Acta naturalia Islandica - 01.07.1964, Qupperneq 10

8
SIGURDUR STEINTHÓRSSON
century, when it changed to its present course. Then the road, which
now runs between Pöst and Dysjarhóll, lay above Pöst (Figs. 2 and 3).
Hvammsmúli is a small promontory extending south oí Eyjafjöll (Fig. 2).
At its base occur the ultrabasic rocks which form the main subject of this
thesis. Then follows a thin succession of basic lavas, and lastly a pile of pala-
gonite tuffs and breceias, which sometimes show conspicuous bedding. The
palagonite is intruded by basic sills and veins showing close columnar
jointing. The extrusive nature of the lower basaltic sequence is indicated
by the scoriaceous top of at least one flow, which is followed by a thickish
layer of scoriae. A dyke (1.5 m wide), cuts both the lavas and the palagonite
trending NE—SW.
The ankaramites at the base of Hvammsmúli occur in well defined out-
crops, bounded by vertical cliffs which at one time were washed and eroded
by the sea, and later by the river Markarfljót (cf. above). The cliffs run parallel
to the dyke mentioned (i. e. NE—SW). Presumably, therefore, the gaps be-
tween the outcrops are sea-eroded zones of weakness which may or may not
have been occupied by dykes. (See Fig. 2).
Whether the main masses be thick lava flows or sills remains uncertain,
because the top is concealed by scree and the bottom by alluvium. But certain
structural considerations as well as the fact that the rock is distinct and different
from the succession above, but has certain features in common with the int-
rusive veins in the tuffs, seem to be rather in favour of the latter.
Recently one of the cliffs was quarried for road material. The quarry, of
course, provides 100% exposure and a good opportunity to study the more
delicate structures of the rock. Its marvels will be described in some detail on
a later page.
The rocks are genenally highly porphyritic with phenocrysts of clinopyrox-
ene and olivine up to 6 mm across. Feldspar phenocrysts occur as well but in
minor amounts. Other minerals are iron ore in the groundmass and as exsolu-
tions in olivine and inclusions in feldspar, serpentine and iddingsite as altera-
tion products of olivine, and occasional spinel in olivine. Aragonite occurs as late
stage filling of vesicles, and biotite was seen in one thin section associated with
aragonite. Apatite was not noted. Neither analcite, pigeonite nor titan-augite
were found. There is therefore no evidence as to the affinity of the magma,
whether it was tholeiitic or alkalic-olivine basalt — proably in between the two.
Two distinct textural types are present in the groundmass, sub-ophitic and
granular. Usually one rock mass is characterized by only one textural type,
but both types may occur in the same rock. The rocks occur in 6 well defined
outcrops (Fig. 2), Kálfshamar, Pöst and Dysjarhóll to the south west and the