Acta naturalia Islandica - 01.02.1946, Page 78

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TRAUSTI EINARSSON
that the supply of lava was not great, so that there was practically
no movement. Under these conditions the front of the flow would
cool, cracks would be formed at the front and sides, and from these
new lava would come forth and form bulbous masses, which would
cool with the formation of a tough membrane at the surface, and
this would stretch by the pressure of the lava within until the in-
creasing rigidity stopped it. Continued pressure would cause the
formation of new “buds,” and new lava would flow through a suc-
cession of pillows connected by short necks or necks of no appreciable
length.”
Now, the conclusion which I have arrived at in my paper, is that
the magma from which the Icelandic rocks in question are derived,
was relatively cold, and often very viscous and in that case the condi-
tions required by Lewis were found. It seems to me that the Icelan-
dic pillow lavas may be satisfactorily explained by Lewis’ theory,
and at any rate I think it will be admitted that the pillow structure
is no conclusive evidence of a sub-glacial origin.
Of interest for comparison with my results is a paper by M. G.
Hoffman on “Structural features in the Columbia River lavas of
Central Washington (Journ. Geol. 1933. P. 184—195).
He describes lava structures which are quite similar to those de-
scribed in my paper: “Throughout the Moses Coulee area many
different kinds of basalt structures can be seen. Columns are thick
and thin, have parallel walls or wavy outline, and few or numerous
horizontal fractures. Many of the flows are not columnar, but massive
and broken by widely or closely spaced joints. Some are irregularly
fractured throughout and broken into odd-shaped blocks 6 inches to
1 foot in diameter. Several exhibit spheroidal weathering. Still others
are largely or entirely ellipsoidal, and a few contain a large per-
centage of shattered basalt glass” (p. 184).
“All structural types grade into one another, some gradually and
some abruptly. Here and there a lens of columnar lava is included
within ellipsoidal or scoiúaceous basalt. In a few places ellipsoidal
lava grades upward into columnar basalt.
Four structural types were found which are gradational from
ellipsoidal basalt to basalt breccias” (p. 188).
“Fuller . . . assumes that all the ellipsoidal lavas in the region were
formed under water...........Neither the studies of Lewis nor field
evidence substantiate this assumption” (p. 191). Hoffman rejects