Page:The Journal of geology (IA journalofgeology11893univ).pdf/360
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 phenocrysts of hornblende and augite are imbedded in the dense bluish-gray groundmass. The next most abundant mineral is nepheline 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 contains minute octahedrons of picotite and in some sections abundant inclusions of a liquid with moving bubbles.
The amphibole mineral is a typical basaltic hornblende. It becomes transparent with a dark reddish-brown color and exhibits a strong pleochroism according to the following scheme:
a, brownish yellow, b and c dark reddish brown. Absorption, c > b > a.
The angle of extinction was examined in sections cut approximately 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 Fe2O3. The dependence of the angle of extinction upon the amount of Fe2O3 in minerals of the amphibole group has been recently established by Schneider 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 pyroxene and magnetite, presenting clearly the outlines of the absorbed mineral.
The group of pyroxenic minerals is represented by two mono-