108

wholly basaltic, was separated from the earlier more silicic in-
trusions by a considerable lapse of time during which the Skonun
sediments accumulated.

The Tertiary volcanics are believed to have originated by
processes of magmatic differentiation in a sub-crustal reservoir,
and from that source to have been intruded into the upper por-
tions of the crust, finally reaching the surface. The facts given
under the preceding heading are best explained by this hypothe-
sis, which we may now elaborate somewhat.

The consanguinity of the various types of the Etheline in-
trusives, which is evident from the descriptions given, indicates
a common parent magma which seems to have been an olivine
free basalt of felsic nature. This common parent magma is
supposed to have differentiated by gravitative adjustment so
that a gradational stratified condition was reached, with dacites
at the top, underlain by andesites, in turn resting on basalts.
This differentiation and stratification took place during the long
period of quiescent subsidence during which the Cretaceous sedi-
ments were accumulated. The movements causing the folding
of the sediments disturbed the equilibrium of the magma chamber
and part of its filling was injected into the roof. The intrusions
began gradually, consequent on the slow but powerful forces de-
forming the region and extended over a considerable period, as
shown by the folding of sills and dykes with the Queen Charlotte
series. The first material to leave the chamber was the upper-
most dacite magma, and this was forced into the first cracks
penetrating the crust. It will be recalled that dacite occurs only
as dykes. There are a number of reasons for this: first, the
marginal rock was the coolest or possessed the least degree of
superheat and, therefore, was most viscous and incapable of
penetrating to the surface; second, the silicic character of the
magma would render it viscous; third, the first fissures may well
be supposed to have been of less magnitude than the later ones;
and fourth, the surface was at a greater distance in this early
period of intrusion owing to a less amount of erosion having
taken place. In the case of the andesites, the same causes oper-
ated but in less degree; so we have many andesite dykes and
some sills, necessitating a more superheated rock. The greater