Acta naturalia Islandica - 01.07.1964, Blaðsíða 21

THE ANKARAMITES OP HVAMMSMÚLI, EYJAPJÖLL, SOUTHERN ICELAND 19
areas, partly enclosing plagioclase laths of greater average length than that
of the pyroxenes themselves (F. Walker 1957). More or less rounded grains
of olivine are enclosed by the pyroxene as well. The form of the pyroxene is
completely governed by the other minerals, and plagioclase laths are seen to
extend into olivines too. The form of the olivine is, however, relatively unaffected
by the surroundings.
Flow structure is common, where the plagioclase laths, which dominate
the groundmass, are orientated in parallelism and bend round obstacles such
as phenocrysts. The sub-ophitic rocks are coarser than the granular ones,
which may reflect more rapid cooling of the latter, or, what is more likely,
more intense movement resulting in smaller and more numerous crystals. Both
types show gradation in grain size. The grain size of the granular rocks seems
to be inversely proportional to the amount of pyroxene in the groundmass.
The magnetite forms patches of similar size and habit as the pyroxene in
the sub-ophitic rock.
There seem to be two factors which govern the resulting texture of the rock,
viz:- chemical composition, and amount and timing of movement with respect
to crystallization. In both types the plagioclases are aligned in parallelism
(seen in suitable sections) indicating flow. In the case of the ophitic rocks, the
movement of the mass, which consists mainly of plagioclase laths, seems to
have come to a halt when the crystallization of the pyroxene took place, whereas
the magma was still moving while the granular pyroxene was crystallizing,
and hence the texture. It is possible in the case of the plagioclase-rich liquid
that movement ceased before all the plagioclase had crystallized out. The re-
maining liquid crystallized on already formed smaller needles which were
aligned in parallelism due to the flow. Hence the relatively large and well
Fig. 4. Kálfshamar (left) and Pöst (right) seen from northwest.