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number of andesites is what would be expected according to the
hypothesis of differentiation from a basalt or an augite andesite
magma.

The tapping of the upper portions of the magma chamber
seems to have sufficed to relieve the pressures that tended to
cause eruption; so that a quiescent period of considerable dura-
tion ensued during which the Skonun formation accumulated.
Then came an era of tremendous volcanic activity and explosions
and effusions of lava took place in great number. During this
period the main basaltic sub-crustal reservoir was in direct com-
munication with the surface.

It is thus seen that the varicus igneous rocks of the Tertiary _

period may best be explained as the result of magmatic differen-
tiation by gravitative adjustment. This general explanation,
however, does not suffice to explain the exceptional alkaline
trachyte associated with the basalts of the Masset formation;
and it can only be said here that owing to its unique occurrence
in the series, and its relatively small volume, it is apparently a
local differentiate, in, or near a vent. That it was near a vent is
indicated by the tachylyte as well as the agglomerates found in
its vicinity. Surrounded as it is by enormous masses of basalt
the field relations of this trachyte afford further proof that it isa
differentiate of the basaltic magma.