Acta naturalia Islandica - 01.02.1946, Blaðsíða 35

ORIGIN OF THE BASIC TUFFS OF ICELAND
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5. SOME GENERAL CONSIDERATIONS
VVith Skeiðarárjökull we have reached the eastern limit of this wide
area of the Palagonite Formation. An isolated small area of similar
rocks is the complex of Öræfajökull which is separated from Fljóts-
hverfi by the mountains around Morsárdalur which are built up of
older dipping lavas, a different kind of basalt with intercalated
altered tuffs and breccias.
The main area here considered from Tindafjöll to Fljótshverfi
is a vast, dissected plateau built up of horizontal layei-s of:
1. Grey porphyritic very fine-grained lavas.
2. Brown tuff of worn grains of translucent porphyritic, often
very porous yellow glass and opaque glass which probably were
thrown out by explosive eruptions and carried to and fro by the
wind over wide areas.
3. Brown tuffs of angular fragments of sideromelan which have
not been transported at all and seem either to represent lava flows
which consolidated wholly as glass and vere split up into innumer-
able pieces or they represent what may be called a volcanic sandflow
of glass fragments.
4. Peculiar basic lavas which partly consolidated as an extreme-
ly fine-grained dark basalt with columnar and block jointing and
partly as translucent glass which may be compact or split up into
relatively large fragments. In some cases the basalt is predominant,
in other cases the glass. These lavas are mostly exceptionally thick
and seem nevertheless often to extend over relatively wide areas.
There is no indication of a tumultuous motion or explosion from
contact with a chilling agency.
5. Occasionally sheets of indurated grey glassy conglomerates
occur in the series.
The whole plateau series appears to be built up on ice-free, dry
land in a relatively short period, so as to leave but very limited
space for the destructive forces. The magma appears to have changed
very little during the volcanic period as indicated by the phenocrysts
but it consolidated in many different ways. This difference, as we
have pointed out, it is very difficult, not to say impossible to ascribe to
external chilling agencies. It must be sought in somewhat chang-
ing physical properties of the extruded magma and in the concluding