Fairbanks.] 



Geology of Point Sal. 



49 



served in the exterior. Apatite is present in some slides. One 

 short, thick rod was noted having OP at one end and a pyramid at 

 the other. 



Chemical and Mineralogical Character. — Some interesting fea- 

 tures appear in connection with this series of intrusives and flows. 

 One is the mineralogical simplicity of an exceedingly complicated 

 series of eruptions. The only primary components recognizable are: 

 plagioclase, augite, hornblende, apatite, and magnetite. During the 

 petrographic investigation the question arose as to whether all the 

 volcanic flows and related dikes were really basic enough to be 

 termed basalts. The feldspar microlites were so decomposed that 

 measurement was difficult. The angles, however, as nearly as they 

 could be determined, were low in all cases, the highest ranging from 

 1 5° to 22°. A sample of one of the dark greenish masses cutting 

 across the spheroidal basalt was taken for analysis, and the result is 

 given in No. II, in the table below. The feldspar in this rock is 

 apparently the same as that in the spheroidal basalt, and the excess 

 of silica and alkalies and lack of lime are quite puzzling. A partial 

 analysis of the spheroidal basalt is also given in No. III. This cor- 

 responds more nearly with what would be expected from the micro- 

 scopic examination. An extreme degree of alteration is shown by 

 all these fine-grained amygdaloidal rocks, which, to a certain extent, 

 invalidates any conclusion which can be drawn from the analysis. 

 Making allowance for this state of things, there is certainly a 

 greater chemical difference in the flows and dikes than was at first 

 supposed. It is probable that none of the rocks contain much less 

 than 50 per cent of silica, and that they range as high as 56, with a 

 proportionate decrease in lime, and increase in alkalies. The most 

 acid facies must be considered as an andesite. 



The gabbro taken for analysis (I) is a comparatively fresh rock 

 with both hornblende and augite, but less magnetite than is usually 

 present. The character of the feldspar in the diabasic facies is the 

 same as in the gabbro. 



Alteration interferes with the drawing of any exact conclusions 

 in regard to magmatic variation in this series, a variation which it 

 might be expected would be considerable. The order of succession 

 is, as far as was made out, amygdaloidal basaltic flows ; less amyg- 



