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THE BASIC MASSIVE ROCKS, ETC.
701

There can thus be no doubt that the feldspar throughout the entire mass of the rock is practically of the same character, since the three specimens tested were taken from three widely separated portions of the gabbro area, and each represents a distinct type of the rock. No. 8786 is very rich in olivine, No. 8589 contains much augite and a large quantity of brown biotite, while No. 10440 is very rich in feldspar and quite poor in pyroxene.

An analysis of the feldspar separated from No. 8786, and partial analyses of the plagioclase from the other rocks were made by Dr. W. H. Hillebrand. They are as follows:

8786 8589 10440a 10440b
SiO2 51.89 52.18 47.59 46.92
Al2O3 29.68 29.20 30.97 31.51
Fe2O3 .32 1.11 1.55[note 1] 1.29[note 1]
FeO .37
CaO 12.62 11.18
MgO .38
K2O .50
Na2O 3.87
H2O (100°) .07
H2O (above 100°) .39
Total 100.09
Sp. Gr. 2.700 2.711 2.712 2.674

The figures under 8786 and 8589 correspond very closely with those of a basic labradorite. Those under 10440a and 10440b are abnormal, in that they indicate that the more basic portion of the feldspar in this rock has a lower specific gravity than the more acid one. The alumina in the four cases, however, corresponds quite well with the proportion of this oxide in basic labradorites. In Ab1An3, which Tschermak makes the dividing line between labradorite and bytownite, the percentage of alumina present is 32.8 per cent. Since the rock specimens from which these feldspars were separated represent the only phases of the gabbro that have retained the normal gabbro characteristics, it is probable that the feldspars themselves represent the variations within whose limits all of the feldspar in the great mass of the rock may be found. A comparison of this plagioclase with that of the very coarse diabase from the boss-like dike forming Pigeon

  1. ↑ 1.0 1.1 All iron determined as Fe2O3.
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