Acta naturalia Islandica - 01.02.1946, Blaðsíða 50

44
TRAUSTI EINARSSON
There is no indication of a notable transport of these glass frag-
ments. Further, the absence or low degree of porosity seems to indi-
cate that these tuffs are not the result of explosive eruptions. Most
probably they either represent lava flows consolidating entirely as
glass and crumbling into pieces on cooling or, what seems more
likely, they are streams of a mixture of glass fragments — erupted
sandflows.
The dolerite on the top of the mountain in hand-specimen closely
resembles the dolerite of Reykjavík as remarked by Thoroddsen. In
thin section (391) it is practically identical with the ophitic dolerite
of Reykjavík.
There remains to be mentioned an interesting feature in the
mountain slope some 100 m east of Austurgil. This is a stream of
breccia running down the slope. It consists of dark lava fragments
of distorted forms and a matrix of brown matter of varying percent-
age. In thin section (369) this matrix is seen to have orginally
been a somewhat porous unfragmented mass of sideromelan contain-
ing a few phenocrysts of olivine. A great deal of the glass is now
altered into faintly birefringent palagonite, only scattered patches
of unaltered sideromelan being left. This is clearly a lava flow con-
solidating partly in a fine-grained form and partly as massive glass,
and as far as I can see this flow ran down the present side of the
valley and should thus be relatively recent.
7. MIDDLE NORTHERN ICELAND
The high mountainous area on both sides of Eyjafjörður was
earlier considered to be built up of a Tertiary Basalt Formation.
According to Pjeturss’ system, however, the higher reaches of these
mountains consist of Quaternary rocks, as we have already men-
tioned in the introductory remarks. It has often been stated, among
others by Paijkull, Kjerulf, Thoroddsen and Pjeturss that the basalts
crowning these mountains are light-coloured and porous and very
similar in appearance to the basalts of Southern Iceland, but it was
not until Pjeturss’ discovery of grey conglomerates of morainic
appearance in the basalts of the North, i. e. at an elevation of about
700 m in the mountains of Fnjóskadalur, that a Quaternary age
was assumed as for the rocks of Southern Iceland.