Page:The Journal of geology (IA journalofgeology11893univers).pdf/274
the opposite conclusion. The amount of plagioclase in all portions of the gabbro mass is so great that it must have occupied a long period in its separation. It is probable that the augite began to separate from the magma that yielded the rock some time before the plagioclase, but that after the feldspar began to crystallize the two minerals grew side by side until all the pyroxenic material of the magma had been extracted from it, when the feldspar continued its growth unaccompanied by the formation of pyroxene. Thus some of the plagioclase is older than some of the augite, though the greater part is younger than the great mass of this mineral.
All the plagioclase grains are traversed by broad twinning lamellæ, the maximum extinction on each side of whose composition plane is about 35°. In order to determine accurately the nature of this plagioclase, the three specimens whose densities are given, were powdered and their feldspars separated by the Thoulet solution. Most of the mineral was precipitated when the density of the solution was between 2.674 and 2.728, the limits in the different cases being as follows: in specimen 8786 between 2.700 and 2.728; in 8589 between 2.700 and 2.711, and in 10440 between 2.674 and 2.712. As a small amount of the plagioclase in each specimen was more or less altered, the average of the above figures may be taken as representing the average density of the plagioclase in the gabbro. The method is justified in the fact that the optical properties of the powder in all cases was exactly the same, and that its precipitation was not in steps or stages, but was continuous between the limits mentioned. The mean density of the feldspar separated from the three rocks was thus 2.701, which indicates a very basic labradorite. In the feldspar of a specimen of the gabbro from the Cloquet river Irving[1] reports 52.40 per cent. of SiO2, while for the most acid member of the bytownite series Tschermak[2] calculates 49.1 per cent. of SiO2. The largest quantities of the powder in the above three cases fell respectively at 2.700, 2.711 and 2.712.