Fairbanks.] 



Geology of Point Sal. 



51 



mile southeastward from Point Lospie, the gabbro has a fairly uni- 

 form character. It is generally a coarsely crystalline rock with a 

 somewhat varying proportion of feldspar. At the western end it is 

 perfectly massive, but toward the east there appears in places a ten- 

 dency to the formation of a gneissic structure, which, in connection 

 with a more or less regular banding, finally comes to be the predom- 

 inant structure. In one or two places, veins or segregated bands 

 of almost pure feldspar appear. Two fine-grained, greenish dikes 

 of ophitic diorite were noted, which are similar to those in the ser- 

 pentine farther east'; also a darker dike, very fine on the edges, and 

 coarser in the center, which a thin section showed contains quartz 

 in addition to feldspar and hornblende. One of these dikes was 

 finely and regularly banded through the arrangement of the ferro- 

 magnesian constituents in thin layers. About half a mile from 

 Point Lospie the gabbro begins to show marked variations in com- 

 position and structure. The banding or gneissic structure is pres- 

 ent in nearly all portions, the bands running in a direction which is 

 usually more nearly east and west than the shore line. A crush 

 zone extends nearly parallel with the shore for several hundred feet. 

 In it the gabbro, with the exception of nodules, has been reduced 

 to a clayey mass and strongly reddened. About half way between 

 Point Lospie and Point Morrito, there begin to appear veins con- 

 sisting of feldspar, augite, and olivine, rarely hornblende, which are 

 very coarsely crystallized, and extend irregularly in every direction. 

 These vary from an inch to ten inches in width. Their origin is 

 somewhat difficult of explanation, for they do not have the appear- 

 ance of being dikes. In many cases there is no sharp line of de- 

 markation between the vein and the walls. The granular pyroxene, 

 feldspar, and olivine of the rock interlock with the constituents of 

 the vein without showing any appearance of a break in the period of 

 crystallization. In this vein crystallization has progressed more 

 slowly to the formation of crystals sometimes two inches in diam- 

 eter. It would seem as if they must be due to the same cause as 

 the veins in the augite-teschenite, that is, to irregular planes through 

 the rock mass in which, owing to movement or tension, crystalliza- 

 tion progressed very much more slowly. They correspond in 

 appearance to many so-called " segregated veins ; " but in these 



