Acta naturalia Islandica - 01.07.1964, Blaðsíða 18

16
SIGURDUR STEINTHÓRSSON
PETROGRAPHY
a. Field criteria.
A few rock types, which sometimes show clear contacts, were recognized
in the field. The characteristic feature of them all, distinctive from the over-
lying sequence, is their highly porphyritic nature. Phenocrysts of olivine and
pyroxene constitute 10 to 60% of the volume of the rock. On average the pheno-
crysts make up about 30%. Gradations are seen, but generally one rock type
is rich in phenocrysts and another relatively poor. On this basis the rocks were
arbitrarily divided into highly porphyritic, porphyritic, sparsely prophyritic
and non-porphyritic groups. The non-porphyritic forms thin bands or „veins“
and inclusions in the porphyritic rocks.
An important field criterion is the colour of the rock. The rocks fall into
two broad groups: light and dark coloured. The dark colour is believed to be
in most cases due to deuteric alteration of the original light coloured rocks
(G. P. L. Walker 1960). Therefore, each dark rock type may have its light
coloured caunterpart and vice versa. The dark colour is due to alteration of
the olivine, especially in the groundmass, and to the dissemination of iron ore.
The primary coloration depends, however, much on the relative abundance and
grain size of the minerals in the groundmass (feldspar versus pyroxene).
The relationship between the light and dark rocks is well seen in the
quarry. Dark rocks occupy the lower region of the quarry face, light ones the
upper levels. A definite boundary is seen, often accentuated by a narrow zone
of brown rock. But there is no actual junction between the two, and no change
in mineral content or texture across the boundary. What most conclusively
shows the later origin of the dark colour is the fact that the dark zone is seen
to traverse non-porphyritic veins and inclusions, leaving the other end unaf-
fected. In other exposures the relationships are quite different, as the light por-
phyritic rock is underlying and intrusive into the dark rock.
The various rock types show different and distinctive weathering. The
highly porphyritic rock, which seldom takes on a dark coloration, because
plagioclase laths predominate in the groundmass, weathers with an even sur-
face when massive but becomes progressively more pitted when the rock is
vesicular. The porphyritic, in most cases dark, weathers to rounded boulders
with a brown surface. The phenocrysts weather better than the groundmass
and stand out. This type of weathering may be a reflection of crude columnar
jointing in the rock.