Fróðskaparrit - 01.01.1975, Side 100

108
Glacial Erratics
dredge hauls 1, 67 and 80. Fine sand or mud sometimes sticks
to the lower side o£ the cobbles, whereas boulder clay is very
rarely seen.
Basalts
On average, plagioclase-phyric basalts constitute 41 percent,
olivine-phyric basalts 9 percent and aphyric basalts 50 percent
of the cobbles of basalt dredged south-east of the Faeroe Is-
lands. The last figure includes near-aphyric basalts with less
than about 1 percent phenocrysts of any mineral and micro-
phyric basalts with no phenocrysts exceeding 1 millimeter in
length. The proportions between the different types of basalts
are fairly constant when the larger dredge hauls are compared.
Most of the basalts have been derived from lava flows and
only a few from dikes or sills judging from the grain size.
Most of the basaltic cobbles are surrounded by a brown
weathering crust which differs much in thickness but seldom
exceeds 10 millimeters.
The porphyritic basalts are usually glomerophyric. The
plagioclase-phyric basalts typically contain 5 to 15 percent
plagioclase phenocrysts, 1 to 3 percent pseudomorphosed olivine
phenocrysts, and sparse microphenocrysts of clinopyroxene. In
10 percent of the plagioclase-phyric basalts, the length of the
largest plagioclase crystals exceeds 5 millimeters. The ground-
mass consists of plagioclase, clinopyroxene, iron-titanium oxides
and variable amounts of completely altered residual glass.
Altered olivine is usually present too, but does not exceed
about 5 percent. The groundmass texture is intergranular or
rarely subophitic. In basalts with less than 1 percent olivine
the iron-titanium oxide grains tend to be euhedral or skeletal.
The olivine-phyric basalts contain between 1 and 30 per-
cent phenocrysts of olivine. A third of the basalts also contains
phenocrysts or microphenocrysts oí plagioclase, whereas pheno-
crysts of pyroxene have not been observed. Fresh olivine is
present in nearly half of the basalts. In a third of the basalts