Acta naturalia Islandica - 01.02.1946, Blaðsíða 34

28
TRAUSTI EINARSSON
It is very difficult to assume a subglacial lava flow in this case
ancl the thickness of the compact, although naturally jointed glassy
mass, is incompatible with the conception of an ultra-rapidly chill-
ing agency.
This breccia at its lower termination rests on a grey hard con-
conglomerate of morainic appearance. I “managed” to find a striated
block standing out of the hard matrix, but I considered the striae
to be possibly of a secondary nature, i. e. made by a glacier creeping
down the slope after the hardening of the conglomerate. In my view
this predominantly glassy conglomerate, resting on crystalline basalt
lavas, is a mud flow. The grey basalts on which the conglomerate
rests are of just the same kind as those mentioned on the east side
of the mountain, which also are covered by a similar conglomerate.
We thus most probably find the same mud flow on both sides.
Descending fui’ther down the slope we find lavas of block jointing,
columnar lavas and at the bottom of the series a thick layer of
breccia which may be identical with that of Kirkjubæjarklaustur.
We now proceed further eastwards to Fljótshverfi where a section
at Núpsstaður shall be mentioned. The lower part consists of lavas
of the usual grey basalt covered by a layer of conglomerate of well
worn pebbles and boulders. No striae were found. Then follows a
very thick interesting layer. At the bottom it is a columnar dark
basalt, soon turning into block jointing with an abundance of yellow
matter and still higher containing patches of brown glassy material.
Here again is a lava flow which consolidated partly with colunmar,
partly with block jointing and partly as glass and zeolites.
On the top af this peculiar layer we have a thick layer of fine
brown stratified tuff which is made of worn grains of translucent
glass and thus is no doubt of distant origin. Covering this at the edge
of the escarpment are again some lavas of grey porphyritic basalt.
The perpendicular wall of the over 600 m high Lómagnúpur east
of Núpsstaður is built up in a similar way although ordinary lavas
are here in less abundance than at Núpsstaður, except at the top of
the series.