Rit (Vísindafélag Íslendinga) - 01.07.1962, Blaðsíða 190

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able masses of fine and medium grain are the result of sub-
aerial explosive eruptions. In the end this may possibly be
caused by water that mixes with the lava at considerable
depth.
3) Especially by relatively low temperature of the magma,
i.e. high viscosity, and by lack of internal stirring, there is
a tendency towards glassy consolidation. Also brecciation is
likely, as a very viscous melt is fragile in response to strong
forces.
With this background we shall now consider the main field
relations. Thereby we must stress that the Fragmental Rocks
consist of a multitude of individual bodies and because of
unsatisfactory outcrops it is very often difficult to come to
a clear result as to how exactly the individual bodies were
formed.
In the lower part of the Basalt Plateau volcanic breccias
are extremely rare but they are not completely absent. Thus,
in the well-known locality for Lower Tertiary lignite, Brjáns-
lækur, we find a river bed in the muddy bottom of which
wood chips have been buried and along which two or three
lavas have flowed (Einarsson 1962). One of these lavas is
in part a typical brown “palagonite breccia” quite similar to
elements found in the much younger main mass of Frag-
mental Rocks. It seems significant that presence of water was
likely in this case. The extreme scarsity of such fragmental
rocks in the Lower Plateau Basalts, being concurrant with
the predominant dryness of the land, as derived from other
evidence (Einarsson 1962), is very suggestive that water, in
one way or another, was the predominant factor in the pro-
duction of the Fragmental Rocks.
In the Young Plateau Basalts fragmental rocks becom
common. It is then probably significant that here fluviatilt
sediments are also common. A layer of breccia is in several
casés found to overlie “glacial” beds, but mostly not imme-
diately. Instead the breccia rests on an eolian deposit or
sometimes a lake deposit. The relationship is decidedly ad-