Acta naturalia Islandica - 01.07.1964, Side 27

THE ANKARAMITES OP HVAMMSMÚLI, EYJAFJÖLL, SOUTHERN ICELAND 25
The contaet between Rocks VI and VII is sharp and marked by a vein of
light coloured rock of Type II, (i. e. groundmass with subophitic texture;
largish plagioclase laths predominant). Towards the contact the groundmass
of Rock VI becomes coarser grained, but is still distinctly granular, with in-
creased amounts of both plagioclase and magnetite, which occurs in very
numerous euhedral grains. This corresponds to the general picture: as stated
earlier the strictly sigmoidal jointing only occurs in a zone which marges into
a softer and coarser grained rock on both sides. The junction between Rocks
VI and VII is in the marginal region of the jointed zone.
Rock VII. The crystalline, sub-ophitic rock of Pöst.
Phenocrysts constitute about 20% of the volume. In the field the rock is
black coloured if fresh, but has a brown weathering surface. It forms poorly
shaped columns with blunt edges, very distinct from the sigmoidal columns
oS Rock VT.
The groundmass is crystalline, similar to that of Rock II. The modes
are similar as well. The plagioclases of the groundmass (0.3—0.5 mm long)
are aligned in parallelism. The plagioclase phenocrysts have the central portion
occupied by numerous crystals of ore, which are orientated and commonly
show cubic form. The marginal portion is free of these inclusions. The two
portions are separated by a compositional break: the core, which is of almost
uniform composition (An75), is rounded in shape, whereas the marginal zone
has good crystal faces and shows continuous zoning comparable with that of
the groundmass-feldspars. The olivines in the groundmass and the margins
of the phenocrysts are altered to seprentine, and magnetite is abundant —
hence the black colour.
STRUCTURES
It seems appropriate to describe the quarry in some detail because owing
to the superior exposure on the newly broken rock face, various relationships
are seen which do not show in the ordinary weathered rocks. The SE corner
of the quarry face is occupied by a light coloured rock with a very fine-grained
granular groundmass. No contact was observed between this rock and the
coarser one higher up. It is therefore thought to be a slightly chilled variety
of same. Similar rock occurs in both the westernmost outcrops of Pöst and Kálfs-
hamar (Figs. 2 and 4), where it occupies the lowest levels of the exposure, but
here the overlying rocks are of a completely different type and show a definite