Fróðskaparrit - 01.01.1975, Síða 101

Glacial Erratics
109
some of the olivine phenocrysts are rod-shaped and up to 5
millimeters long. The groundmass contains at least about 5
percent olivine which is usually completely altered. The con-
tent of iron-titanium oxides is lower than in the plagioclase-
phyric basalts, while zeolites are more common both in the
groundmass replacing residual glass and in vesicles. The ground-
mass texture is intergranular or more rarely ophitic or sub-
ophitic.
Most of the strictly aphyric, near-aphyric and microphyric
basalts contain less than about 5 percent olivine and correspond
closely in mineralogy and texture to the plagioclase-phyric ba-
salts, while the rest are more rich in olivine and similar to the
olivine-phyric basalts.
This grouping of all the basalts according to the olivine
content roughly corresponds to the two groups of basalts on
the Faeroe Islands named quartz tholeiites and olivine tholeiites
by Noe-Nygaard and Rasmussen (1968).
Tuff carbonate sediments
These sediments are brownish in various shades and rather
soft. They vary widely in composition from a carbonate rock
with a few grains of volcanic ash to a pure tuff. The carbo-
nate is finely crystalline and brownish. The volcanic ash, which
is strongly altered, varies in grain size from fine to coarse sand.
In most samples the grain size is rather uniform and bedding
is either very faint or absent indicating a derivation of these
cobbles from ash layers at least 5 to 10 centimeters thick. A few
carbonate sediments contain burrows.
One cobble shows a few millimeters thick ash layer grading
upwards into a carbonate rock with a few, small ash grains
and sharply bounded downwards by a similar carbonate rock.
A few indeterminable microfossils have been found in the
carbonate matrix in another sample. In these cases the carbo-
nate is clearly of sedimentary origin. However, two other
cobbles (from stations 65 and 87) look like concretions in