Rit (Vísindafélag Íslendinga) - 01.07.1962, Qupperneq 64

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the NW and a vertical displacement is less evident; at least
in the northern part where we can trace the Hreppar basalts
from the Gullfoss area far to the SW within the wider de-
pression. Only west of this comes a greater sinking of the 20
km main zone. But at the southem end of Hreppar the ver-
tical drop into the graben is clear.
On both sides of the graben it is clear that the subsidence
is negligible in the north, i.e. about north of a line connect-
ing Ok with Gullfoss, but increases southward where its
amount in the narrower Thingvellir zone is unknown.
The depression and its prolongation northwards is occu-
pied by the Basic Tuffs. These occur either as individual
steepsided, flat-topped mountains or as clusters of such or as
irregular elongated ridges with the general tectonic trend
SW—NE. Some of the flat-topped mountains are covered by
a thin sheet of horizontal lavas and in a few cases volcanic
craters are found here.
On the lower ground between the mountains glacially scour-
ed, rather thin lava sheets are extensive. They consist of quite
similar light-grey basalts as are found as cover on some of
the mountains. These fresh basalts have long been called
dolerites, a term only correct in such cases where the lavas
are coarse as for instance in the surroundings of Reykjavík.
But for the sake of convenience we will retain this term.
Finally much of the depression area has been covered with
post-glacial extensive lava flows. The older ones are usually
of the smooth helluhraun type and flowed from shield vol-
canoes, while the younger are the result of fissure eruptions
or eruptions in smaller individual craters and are usually of
the rough apalhraun type. Such fragmental rocks as charac-
terize the Basic Tuffs have only very exceptionally been form-
ed in the post-glacial time.
Much of the depression is cut up into a great number of
elongated blocks with the tectonic trend SW-NE, vertically
displaced relative to each other. The fault scarps are mostly
covered with glacial debris but the freshness of the forms
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