Acta naturalia Islandica - 01.07.1964, Qupperneq 30

28
SIGURDUR STEINTHORSSON
A second intrusive phase is clearly indicated by an apparently composite
xenolith seen in the rock immediately east of the non-porphyritic „veins" (Pl.
Vn b). One half of it is a non-porphyritic rock, the other half porphyritic,
grey in colour. The porphyritic portion is ramified by non-porphyritic „veins“
which are cut short by the margins of the inclusion. The surrounding rock is
highly porphyritic of Type III (p. 23). The edges of the xenolith are fused,
especially towards the top, which obscures the relationships there. An app-
roximately 20 cm thick band of the intrusive rock separates the xenolith from
the normal porphyritic rock below. The intrusive rock is more porphyritic
undemeath the xenolith than on the sides, possibly representing an instance
of filter-pressing. The fact that the xenolith is more fused on the sides and
round the top than at the lower margin may be a reflection of this: the liquid
was squeezed out from underneath the xenolith, leaving the phenocrysts, and
hence could not react at the lower margin.
The porphyritic part of the xenolith at least has not been carried far.
In fact, it might be disturbed by the intrusion but otherwise more or less in
situ, because it is the same rock as is seen other places in the quarry. The non-
porphyritic portion is harder to place, but it is certainly a proper xenolith
floating in the magma. Pl. VlIIa is taken much farther west on the quarry
face. The highly porphyritic rock is clearly intrusive. In the outcrop just SW
of the quarry the same relationship is seen again where differential weather-
ing helps to bring out the two types, (Pl. VTIIb).
Small inclusions or clusters of a more or less pure feldspar rock occur in the
quarry. The composition and size of the feldspar crystals is similar to that
of the feldspar phenocrysts in the rock, but the magnetite inclusions (cf. Pl. II a)
are missing.
A last feature to note in the quarry is the occurrence of aragonite crystals
filling some vesicles but not others. No systematic distribution of filled and
empty vesicles was discovered.
CORRELATION
In Dysjarhóll light porphyritic rock is seen to underlie and intrude the
main rock, which is black in colour. The same light, porhpyritic rock appears
in the S and SE cliffs of Pöst as coatings on the main rock, 3—5 cm thick.
The magma has been squeezed up joints and solidified there. Where the joint
units have weathered away the veins are seen as coatings on the joint face.