98

brown biotite phenocrysts of ragged outlines and up to 2 mm.
across, forming less than 10 per cent of the rock.

Under the microscope the biotite is strongly pleochroic,
and often partly replaced by chlorite, and phenocrysts of sodic
andesine also occur. Augite is very rarely found as a phenocryst.
The groundmass is probably andesine, apparently slightly more
calcic than the phenocrysts, and is considerably altered to chlorite
and calcite. Apatite and magnetite are accessory minerals.

In composition, texture, and alteration this rock shows its
kinship with the Etheline intrusives.

General Petrology. The Etheline formation is characterized
by a complete gradation from dacite to augite andesite—a rock
near a basalt in composition—and the series possesses a few pecu-
liarities common to all the types. The textures of the rocks are
remarkably similar and are in sharp contrast to the textures
both of the Jurassic Yakoun volcanics, and of the later Tertiary
Masset flows. The minerals forming the rocks are strikingly
similar, and a complete gradation of plagioclase feldspar of
similar habit is observed. The augite is very much the same
throughout the series. The consanguinity is shown not only
in the presence of certain minerals, but in the absence of others.
Thus, biotite has been noted in three instances only and horn-
blende in one only. Subordinate minerals are characteristically
rare. Magnetite, ilmenite, and apatite only have been noted.

The general family resemblance of the rocks is also shown
in their alteration. The characteristic change is a chloritic
replacement of augite, and a replacement of the minerals and
rocks as a whole with calcite. Impregnations with finely granu-
lar pyrite are also characteristic.

Alteration. A striking feature of the Etheline formation
is the progressive alteration exhibited, this being greatest in
the most silicic rocks, the dacites, and uniformly becoming less
as the sub-silicic end of the series is approached, There are,
of course, exceptions; but the general rule seems to hold for most
of the specimens studied.

The very great alteration of augite and other ferro-magnesian
minerals to chlorite, in these Tertiary intrusives, is a strong in-
dication that processes more intense than usual have affected