700 THE JOURNAL OF GEOLOGY. 



the opposite conclusion. The amount of plagioclase in all por- 

 tions 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 pyrox- 

 enic 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 

 lamellae, the maximum extinction on each side of whose compo- 

 sition 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 2.71 1, 

 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 men- 

 tioned. The mean density of the feldspar separated from the 

 three rocks was thus 2.701, which indicates a very basic labrador- 

 ite. In the feldspar of a specimen of the gabbro from the Clo- 

 quet river Irving 1 reports 52.40 per cent, of Si0 2 , while for the 

 most acid member of the bytownite series Tschermak 2 calculates 

 49.1 per cent, of Si0 2 . The largest quantities of the powder in 

 the above three cases fell respectively at 2.700, 2.71 1 and 2.712. 



* x Copper-Bearing Rocks, p. 439. 



2 Lehrb. d. Mineralogie, 2te Aufl. 1885, p. 439. 



