Acta naturalia Islandica - 01.02.1946, Blaðsíða 62

56
ORIGIN OF THE BASIC TUFFS OF ICELAND
thin section (198) we find a number of angular fragments of fresh
sideromelan enclosing phenocrysts of olivine, in a predominating
groundmass of comminuted sideromelan. The glass fragments are
rather porous and there is flow structure in some of them which is
absent in others. On account of this slight heterogenity and the
abundance of comminuted matter I think it is clear, that the tuff
did not reach the surface as a fluid magma, but as a calmly flowing
wet mixture of glass fragments.
Hólsfjöll. Of wide occurrence in the vast masses of fragmental
material occupying N. E. Iceland between Melrakkaslétta and Vatna-
jökull is a peculiar whitish tuff. Thoroddsen mentions this and also
points out the occurrence of a similar variety in the Fiskivötn area.
The mountain Haugur is largely built up of this tuff and from that
area I have studied a thin section (83). It consists of grains of brown
translucent glass enclosing a number of phenocrysts of plagioclase.
All interspaces between the grains, the contours of which are seldom
clearly seen because of the advanced state of alteration, are filled
with white zeolites and some calcite which causes the whitish appear-
ance of the tuff.
Another slide from the top of Þjóðfell (68) consists of fragments
of porous yellow glass, enclosing a number of phenocrysts of plagio-
clase and olivine. The glass shows mostly flow structure and is very
much altered and mostly birefringent. There is also one fragment
of opaque glass. The vesicles of the glass and the interspaces are
largely filled with calcite. The heterogenity and whole appearance of
the slide indicate an explosive eruption of this matter, or more
generally, an eruption in the solid fragmental state. Slide (71) from
the top of Kjalfell, west of the farm Möðrudalur, is of nearly the
same composition as (68) except that augite also occurs as a pheno-
cryst and that the alteration of the very porous yellow glass has
proceeded much farther, only a very few fragments being wholly
isotropic.
This tuff of Kjalfell is of interest for comparison with other alter-
ed tuffs. Thick masses (up to 300 m) of green tuffs occur down to
the lowest visible horizon of the basalt plateau of Esja and in the
old basalts in Suðursveit in Eastern Iceland. A slide from Esja is in
many respects very similar to (71) only more altered and it is
difficult to decide whether this alteration product is derived from