Fairbanks.] 



Geology of Point Sal. 



69 



With the decrease of augite we reach the type of rock to which 

 he term "troctolite" has been given (German, Forellenstein). This 

 rock sometimes occurs as facies of the normal gabbro, and some- 

 times as homogeneous independent dikes. A dike of this kind 

 appears intrusive in the serpentine south of the Lion's Head. The 

 greater portion of the troctolite is found in the near vicinity of the 

 peridotite area west of Point Morrito. While it occurs here well- 

 defined from .the true gabbros, yet it is genetically connected with 

 them rather than with the peridotite. The rock is here remark- 

 ably gneissoid, owing to the arrangement of the olivine granules in 

 long, platy aggregates. It is an interesting fact that the olivine 

 grains never occur alone, except as inclusions in the other constit- 

 uents. The feldspars are invariably allotriomorphic. The rocks 

 are rarely free from small amounts of augite and hornblende. 

 Magnetite is absent from all the specimens examined. 



Olivine norite-gabbro is quite common, occurring almost always 

 as dikes in the complicated area west of Point Morrito. Some of 

 these have a subordinate amount of feldspar, and might perhaps 

 be considered as transitions to the peridotite. South of the Lion's 

 Head a dike of this composition was seen intrusive in a peridotitic 

 rock. The latter consists of augite, olivine, feldspar, and horn- 

 blende, the two last components in small amount, while magnetite 

 is abundant. The dike is about two inches wide, but possesses a 

 typical granular structure with the following composition: augite, 

 hypersthene, feldspar, olivine, and a small amount of magnetite. 

 The constituents of the two rocks interlock along the contact in 

 such a manner as to lead to the view that the older was not fully 

 •consolidated at the time of intrusion. 



A hornblende-bearing facies of this type is illustrated by an 

 interesting specimen from the canon below the dairy. It is some- 

 what more coarsely crystalline than the others, and contains an 

 abundance of granules and irregular plates of a pale yellowish 

 brown hornblende with weak pleochroism. It is intergrown in 

 several cases in a granophyric manner with the augite. Some of 

 the hornblende has a poikilitic habit, with inclusions of the other 

 constituents. Several large irregular crystals of olivine are pres- 

 ent, and one section of twinned augite approximately idiomorphic. 

 There is in addition a very little magnetite. 



