Acta naturalia Islandica - 01.02.1946, Blaðsíða 22

16
TRAUSTI EINARSSON
old moraines and is possibly identical with Pjeturss’ lower moraine
at Barkarstaðir. Beautifully striated boulders are also easily found
here.
Yet, comparing the different boulders we find that their striae
are always largely parallel and strike along the mountain slope, as if
made by a late glacier creeping down the valley of the Markarfljót
— as undoubtedly they are. By chance the boulders could not be put
in such position in a moraine, but protruding out of the hard con-
glomerate they would be striated in such a way by ’a late glacier
of the Markarfljót valley.
I was not able to detect a single stone in the conglomerate which
I considered to have originally lain in it as a striated boulder, and in
my view a proof of the morainic nature of the conglomerate is
lacking.
The conglomerate is covered by a very coarse breccia consisting
mainly of unworn blocks of a porphyritic basalt, containing unusually
large phenocrysts of plagioclase in great abundance.
Parts of a lava of the same basalt are alstí seen and the breccia
in all probability represents the crumbling of this lava, which pro-
bably was heavily jointed and easily broken down (cf. later).
Higher in the section occurs a confused mass, which in the main
consists of irregular sheets of this same porphyritic basalt, and the
first regular lava sheet, still of the same kind of basalt, is not met
with until we reach the edge of the escarpment, 230 m. above sea level.
To reach the edge of the main plateau mentioned at the outset,
we have now to walk up a gently sloping terrain with but very
little barelaid rock. Not until reaching the brook Merkiá at a height
of about 400 m, could I follow clearly the volcanic series again.
We find here a porphyritic lava similar to those at the base of the
series. It is covered with many thin strata of sediments: a brown
sandstone with veins of bog-iron, a darker stratified sandstone with
thin layers of white rhyolite-pumice and a layer of very coarse sand-
stone. These strata are all of nearly the same composition, namely
subangular fragments of sideromelan and a few worn crystals of
plagioclase (bytownite-labradorite) and augite, which appear to be
phenocrysts originally enclosed in the glass. These sediments are most
probably wind-blown sideromelan “ash”, thrown out by some nearby
volcano. They do not on the other hand suggest any appreciable
period of volcanic rest and weathering.