Acta naturalia Islandica - 01.02.1946, Blaðsíða 59

ORIGIN OF THE BASIC TUFFS OF ICELAND
53
the valley. It is of the same character as the plateau we have been
considering and is no doubt a part of the same plateau. A short dis-
tance south of the farm Klambrasel a brook has opened a section of
this series, Fig. 11.
Resting on the lowest visible lava is a eonglomerate, about 2 m
thick, consisting almost exclusively of porous, scoriaceous stones in a
very sparse cement. This is probably a directly erupted material.
It is covered by a conglomerate of well-worn pebbles in a grey matrix
which has some horizontal jointing as seen by its form of weathering
and its breaking into flakes. Beyond that, the mass is entirely with-
out structure and of the common moraine-like appearance. I could
find no striated stones and in a thin section (528) the matrix is seen
to consist of large and small fragments of porous opaque glass,
small fragments of brown translucent glass, fragments of very
fine-grained lava, and a brown amorphous palagonite filling all
interspaces and it is at least doubtful whether this palagonite is an
alteration product and not of a primary origin. This conglomerate
thus has all the appearance of being a volcanic product and not a
moraine.
Higher in the section there is a similar grey conglomerate of a
thickness of 5—6 m. The relatively few stones it contains are not
larger than 10—15 cm. but mostly much smaller. The groundmass
(534) consists again of rounded grains of a very fine-grained basalt,
opaque glass and a few grains of brown translucent glass.
A short distance below this layer the basaltic lavas are traversed
by a “dyke” which is seen in both walls of the gorge. Its thickness is
about 1 m and it consists of a chaos of worn and angular blocks of
basalt of a diameter up to % m. The grey matrix (533) is of sub-
ordinate abundance and consists of rounded grains of a very fine-
grained basalt — also fewer grains of a coarser basalt — opaque
glass, and sideromelan, the glass being of a high percentage. Thin
apophyses extend into the walls of the fault and the dyke is beyond
doubt of volcanic origin.
Although this material is coarser than the grey conglomerate higher
in the section, it is typical of the grey conglomerates, both as concerns
the condition of the boulders and the composition of the matrix, and
the dyke renders perhaps the clearest proof of the volcanic nature
of the conglomerates.