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the west. From there to Hestvík these “eastem” dolerites and the
Mosfellsheidi “westem” dolerites are hidden in an 8 km wide zone
under a complex of Pleistocene rocks on the south side of Thing-
vellir Lake, as will be considered under d). Reverse magnetization
is found at a high stratigraphic 'level in these rocks.
It now seems most likely that these main shieldvolcanoes cor-
respond in time to the volcanic activity at the mouth of Skaga-
fjördur and adjacant areas whidi was mentioned at the beginn-
ing of this paper, and the age of which is about 1 My. This
would be the first indication of such coupling of the volcanic phase
in the north and the south of the Median Zone. There are several
shieldvolcanoes in the Median Zone between the mentioned ends
(e.g. Ok, Skersli, Skálpanes, and Baldheidi; all close to the Lang-
jökull), which might belong to the same volcanic phase, but these
will not be included in the present study.
The dolerite gap of 8 km at Thingvellir Lake, compared with
the distance of 23—24 km between the eastem and western dolerite
centers, would not allow any backward displacement that could
bring the centers into such a picture of SW-NE axes which charac-
terizes the late activity described in a). The dolerite shields are
not axial, but rather border or areal features like the valley vol-
canoes in b).
The crater of Lyngdalsheidi has been destroyed, but coarse plugs
are found instead. Around the valley of the Sog river a very similar
state of erosion of these lavas is found as in the Reykjavík area
(2, pp. 112-114). The lavas rest on a tillite at Sog. Thus, there
are many similarities which suggest a similar if not identical age
of the eastern and western dolerites. Both may be the result of a
certain state of the stress field, under which volcanism appeared
preferably at the crossing points of conjugate lines. It is pretty
clear that the valley volcanoes would have become shieldvolcanoes
if space and absence of water had allowed, and those volcanoes
prove that huge amounts of magma emerge at such points. For
the postglacial time there is, sofar I know, no such case where it
can be demonstrated that a fissure (or linear) eruption has pro-
duced a major shield. On the contrary, the complete absence of
elongation of the shield speaks against this. In the case of Selvogs-
heidi, on the other hand, it is pretty clear that this shield is situ-