6

the overlying one Mesozoic. The upper part of the series that may be in
part Triassic could not be distinguished lithologically from the lower sedi-
ments, and as it yielded no fossils it was included with the Dease series.

MCLEOD SERIES

Volcanic and sedimentary rocks of the McLeod series form the entire
northern side of Hotailuh mountains and extend from Thenatlodi peak
on the west to the headwaters of McBride river on the east. Smaller
patches of the McLeod series outcrop in Big Eagle valley south of Black
mountain. North of the Cassiar batholith the series forms a belt 40 miles
long that extends northwest from Beale lake past the head of Quartz creek.
The rocks are quite resistant to erosion and form most of the higher peaks
in Cassiar and Hotailuh mountains.

The McLeod series is a rather complex interbedded assemblage of
voleanic fragmental rocks and flows, and minor amounts of sandstone,
greywacke, cherty quartzite, and conglomerate. It is usually possible to
distinguish between voleanic and sedimentary members of the series in
the field, but they are so intimately associated that they cannot be
separated on the accompanying map.

There are intrusive bodies of augite porphyrite within the belt of
volcanic rocks on the north side of the Cassiar batholith. In some places
the augite porphyrite is partly altered to serpentine and because of this
relationship it will be described along with the other basic rocks associated
with serpentine.

The basal member of the McLeod series in the vicinity of Black moun-
tain is a conglomerate that contains rounded cobbles of limestone and
quartzite in a tuffaceous matrix. Several miles to the southeast the basal
member is a fine-grained andesite.

The tuffs, agglomerates, flows, and interbedded sediments exposed in
Hotailuh mountains are at least 10,000 feet thick, The base of the series
is not exposed. The rocks have been intruded on the south by granite,
and on the north dip under recent alluvium and glacial drift between
Cassiar and Hotailuh mountains. Several fairly complete sections show
a gradation from predominantly sedimentary rocks in the south, to pre-
dominantly voleanic rocks in the central and northern parts of the
mountains.

Extrusive rocks which make up the greater part of the McLeod series are
well exposed in Hotailuh mountains east of Tanzilla river. Massive flows
up to 400 feet thick are intercalated with tuffs and agglomerates. Although
these rocks are varicoloured on weathered surfaces they are green on fresh
fractures.

The lavas are fine-grained andesites. Some are amygdaloidal and
many are porphyritic, feldspar and pyroxene forming the phenocrysts.
Microscopic examination shows that the feldspars, oligoclase-andesine in
composition, are altered to saussurite and that the pyroxene phenocrysts
are partly altered to chlorite. The groundmass is made up of a fine felt
of feldspar laths, chlorite, calcite, magnetite, hematite, and, possibly, some
glass. Quartz and calcite are present as amygdule fillings.