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usually equant and intersertal, indicating crystallization after
flowage had ceased.

Alteration varies in these basalt porphyrites, some being
almost wholly unaffected while others have cloudy feldspar and
chloritized augite. Decomposition is never comparable with
that in the Etheline volcanics.

In the thin section made from one of the basalt porphyrites
collected by C. H. Clapp, several of the labradorite phenocrysts
exhibited a peculiar regular mottling, the cause of which is not
clear.

The phenocryst best exhibiting this mottling is cut nearly
normal to 010 and 001. The composition is AbgosAnyz (deter-
mined by index of refraction, optical character, and by the
statistical method). It contains numerous inclusions of augite
up to 0-3 mm. long, usually under 0-1 mm., and generally
altered to chlorite and magnetite.

The peculiar mottling consisting of connected, rectangular
or square areas, and forming a net work throughout
the section, gives the impression of a crystalligraphically inter-
grown substance. This substance has a slightly higher index
of refraction than the rest of the phenocryst, and is probably
plagioclase, more calcic than AbgssAnz. No satisfactory explan-
ation of this unusual texture in labradorite occurs to the writer.

Basalt Amygdaloid. Amygdaloids are of frequent occurrence
in various parts of the Masset formation, and are most inter-
estingly developed at Tian point, on the west coast where highly
amygdaloidal basalts have the cavities filled with chalcedony,
quartz, calcite, and black sticky tar, the latter a substance of
great rarity in such a connexion. These rocks have not
been studied in thin section owing to the unfortunate loss of the
specimens by shipwreck, but their field appearance is that of
the ordinary basalt as described above, modified, of course, by the
texture. They are further treated of in the description of the
tar occurrence in the chapter on “‘Economic Geology.”

A specimen of basalt amygdaloid, collected from a talus
at the foot of a high cliff of flows on Tarundl creek, is a light
greenish altered looking rock, composed of about 5 per cent of