Acta naturalia Islandica - 01.02.1946, Síða 25

ORIGIN OF THE BASIC TUFFS OF ICELAND
19
talline lavas in the middle of the period. At that time, however, some-
where else sideromelan was produced, probably as ash, and blown
by the wind to our locality. Also mudflows occurred as a volcanic
phase, consisting mainly of glass fragments. In one case we have
observed what may be a transition from a crystalline to a glassy con-
solidation.
As to the formation of sideromelan no signs of a connexion with
ice are visible.
On the other hand the peculiar, thick lava at the edge of the plateau
points to a quite different explanation, for this lava indicates that
the same magma consolidated in three different ways without any
detectable direct influence of glaciers, namely with columnar jointing,
block jointing, and as a mass of brown glass, and significantly enough
the patches of brown glass are surrounded by lava and have there-
fore hardly been in contact with a chilling agency.
This observation suggests that the formation of sideromelan is
to be considered as dependent on certain physical properties of the
magma rather than being caused by external chilling agencies. The
same suggestion is derived from the breccia at Vörðufell. Ice may
be assumed to cause the formation of minute glass fragments, but
it can hardly produce a compact mass of glass. Or what about the two
generations of glass, the one enclosed in the other? It is at least
not naturally accounted for by glacial influence.
We have, however, at this stage still too scant a material to enter
into a further discussion of the possible modes of formation of sider-
omelan.
3. EYJAFJÖLL
This is a neighbouring plateau crowned by the snowcapped recent
volcano Eyjafjallajökull. It is separated from Tindafjöll by the valley
of Markarfljót and it may be more correct to consider the two com-
plexes as a single one dissected by the river. Yet, this is a question
wich I shall not further discuss here. We are concerned with the com-
position of this second complex which is a volcanic series closely
similar to that of Tindafjöll.
At the farm Hvammur I made a closer study of the series.