Acta naturalia Islandica - 01.02.1946, Síða 24

18
TRAUSTI EINARSSON
met with a breccia which probably results from the breaking up
of a lava of this kind.
We have now reached the edge of the plateau. Here we find regular
columnar lavas of a light coloured porphyritic olivine-basalt. (260)
shows a few macrophenocrysts of bytownite and scattered micro-
phenocrysts of bytownite, augite and olivine, in a very fine-grained
groundmass.
The plateau rises very gently towards the central parts and indi-
vidual lavas cannot be discerned, but everywhere in the surface we
find the same kind of grey, porphyritic fine-grained basalt.
In the central part this basalt is covered by hills and high peaks of
brown volcanic breccia and tuff, and according to Thoroddsen rhyo-
lite also occurs here.
Near Vörðufell, one of the lower outer hills, the volcanic breccia
(257) contains fragments of dark vesicular glass with micropheno-
crysts of plagioclase and augite, this glass being enclosed in a com-
'pact groundmass of somewhat altered sideromelan, which also en-
closes phenocrysts of olivine. This groundmass does not appear to
have ever consisted of fragmental glass, it has clearly consolidated
from a melt as a compact glassy mass. The melt then contained frag-
ments of vesicular glass as well as crystals of olivine and during
consolidation a great number of small crystals were formed. This
relation of glass within glass is of great interest and we shall find it
again in other localities.
Brown tuff, (258) which is also found here, consists on the other
hand of small fragments of glass and is probably blown to the place
as “ash” whereas we must assume that the compact glass of (257)
results from a neighbouring extrusion of magma.
I was not able to detect any signs of a glacier with which this
breccia might be connected and proofs of its sub-glacial origin are
certainly lacking.
As a result of this study I have come to the conclusion that the
whole mass of Tindafjöll presents an unbroken volcanic series of
certain peculiar characteristics, built up without any great change
of magma (the rhyolite of the peaks of Tindafjöll is not considered
here) in a relatively short period, for there is no sign of a prolonged
denudation.
Early and especially in the last phase of acticity the magma con-
solidated as translucent glass, whereas it formed fine-grained crys-