Acta naturalia Islandica - 01.07.1964, Qupperneq 19

THE ANKARAMITES OF HVAMMSMÚLI, EYJAFJÖLL, SOUTHERN ICELAND 17
The sparsely porphyritic rock weathers very well. It is hard and tough to
hammer. The groundmass is granular, rich in pyroxene, and in thin section the
rock is similar to the other rocks but for the relative paucity of phenocrysts. In
this rock a distinctive type of columnar jointing is developed in a ca. 10 m
wide zone running NW—SE (See Pöst, Fig. 2). The columns are commonly
bounded by 3 faces, which meet in sharp, acute edges. This zone grades on
both sides into softer rock with less well defined columns.
The non-porhyritic rock occurs as ,,veins“ or inclusions in the porphy-
ritic type. The veins show contact with the surrounding rock, the only differ-
ence being the absence of phenocrysts in the vein. The non-porphyritic weathers
slightly better than the host (Pl. Vlb).
A suggestion of columnar jointing is fairly common in these rocks. Usu-
ally the columns are fat and ill defined, with rounded and blunt edges. The join-
ting of the hard, sparsely porphyritic rock mentioned above is an exception;
here the individual units are roughly triangular in cross section, bounded by
sigmoidal surfaces, and lens out towards both ends. This type of jointing is
believed to be a reflection of some special conditions of cooling, probably ac-
compained by movement. It is of note that a trace of this NW—SE zone of
sigmoidal jointing, which grades into normal rock on either side, is seen to
affect other rock types in direct continuation NW and SE of the type cliff.
The extension of the zone is indicated on Fig. 2.
There is no obvious evidence of movement to be seen in the field, and
the only difference between the types is slightly coarser grain outside the zone.
But the fact remains that inside the zone the columns are more sharply defined
than outside it, and the zone extends through two rock types.
b. Textures.
Two apparently well defined types of texture are recognized in the ground-
mass, the overall texture being porphyritic, viz:- (a) granular, and (b) sub-
ophitic. (Pl. Vb).
The granular groundmass is characterized by a high proportion of py-
roxene, which forms small subhedral grains embedded in the mass of small
plagioclase grains or laths. The plagioclases exceed the pyroxene grains in
length, and govern the form of them. The olivines of the groundmass are of
larger size than the other two. Their rounded outline seems to be independent
of the surroundings, suggesting earlier crystallization. The ore shows poiki-
litic intergrowth with the other minerals of the groundmass, and commonly en-