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THE JOURNAL OF GEOLOGY.

Point, show it to be a little more acid than the latter, though not enough so as to cause it to be placed in a position in the plagioclase scale far removed from that of the feldspar of the diabase[1]. The corresponding figures for the two plagioclases are:

SiO2 Al2O3 Fe2O3 FeO CaO Na2O
Gabbro 51.89 29.68 .69 12.62 3.87
Diabase 53.75 30.39 1.26 10.84 3.76

The augite is generally older than the plagioclase, although the latter mineral seems sometimes to mould the contours of the former one. The pyroxene occurs either in the interstices between the labradorite grains, or as narrow rims around the olivine, forming a mantle that surrounds these and separates them from the feldspar (see Fig. 1).[2] The mineral is very light colored, sometimes being almost colorless, but it is usually tinged

Fig. 1.Section of the olivine-gabbro, exhibiting the tendency of the pyroxene to include olivine grains. Section 1103. ✕ 20.

with pink. It is moreover possessed of a diallagic parting, accentuated by dark decomposition products, the most abundant of which are tiny, irregular black and brown dots. These are scattered everywhere throughout the pyroxene, but are accumulated most thickly in the neighborhood of the cleavage lines. In some of the pyroxene pieces are the peculiar platy inclusions

  1. ↑ Bull. U. S. Geol. Survey, No. 109.
  2. ↑ Cf. M. E. Wadsworth, Bull. No. 2, Geol. and Nat. Hist. Survey of Minn., Pl. III. Fig. 1.In this figure the author pictures a pyroxene and olivine bearing the same relation to each other as the diallage and olivine shown in Fig. 1 of this paper.
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