Acta naturalia Islandica - 01.02.1946, Blaðsíða 61

ORIGIN OF THE BASIC TUFFS OF ICELAND
55
glacial chilling and in fact the arguments found against that theory
are partly derived from observations of these younger masses and
are equally valid for them as the older tuffs. But it is probable that
some of the youngest tuffs were extruded beneath ice-sheets.
The younger tuffs deserve a detailed treatment but it would carry
us beyond the limits of this work. Here only a few features can be
described.
Skriöci is a 1006 m high mountain north of Laugarvatn in S.
Iceland. The surrounding country is about 600 m high and above this
the mountain is in its steep slopes seen to be built up of fragmental,
Fig'. 12. — The mountain Skriða. Loose brown tuff and breccia covered by lavas
of grey basalt.
mostly glassy material up to the edge at about 850 m. Here we find
several lavas of fine-grained grey porous basalt, forming a gently
sising shield in the centre of which there is a large and deep crater.
This crater has no doubt produced the basaltic lavas, but what is
of more interest is that its last eruption produced a stream of pure
sideromelan. As shown in Fig. 13 the coarse, brown, largely stratified
layer of tuff which extends from the edge of the inner crater, has
overflowed the eastern and northern rim of the outer crater from
which it spread out over the eastern slope of the gentle dome, in just
the same way as
would a usual lava
flow. I think it is im-
possible to escape the
conclusion that this
material also flowed
calmly from the cra-
ter out over the
dome, either as a
magma which con-
solidated as glass, or
as a wet mixture of
glass fragments. In
/7o\xs o/ g'/czs-S' /z-cíg'jm gn ío'
Fíg. 13. —
A sketch of the crater on Skriða.