Acta naturalia Islandica - 01.02.1946, Blaðsíða 51

ORIGIN OF THE BASIC TUFFS OF ICELAND
45
But quite apart from the question of age which according to our
previous discussion of the grey conglomerates in the South cannot
with certainty be based on a glacial interpretation of the conglomer-
ates, it seems nevertheless a sound assumption that there is a close
relationship between the volcanic series of the South and that form-
ing the highest reaches in the mountains of Middle Northern Iceland.
In fact, the volcanic plateau which we have studied from Tindafjöll to
Fljótshverfi is not only continued in the region of Hreppar, to the
west, but also without a disturbance of any notable magnitude to
the north between Hofsjökull and Tungnafellsjökull, to the high
ground of Middle Northern Iceland. The country is essentially a
slightly inclined plateau with an elevation of about 4 ■—■ 500 m in
Southern Iceland, 6—700 m south and east of Hofsjökull, 7—800 m
north of Hofsjökull, then rising almost imperceptibly to 11—1200 m
around Djúpidalur and Fnjóskadalur (here heavily dissected), at-
taining its greatest height of about 1500 m. around Glerárdalur near
Akureyri. Thence it declines again down to about 1100 m or lower
around Ólafsfjörður and Fljót. The rocks forming the uppermost
part of a thickness of several hundred meters of this large plateau
are the same in the south as in the north and there is no reason why
they should not be considered as parts of the same formation. We
shall now consider several sectioris in Northern Iceland.
Head of Eyjafjörður. In the eastern, about 700 m high slope of the
valley of Eyjafjörður opposite Hafrárdalur I could distinguish 4
or 5 separate layers of brown tuff alternating with banks of basalt,
but I have had no opportunity to make any closer study of these
layers. On the opposite side of the valley, in the gully of the river
Hafrá, there are just above an elevation of 340 m, i. e. below a pile
of at least 700 m of basalt lavas, two layers of brown tuff, the higher
one being 30—40 m thick. In the alluvial cone of the river we find
an abundance of pebbles of tuff which no doubt are derived from
these layers. In a thin section (592) of one of these pebbles we find
worn grains of dark porous glass, yellow translucent glass, a few
separate crystals of plagioclase which clearly are phenocrysts of the
glass, and a few grains of basaltic lava in a predominating ground-
mass of birefringent palagonite. The vast majority of this rock
is worn, probably wind-blown sideromelan, now a great deal altered.
Staöarbyggðarfjall on the eastern side of Eyjaförður just south of
Akureyri has a height of 1200 m. It is almost entirely built up of