Acta naturalia Islandica - 01.02.1946, Síða 29

ORIGIN OF THE BASIC TUFFS OF ICELAND
23
With increasing distance from the centre of the complex, east of
Ráufarfell, the lavas, however, not only dwindle but become quite
negligible. The entire complex is here built up of piles of brown
fragmental material.
This fact possibly means that the main centre of eruptions was
at the site of the present volcano, Eyjafjallajökull. In its neighbour-
hood the lavas were piled up, whereas to greater distances only the
finer fragmental materials was carried (blown). It is in this con-
nexion interesting that in the Westmann Isles, just off the coast, finer
fragmental glassy material builds up the larger mountains, although
coarser material and basalt, probably intrusive, are also abundant.
The material of the thick piles of brown tuff all the way from
Raufarfell to Vík in Mýrdalur seems to have suffered considerable
transportation. A thin seetion (289) of tuffs at Vík (taken from
the hill on which the church stands) is, I think, rather typical. It
shows larger and smaller worn grains of translucent and black
porous glass, enclosing a number of small needles of plagioclase.
The finer mass is altered and faintly birefringent. ..
Following the main road ea^st of Vík it was not before the
Kerlingardalsheiði some 10 km east of Vík, that I noticed tuffs of
a different character and a more primary appearance, and we may
here have reached the domain of a new volcanic centre.
This brown tuff or coarse sandstone is also in a thin section — (290)
which is taken near the highest part of the road — very different
from the transported tuffs to the west. This slide consists entirely
of angular fragments of fresh translucent yellow glass, enclosing a
number of the usual phenocrysts. The shape of many of the larger
fragments suggests that hardly any transport can have occurred, the
fragments being a veritable network of glass. In some cases also,
neighbouring fragments seem to fit so well into one another as to
indicate that the fragmentation took place on the spot. The un-
arranged admixture of large and small fragments indicates that they
did not settle from the air. This tuff seems either to represent a
lava flow that consolidated entirely as glass which on cooling crum-
bled into innumerable pieces or perhaps more likely it is a volcanic
sandflow spreading out over a limited area around the vent. There
are no signs of a tumultuous motion as might be expected if a chilling
glacier had produced this glass.