4 



Fairbanks.] Geology of Point Sal. 7 1 



tions. The pleochroism is rather weak. Crystals of diallage are 

 occasionally present. Some sections contain no diallage, and might 

 be termed saxonite. Intimately related geologically to the hyper- 

 sthene peridotite is a large area consisting of diallage and olivine 

 with or without feldspar. Where the olivine is greatly in excess, 

 and much decomposed, the feldspar has been changed to a white, 

 translucent but isotropic substance. In some fades in which the 

 diallage is in excess, the olivine forms a sort of cementing paste 

 in which it is imbedded. The pyroxenes show no true crystal 

 outlines; yet there is often a rude approximation to them. With 

 an increase of olivine there is an increased amount of alteration 

 noticeable, the olivine being reduced to a brownish serpentine, with 

 here and there fresh cores. The pyroxenes appear fissured and 

 permeated with seams of serpentine. 



An interesting section was obtained from the southern slope of 

 the Lion's Head. It consists of partly serpentinized olivine and 

 diallage in nearly equal proportions. The apparently separate 

 individuals of the latter are almost similarly oriented, as shown by 

 the extinction angles. An examination of the hand specimen 

 showed the existence of lustrous mottled surfaces an inch or more 

 in diameter. This indicates that the extinction phenomenon is pro- 

 duced by large poikilitic crystals of diallage. The slight discord- 

 ance in orientation appearing in the section may be accounted for 

 by strain and movement brought about by the hydration of the 

 olivine matrix. 



It is rather difficult to distinguish the wehrlites from the picrites 

 unless the feldspar present in the latter be made the basis. The fine- 

 grained peridotites west of Point Morrito containing feldspar are 

 not clearly set off from the wehrlites through any peculiarity of the 

 pyroxene. In the picrites proper the pyroxenic constituent is sup- 

 posed to be augite, but in the rocks mentioned it is more nearly 

 diallage than augite, the smaller individuals still retaining their 

 augitic character, while the larger are undoubtedly diallage. Thin 

 sections of these rocks show the olivine to be in an advanced staee 

 of alteration. It is represented by an almost colorless or grayish 

 aggregate of felted serpentine shreds, among which appear occa- 

 sionally grains of altered feldspar (isotropic) and a pale colored 

 augite or diallage. 

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