Acta naturalia Islandica - 01.02.1946, Side 33

ORIGIN OF THE BASIC TUFFS OF ICELAND
27
From now on we ascend the steep slope of the mountain and in it
we observe a few ordinary lava flows but mostly “block lava”. On
the large surface of the mountain are also either “block lavas” or
remnants of more ordinary lavas of a porous grey porphyritic basalt.
A peculiar feature of these porous lavas is that their vesicles are
fillecl with a loose mass ivhich to the naked eye resembles humus but
is in reality fragments of sideromelan. Thus in the midst of lavas
sideromelan is formed in the vesicles and here quite certainly without
the influence of ice or any other chilling agency.
We have already described Kaldbakur as a somewhat irregular
dome. This dome rises above a vast, also somewhat uneven, i. e.
eroded tableland, the southern section of which we have studied be-
tween Þverárnúpur and Kirkjubæjarklaustur. But the plain extends
also northwards, until it meets quite abruptly a ragged ridge of
mountains which extends in the here usual tectonic direction SW—
NE from Torfajökull to Vatnajökull. These mountains are accord-
ingto Thoroddsen and Nielsen15) built up of Palagonite breccia. Their
tectonic origin is emphasized by Nielsen. Many of the mountains are
flat-topped and steep-walled and it seems likely that they are an
uplifted part of the larger plateau surrounding Kaldbakur and Geir-
landshraun.
The idea presents itself that these two domes are in reality an-
cient volcanoes, and thes was already suggested by Thoroddsen. Kald-
bakur lies on the very active volcanic line which is marked by Gríms-
vötn, Hágöngur, and Rauðhólar, but more direct evidence is also
available. The western slope 'of Kaldbakur from about 600 m down
to about 400 m elevation is covered by a thick layer of a brown mass
with scattered blocks of dark basalt. On the surface we observe low
elongated ridges stretching down the slope, as if the material had
flowed downwards. This layer is but little dissected, and only at
the lower end did I see a good section of it, about 30 m thick. It is
stratified with the general dip of the slope. At the bottom we have
scoriae and a mixture of irregular lava blocks and the usual brown
matrix, the percentage of the matrix increasing rapidly a short dist-
ance from the bottom and then remaining constanet. In thin section
(280) the brown matrix is seen to consist of compact sideromelan,
such as we have formerly found in the breccia at Hvammur. It is
clear therefore, that a very thick and no doubt viscous lava flowed
down the slope and consolidated mostly as glass.