Fairbanks.] 



Geology of Point Sal. 



65 



Olivine is almost universally present in the gabbros and perido- 

 tites, but generally absent from the diabasic dikes. It is in a fairly 

 fresh condition in the feldspathic rocks, but appears in all stages of 

 alteration in the peridotites, until the extreme is reached in the 

 dunites, where in some cases its former presence is recognized only 

 by the mesh structure. It never possesses idiomorphic boundaries, 

 but has the form of rounded grains or irregular aggregates of indi- 

 viduals differently oriented. 



The rocks as a whole are remarkable for the small amount of 

 iron ores present. Magnetite is absent from the normal gabbro and 

 peridotite, and occurs in abundance only in the narrow dikes which 

 appear to have been among the latest of the intrusions. The peri- 

 dotites are remarkable also for their small amount of chromite and 

 picotite. In one slide only do there appear a few reddish grains, 

 which are probably referable to one of these minerals. 



Having given now an outline of the characters of the rock- 

 forming minerals, a brief description will be added of the various 

 rock facies. The blending of one variety into another adds to the 

 complexity of the subject, but typical examples of the different 

 facies already given will aid in understanding them. 



The anorthosites, those rocks poor in the ferro-magnesian sili- 

 cates, occur in the gabbro in the form of bands occasionally reach- 

 ing a width of six inches. Bands one to two feet wide, which 

 appear more like veins, occur in the massive gabbro. There are 

 also ill-defined areas in which the dark silicates are sparingly repre- 

 sented. In different parts of this facies appear small amounts of 

 olivine, augite, and brown hornblende. In one vein the feldspars 

 appear most curiously dovetailed into each other, and many indi- 

 viduals, particularly those not showing twinning, present a sort of 

 zonal structure with a jagged outer rim extinguishing at a different 

 angle from a similarly jagged interior. 



The dikes possessing an ophitic structure range from true dia- 

 bases to quartz-diorite. A dike of the latter was noted intrusive in 

 the gabbro. It is four feet wide, dark, and very fine grained on the 

 edges. Judging from the extincrion angles, the feldspars evidently 

 belong in the more acid portion of the labradorite series. They 

 have well-marked zonal structure. The hornblende is without any 



