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to 0-5 by 0-07 mm., finely twinned after the albite law, some-
times after the Carlsbad as well. It forms from 40 per cent
to 85 per cent of the rock. Augite is colourless, pale greenish
or violet, faintly pleochroic or not at all. It is often almost
entirely unaltered. In some rocks it varies greatly in size
from 0-02 mm. to 1-5 mm., the larger grains being oikocrysts
poicilitically enclosing chadacrysts of labradorite. The smaller
grains are interstitial between the feldspars. Alteration of the
augite, when present, is to chlorite, calcite, and finely granular
magnetite. Augite makes up 15 to 30 per cent of the rock.
Primary magnetite (ilmenite?) is in considerable abundance
in some specimens, in single and grouped euhedral grains.

The texture is in most cases intersertal, but in some it is
ophitic, and others exhibit both fabrics. In all cases a marked
fluidal development of the labradorite laths is manifested,
even in the poicilitic basalts the labradorite chadacrysts in the
augite oikocrysts being arranged parallel to the general direction
of flow.

Basali Porphyrite. Several varieties of this type of flow
rocks have been studied, which vary only in texture. In the
field they are black or dark grey, heavy rocks, differing in the
amount of phenocrysts they contain, and in the granularity of
the groundmass, which ranges from glass (forming the vitro-
phyre, a variety described below) to distinctly, though finely
crystalline.

In thin section labradorite appears as a phenocryst, usually
accompanied by augite, in a groundmass composed of the same
minerals with magnetite (ilmenite). In some cases, badly
altered grains, of the general size of the phenocrysts, are seen,
which were probably originally olivine. The labradorite varies
in size from 0-25 mm. to 1-0 mm., and is frequently equant,
though sometimes tabular. Its composition is from AbsgAnzo
to AbsAng. Augite is generally smaller than the feldspar
phenocrysts, and is usually equant and euhedral. In many
rocks the phenocrysts have a marked glomeroporphyritic
tendency. The groundmass consists of labradorite and augite
with magnetite, the feldspar being typically lath shaped and
often arranged in marked flowage lines, while the augite is

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