Fróðskaparrit - 01.01.1975, Blaðsíða 102

110
Glacial Erratics
shape, and the carbonate in these samples probably has been
secondary introduced.
The volcanic ash grains are compact or slightly vesicular.
Their outline is somewhat irregular and in most sediments they
show some evidence of corrosion by the carbonate matrix. Most
of the ash grains consist of glass completely altered to clay
minerals or palagonite or replaced by carbonate. Some ash
grains, however, are microcrystalline and plagioclase and iron-
titanium oxides can still be identified in them, which shows
that the ash is basaltic. Larger crystals are usually sparsely
distributed in the ashes. Most consist of plagioclase (An 67
according to a single measurement), while microphenocrysts
of pyroxene and iron-titanium oxides are fairly common too.
Pseudomorphosed olivine has been identified in a few cases.
The pyroxene is usually completely replaced by calcite, but is
brownish gray when fresh.
The phenocryst content in the ashes suggests that they are
similar in chemistry to the olivine-free and olivine-poor tholei-
ites from the Faeroe Islands.
Other sediments
Slightly more than 90 percent of the cobbles in this group
consists of various sandstones, while the rest consist of dark
siliceous silstones, dark shales and limestones. One cobble
consists of chert and chalk, and one of chert alone.
The sandstones are typically fine grained, light or medium
gray and rich in quartz. A few are micaceous. Most of the
quartz-rich sandstones are cemented by quartz, while a third
partly or solely are cemented by carbonate. A quarter of all
sandstones constitutes a distinct group of arkoses with a pale
red or grayish red colour. Some of the arkoses are inhomo-
geneous in grain size, but most are fine grained and in hand
specimens easily confused with microgranite.