Acta naturalia Islandica - 01.02.1946, Síða 45

ORIGIN OF THE BASIC TUFFS OF ICELAND
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4. Conglomerate with rounded peb-
bles of basalt, not larger than a fist, in
a brownish matrix. This conglomerate
is not a chaotic mass but some-
what stratified and there are alter-
nating layers of conglomerate and thin
stratified iayers of fine-grained brown
tuff. Pjeturss asserts that striated
stones are easily found in this con-
3. A lava of amygdaloidal basalt.
glomerate, yet I could not detect any Fig. 3. — Sediments in Austur-
stones with clear glacial striae. gil- Explanation in the text.
Scratched stones may be found, but
certainly they do not prove the conglomerate to be a moraine.
The matrix of this conglomerate, in thin sections (362 a) and
(362 c), is seen to consist mainly of yellow faint polarizing glass
although there is also an abundance of minute fragments of unde-
terminable crystals, and worn grains of basalt and opaque glass are
scattered throughout the matrix. The yellow glass does not consist
of distinguishable fragments but appears as a compact cement. The
strata of tuff intercalated with the conglomerate consist of rounded
grains of opaque glass and of grains of an extremely fine-grained
lava in a rather sparse groundmass of a brown amorphous substance.
The stratification as well as the composition of this conglomerate
seems to fit in very badly with the hypothesis of a moraine and as
already stated, I could not find glacially striated stones' in it.
This composite conglomerate is directly covered by:
5. Many thick layers of brown tuff or mudstone, greenish coarse
sandstone, and at the top a coarser brown tuff.
All these strata consist to a high degree of fragmental trans-
lucent glass. The lower brown tuff (363) consists of small rounded
grains of a black fine-grained lava, separate crystals, so large that
they were no doubt originally phenocrysts, and yellow glass.
The green sandstone (364) consists mostly of rounded and sub-
angular grains (average diam. 0,3—0,4 mm) of greenish palagonite,
dark glass, and separate crystals of augite, olivine and plagioclase,
obviously phenocrysts from the glass. No lava fragments are seen in
the slide. The grains are cemented by palagonite, zeolites, and cal-
cite. The topmost layer of brown tuff (365) differs from (364) by