100

encountered, piled up in a great succession of flows and in ag-
glomerate and tuff beds, over 5,000 feet thick, and covering an
area of over 1,000 square miles.

The various types of basaltic rocks examined in thin section
can be considered only as samples of the formation, although
their unvarying composition is good testimony to the general
homogeneity of this great mass of volcanics.

Basalt Dykes. Basalt is rare as an injected type, but a few
dykes have been noted. They are dark grey, very finely crystal-
ine rocks.

Under the microscope the essential minerals are labradorite
and augite, with subordinate magnetite, and secondary chlorite
and calcite. The labradorite has the composition AbsAngo,
and is in sharply crystallized, lath shaped, and tabular finely
twinned forms. It forms about 60 per cent of the rock, averages
0-12 by 0-04 mm. in size, and is very fresh. The augite has a
violet tinge, and is faintly pleochroic. It is usually fresh, but
is occasionally altered to chlorite. Magnetite forms individual
grains, and is often in parallel and reticulate strings of octahedra.
The texture is intersertal, the augite in small equant grains filling
the interstices between the labradorite laths. Labradorite and
augite crystallized in part simultaneously, but the latter kept
on forming after the labradorite had ceased to grow.

Both in composition and habit the dyke basalt is clearly re-
lated to the effusive basalt, and the dykes are doubtless the
feeders for the flows, in part at least.

Basalt Flows. These flows are fresh-looking rocks, varying
in texture from almost vitreous to very finely, but distinctly
granular. This type is of very wide distribution and, with the
basalt porphyrite, makes up by far the larger part of the forma-
tion.

Under the microscope the essential minerals are labradorite
and augite, with subordinate magnetite and perhaps olivine.
Some rocks contain small amounts of residual glass, and these
form a link with the vitrophyric type. Secondary minerals are
chlorite, calcite, kaolin, limonite, hematite, epidote, and prob-
ably serpentine. The labradorite is usually in sharply crys-
tallized, unaltered, rectangular laths, from 0-05 by 0-005 mm.