79

built up in this way. None of the actual vents from which this
great basaltic accumulation came were seen, and it is improbable
that any volcanic cones now remain. The tachylyte of Ship
Kieta island is, however, evidence that one at least of the vents
was in that vicinity. From the well stratified nature of some
of the tuffs it is inferred that their accumulation was subaqueous.

Origin. From the character of the rocks, it is plain that
the Masset formation is the result of extensive explosive volcanic
action, accompanied by the extrusion of great flows of lava.
What is now Graham island, may be conceived to have been an
archipelago in this volcanic epoch, or an upland with large
lakes, as many of the tuffs and agglomerates are clearly water-
lain. Into these water basins were also carried ferruginous
detrital sediments, derived from the subaérial decomposition
of the basalts, in a way analagous to the hematitic deposits
of the Island of Molokai in the Hawaiian group.!

Age. The Masset formation is younger than the Skonun
sediments, which may in part be Pliocene in age, and is prob-
ably conformable with them. It is older than the Glacial
epoch, as many of the outcrops are glacially scoured and the
basalts of the Slatechuck range have been greatly sculptured by
ice action. Until the age of the Skonun sediments is definitely
determined, the Masset formation may be considered Pliocene
in age.

COMPARISON OF THE YAKOUN AND MASSET FORMATIONS.

A comparison of igneous rocks of different ages in the same
district is always interesting from the scientific view-point, and,
in the case of the Masset volcanics of Tertiary age and the
Yakoun volcanics of Jurassic age, there is an important economic
reason why these two formations should be compared and clearly
distinguished. Between them lies the coal-bearing Queen
Charlotte series, resting unconformably on the Yakoun volcanics,
and unconformably overflowed by the Masset volcanics. To the
prospector the ability to recognize which formation of volcanics—

1 Lingdren, W., “Mineral deposits." McGraw-Hill Book Co., New York, N.Y., 1913,
p. 250.