 

 minerals in the last, allotriomorphic masses of reddish brown horn- 

 blende. The latter appears to be the freshest of any of the con- 

 stituents. 



The pyroxenite occurs at Point Morrito as segregations in the 

 wherlite. It is seldom wholly free from olivine, and occurs in irreg- 

 ular patches of considerable size imbedded in a matrix consisting 

 chiefly of olivine. 



The dunite is represented by the dark homogeneous serpentine. 

 It forms by far the larger part of the peridotite area, being mainly 

 developed between the Chute and the eastern end of the complex. 

 Under the microscope its derivation from olivine appears clear. In 

 specimens in which the alteration is not complete there are scat- 

 tered spots of small size of a brilliantly polarizing, highly refract- 

 ing mineral. These grains are inclosed in a mesh of interlacing 

 fibers of serpentine. Numbers of them near together extinguish 

 simultaneously, showing them to be portions of a single crystal. 

 With the analyzer removed the reticulated mesh structure, which is 

 considered characteristic of olivine, is apparent throughout the 

 whole slide with the exception of the clear spots. In some slides 

 no olivine remains. 



A remarkably banded rock occurs near the Head. This con- 

 tains hornblende, augite, olivine, hypersthene, and feldspar. Owing 

 to the small proportion of feldspar it might be considered as belong- 

 ing between the two main groups. The large irregular plates of 

 yellowish brown hornblende contain inclusions of all the other con- 

 stituents. The diallagic augite is gathered in bands free from the 

 other components, the individuals being arranged with their vertical 

 axes approximately normal to the edges of the band. 



The petrography of the "segregated veins" or dikes which 

 appear in different portions is interesting. Without doubt these 

 occurrences can be divided into two classes, one of which might be 



