Acta naturalia Islandica - 01.02.1946, Síða 27

ORIGIN OF THE BASIC TUFFS OF ICELAND
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merate consisting of pumice, scoriaceous material and glass frag-
ments and is most probably of explosive origin, and partially wind-
blown.
Next are some lavas (2) of a fine-grained porphyritic grey basalt
the heat of which has reddened the sandstone below. Then again
we have brown tuff and breccia similar to those at the base of the
section (3). These sediments are covered by a peculiar, thick layer
consisting of unworn cubes or polyhedrons of dark, very fine-grained
basalt, about 1 dm3 in size, in a brown matrix.
This layer reminds one of the thick lava at the head of the
brook Merkiá, the main difference being a far greater amount of
brown matter at Hvammur.
This matrix is seen in thin section (268) to consist of a compact
mass of brown translucent glass. No glass fragments are seen in the
slide.
Another slide (266) of a brown matter enclosed in a lava cavity
at the same place shows again a compact mass of sideromelan en-
closing phenocrysts of bytownite and some augite.
The nature of this layer is clearly very different from that of the
lower breccias and tuffs. While these layers consist of transported
fragments we have here a material which shows no signs of trans-
port. This has not been a watery mixture of lava blocks and glass
fragments. Nor are there, as far as I can see, any direct signs of the
molten lava having been in contact with a chilling agency. It is, I
think, clear that this layer represents a lava flow that consolidated
at its present place partly as a “cube lava” and partly as a compact,
yet irregularly jointed mass of translucent glass.
Above this breccia there is again a thick layer of brown tuff, ob-
viously transported, and lastly there are several lava banks of a
light porphyritic basalt up to the edge of the plateau.
Above the edge such lavas are everywhere in the surface of the
plateau, up to the central regions. Here the plateau is surmounted
by hills, probably of fragmental material, but because of fog I was
unable to study these hills closer.
This series is thus built up of alternating layers of fine-grained
porphyritic lavas and tuffs which consist mostly of translucent
brown glass fragments, most probably being blown to this place.
There is only one layer of glassy material which is in its place of