Acta naturalia Islandica - 01.07.1964, Blaðsíða 26

24
SIGURDUR STEINTHÓRSSON
Roclt V. The finest grained rocks above the quarry and in Pöst.
The groundmass is granular, too fine-grained for modal analysis, but
pyroxene is vastly predominant. Olivine occurs in larger rounded grains in the
groundmass. Tabular crystals of ore are numerous, scattered all over the ground-
mass. They vary in size from place to place and may in some cases form
minute and extremely numerous rounded grains in the groundmass.
Phenocrysts of pyroxene and olivine constitute ca. 30% by volume. Piagio-
clase phenocrysts are present as well but few. Gradations in grain size are
seen; in the finest grained rock the phenocrysts carry no reaction rim, or a
very thin one, presumably indicating rapid cooling with crystallization about
many centres. The phenocrysts sometimes are rounded in shape. The pyroxenes
in particular have embayments filled with groundmass, and a rounded, ground-
mass-filled cavity in the middle of the olivines is rather the rule than the
opposite (Pl. III b).
The few feldspar phenocrysts show clear signs of unstable relation to
the groundmass, by their rounded outline, occasional embayments and numerous
inclusions of various kinds.
The crystals are, however, very fresh. The edges of the olivines are slightly
indented (jagged) and serpentinized, and the rounded conchoidal fractures
parallel to the margins are characteristic.
With coarser grain size the outer zone of the phenocrysts becomes more
pronounced. and alteration generally more advanced. The pyroxene carries a
reaction rim in the coarser grained types of this rock, but in the more fine-
grained varieties the pyroxenes are rounded and have no rim, (Pl. Vla).
It is thought that this rock represents a variant of Rock IV. The relation-
ships have been discussed on an earlier page.
Rock VI. The sparsely porphyritic rock in Pöst.
Phenocrysts constitute ca. 15% of the rock. In the field it is dark, massive,
and tough to hammer. Columnar jointing is pronounced; the columns are
sigmoidal with the faces meeting in sharp edges. The main characteristic of
the rock is the jointing and the relative paucity of phenocrysts. A very marked
increase in the amount of phenocrysts is, however, seen from SW to NE
along the NW cliffs of both Pöst and Kálfshamar.
The groundmass is granular, of the same type as that of Rocks I and
IV. Olivine and pyroxene phenocrysts may reach 3.2 mm in diameter. Some
are euhedral, others rounded or irregular in shape. They sometimes include
plagioclase laths and aragonite in cavities. A reaction rim is present but rela-
tively thin.