Fairbanks. J 



Geology of Point Sal. 



47 



A section from the glassy facies of one of the basaltic dikes, near 

 the eastern edge of the gabbro, showed through the most of the 

 slide a pale green isotropic substance mixed somewhat with zoisite 

 -and granular secondary feldspar. A hand specimen of a different 

 facies of this dike also appears glassy, and contains, in addition, 

 little pea-like bodies (varioles) four to five millimetres in diameter. 

 This rock consists of an aggregate of polarizing grains, probably a 

 devitrification product. In this were imbedded the indistinctly 

 radial spherulitic bodies which polarize feebly. This is the only 

 spot where a purely glassy facies was noted. The basalt must 

 have been in a state of complete fusion when erupted, and have 

 cooled under almost uniform conditions, as the porphyritic crystals 

 are not numerous or large. The mineralogical composition and 

 structure are remarkably uniform. The almost glassy texture of 

 the dikes in the gabbro indicate that their intrusion was subsequent 

 to the complete solidification and cooling of the latter. 



There are numerous transitions between the basaltic dikes and 

 those which, though very fine grained, should, on petrographic 

 grounds, be classed with the diabases. The most of the sections 

 prepared from the diabase contain the ferro-magnesian silicates 

 augite and hornblende in varying proportions. In some cases the 

 hornblende is primary, but, as a rule, it has been formed through 

 the replacement of the augite. Excepting the secondary minerals, 

 all the diabase dikes have a fairly uniform composition. In some 

 cases there is a tendency to the separation of the augite in two gen- 

 erations. A specimen obtained about 500 feet above the water, 

 showed the presence of small granular crystals and aggregates of 

 green hornblende, a little granular colorless pyroxene, irregular lath- 

 shaped feldspars, and granular magnetite. The feldspars show high 

 •extinction angles. Another slide contained long, slender crystals of 

 hornblende, often twinned with symmetrical extinction angles of 

 about 1 5 . A slide from one of the greenish dikes, intrusive in the 

 sheeted basalt north of the seal rocks, contained augite, feldspar, 

 chlorite, zoisite, quartz, and magnetite. The feldspars form irregu- 

 lar laths. The augite, which has a pale brown color, occurs partly 

 in the form of idiomorphic crystals, partly as aggregates of differ- 

 ently oriented individuals, and partly as disseminated grains. The 



