352 V. DOLMAGE.

Younger than these, and cutting sediments of known Cre-
taceous age outside the district here considered, are many small
intrusions forming sills and dikes, which Leach has grouped
together into what he calls the Bulkley intrusives. These intru-
sions are of two different rock types, acid members of a peculiar
composition, being high in soda, low in potash and medium high
in silica; and basic members lamprophyric in composition. In
hand specimen the acid type has a reddish pink color, and is seen
to be composed of a close aggregate of pink tabular feldspar
phenocrysts, uniform in size, having an average length of 4 milli-
meters evenly distributed in a scant light gray groundmass,
sprinkled with small grains of iron oxide. Under the micro-
scope the phenocrysts were found to be albite, and their pink
color is due to abundant decomposition products, mainly hematite.
The groundmass consists of quartz, sericite, orthoclase, chlorite
and hematite. The quartz is the most abundant, while the ortho-
clase is almost negligible in amount. The hematite forms pseudo-
morphs probably after amphibole.

This rock occurs only in the western half of the area where it
forms the core of the western mountain group called the Star
Mountains. It forms an exceedingly irregular mass with nu-
merous long tongues or lobes projecting out in three dimensions.
It would be difficult to classify it with any of the common types
of injected bodies but it seems to fill all the conditions of Pro-
fessor Daly’s definition of a chonolith.

The basic members of the Bulkley eruptives occur in all parts
of the area, usually but not always forming dikes, and have
played an important part in the mineralization of the district.
They contain such rocks as diabases, kersantites and odenites,
diabases being the most common.

ECONOMIC GEOLOGY.

A considerable amount of mineralization has been produced
along the contacts of all the above-described intrusions, and
while no large ore bodies have as yet been discovered in the dis-
trict, there are many small deposits, some of which are rich in
copper, gold and silver. According to their geographical dis-