Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 187

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cycle, and partly towards its end the wide lowlands were
covered with a thin sheet of lavas: the dolerites.
2. Mode of formation of tlic Fragmental Rocks.
We have earlier referred to the main theories that have
been put forward to explain the mode of formation of the
great amounts of fragmental volcanic rocks of basaltic com-
position in Iceland. We shall now consider this question a
little further.
In the writings of Th. Thoroddsen and H. Pjeturss a “Pala-
gonite Formation” was a vaguely defined assemblage of more
or less fragmental rocks overlying the main bulk of the Pla-
teau Basalts. Rocks of very different ages and different ori-
gins were gathered under this name. Thus in Thoroddsen’s
geological map of 1901 the Hreppar Plateau Basalts, Vörðu-
fell, Mosfell, Bjarnarfell, Bláfell, Ármannsfell, Súlur, and
Hrúðurkarlar have the same signature (“Palagonitic Breccia,
Tuffs and Conglomerates”) being parts of the Palagonite For-
mation. These rocks range, according to the present analysis,
from the Upper Pleistocene and perhaps hack to Miocene
times. On the other hand the Young Plateau Basalts at Hval-
fjörður are shown as “Tertiary Basalts”. And when Pjeturss
announced the discovery of indurated moraines in the Hrepp-
ar basalts he claimed to have proved a Pleistocene age of the
Palagonite Formation.
As long as the stratigraphy of the Fragmental Rocks was
little known, an explanation of their origin could only be
formulated in the most general terms: they were either of
sub-marine, explosive, or sub-glacial origin. The last alter-
native was based on the occurrence of tillites within the Frag-
mental Rocks, hut also on the nature of the occurring glass.
It is the merit of Peacock (1926) to have stressed the fact
that the glass occurring in the Fragmental Rocks is in the
unaltered state essentially sideromelane, not tachylite. In con-