Acta naturalia Islandica - 01.02.1946, Síða 65

ORIGTN OP THE BASIC TUFPS OP ICELAND
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ent were thus formed early in the geological history of Iceland —
long before the Quaternary period. Volcanic tuffs are also found on
the lowest visible horizons in the fundamental basalt plateau as e.
g. in Esja and in Suðursveit in S. E. Iceland, and it is at least
probable that these altered tuffs contained sideromelan.
Wide areas of this plateau are covered with a thick layer, up to
500—600 m, of brown tuff and breccia, mainly consisting of sidero-
melan, but also of opaque glass and scoriae, and very fine-grained
blocks of lava. These piles of fragmental material are only erosional
remnants of a wider cover, which certainly was originally of a far
greater extent than to-day.
Many of these remnants of the old cover carry a layer of ophitic
dolerite lavas. Often there are fresh craters in these lavas. At Bú-
landshöfði, in Snæfellsnes, a thick pile of brown tuffs and breccias
rests on beds with arctic molluscs.
We have therefore a good reason to assume, that the main cover
of fragmental material is of Quaternary age, probably Early Quater-
nary.
Still younger are the fresh flows of tuff on Kaldbakur and Skriða
and the heaps of fragmental material piled up on vents and fissures
ranging even into post-glacial times.
We must at this stage point out the main difference between this
conception of the geological structure of Iceland and that of earlier
authors, especially that of Thoroddsen.
Thoroddsen speaks of Middle Northern, North Western and East-
ern Iceland as „Basalt regions” in which brown tuffs and breccias
sporadically also occur, intercalated between the lavas. Southern
and Central Iceland, on the other hand, is a „Palagonite region” in
which basaltic lavas occur intercalated between the brown tuffs
and breccias, the higher percentage of fragmental material in South-
ern Iceland than in the “Basalt regions” thus leading to a splitting of
one and the same plateau series. Thoroddsen found that the Pala-
gonite Formation is younger than the Basalt Formation. What he
really discovered was, that the thick cover of fragmental brown
material is younger than the composite basaltic masses of Middle
Northern Iceland and the Eastern fjords, i.e. it is younger than our
composite serips, but thereby he does not prove, that the composite
series of Southern Iceland is younger than that of e. g. Middle North-
ern Iceland.