344 THE JOURNAL OF GEOLOGY. 



which are rich in a partly chloritized base, and in which nepheline 

 occurs in well-shaped crystals. 



The second group of rocks, as already mentioned, falls under 

 the head of nepheline-basanite poor in olivine. And since 

 the specimens bear sanidine phenocrysts beside plagioclase, 

 it forms a transition to phonolite. The rock-specimens have 

 a more andesitic than basaltic appearance. Numerous pheno- 

 crysts of hornblende and augite are imbedded in the dense bluish- 

 gray groundmass. The next most abundant mineral is nephel- 

 ine in the form of phenocrysts, in part well-shaped crystals, in 

 part rounded, the largest of which are 0.5 cm. in diameter. The 

 nepheline differs from the feldspar in having a grayish color and 

 greasy lustre. Phenocryst of feldspar and crystals of olivine are 

 scarce. Beside these components, the rocks contain apatite, 

 some titanite and iron ores. Under the microscope olivine is seen 

 to be scarce. It is fresh and shows the normal properties. It con- 

 tains minute octahedrons of picotite and in some sections abun- 

 dant inclusions of a liquid with moving bubbles. 



The amphibole mineral is a typical basaltic hornblende. It be- 

 comes transparent with a dark reddish-brown color and exhibits 

 a strong pleochroism according to the following scheme: 



a, brownish yellow, t) and c dark reddish brown. Absorp- 

 tion, c > fi > a- 



The angle of extinction was examined in sections cut approxi- 

 mately parallel to the clinopinacoid (010) and was determined to 

 be very small. This fact and the dark reddish-brown color are in 

 all probability due to a high amount of FcgOg. The dependence 

 of the angle of extinction upon the amount of FcgOg in minerals 

 of the amphibole group has been recently established by Schnei- 

 der and Belowsky. The basaltic hornblende shows the well- 

 known dark borders produced by reabsorption by the magma in 

 an early stage of consolidation. In many cases nothing of the 

 original mineral is preserved; the whole hornblende is replaced by 

 a fine grained aggregate of p3^roxene and magnetite, presenting 

 clearly the outlines of the absorbed mineral. 



The group of pyroxenic minerals is represented by two mono- 



