Acta naturalia Islandica - 01.02.1946, Blaðsíða 31

ORIGIN OF TIIE BASIC TUFFS OF ICELAND
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watery mass of fragments. Thus the interpretation of this section
seems clear: A very thick lava flow consolidated partially as fine-
grained lava and partially as a mass of glass which broke into
pieces because of internal tension.
Among the glass fragments in the slide two exceptionally large
ones are of special interest. They are mostly opaque, but partially
translucent and then indistinguishable from the neigbouring frag-
ments of sideromelan. The two varieties of basic glass, tachylyte and
sideromelan, are thus found in contact, and a sharp contact at that
with each other.
Proceeding westwards the brown breccia increases in the section
and at Foss the over 100 m high escarpment is nearly wholly made
up of this breccia: Angular lava cubes or polyhedrons in a brown
matrix and it closely resembles the breccia clescribed at Hvammur.
At Foss the matrix (287) is very similar to (286). Some of the
glass fragments are here again partially opaque. The fragments
are on the whole large, up to 10 mm, and there is no sign of a
comminution of them as would result from transport. In some cases
neigbouring fragments seem to fit into one another, indicating that
they were not transported, either as “ash” through the air or as a
mud flow, but represent a crumbled thick mass of glass. The ex-
ceptionally thick lava flow simply seems to have consolidated partially
as translucent glass, not because of some chilling external agency,
but because of its own properties.
Underlying the series is at Breiðabólstaður a grey conglomerate
which I assume to be a mud flow.
At Kirkjubæjarklaustur the whole escarpment, over 100 m high,
is made up of brown tuffs in which fragments of lava and scoriae
occur, especially near the top. Lower, this tuff contains large frag-
ments of glass and unworn pebbles of lava of the size of a fist. A
thin section (281) from a specimen taken at the small waterfall
shows large angular fragments of sideromelan and a fragment of
opaque glass, with enclosed phenocrysts. Some zeolitic matter occurs
as cement. Another thin section (282) from the top of the series
west of the farm contains large (about 5 mm) beautiful augular
fragments of sideromelan, laterally altered. Smaller fragments are
wholly altered into birefringent palagonite. Zeolites are abundant as
cement and cause a whitish appearance in the hand-specimen. As at
Þverárnúpur we find fragments which are partially opaque.