Náttúrufræðingurinn - 1966, Blaðsíða 53

NÁTTÚRUFRÆÐINGURINN
47
sand intercalates two basaltlayers. Tlie underlying basalt (C in fig. 1.) is
amygdoidal, its cavities filled mainly witli zeolites, the overlaying one (A) is
compact and 3—5 m thick. In this layer we observed numerous tubular
moulds (Fig. 2) and as far as we could find out during our very short stay,
tliese tubes liave been left by trunks of trees buried by the lava. Common
diameter of these tubes is 25 to 35 cm. One of the tubes was filled with green
doughy stuff, supposed to be celadonite. The trees buried here were probably
conifers.
II. Some words about Eldgjá.
In a previous paper (Eldgjá, Náttúrufræðingurinn 25, 1955, pp. 148—153)
the author dividecl the volcanic activity which formed the northern part of
the famous fissure volcano Eldgjá (Fig. 3) in three main phases:
1. A rnainly explosive phase forming a layer of teplira and tephratic lava
2. An effusive phase producing enormous quantities of lava
3. A mixed phase exlruding a small amount of lava and forming craters
along the bottom of tlie chasm.
In the light of later experience won while watching the Askja eruption 1901
and the effusive phases of the Surtsey eruption the author now realizes tliat the
teplira layer produced by the Eldgjá eruption resulted mainly from a very
vigorous lava fountain activity ancl not from explosive activity.
The garland contours of the northern part of Eldgjá are — as formerly main-
tained by G. Robson in his unpublished monograpli of Eldgjá — to a great
extent the residt of slipping of rock from tlie walls of the chasm, although
explosive activity at the very beginning of the eruption may have played some
role. During the summer of 1966 the author observed such slipping causing
garland contours of the outer wall of a semicircular chasm formed on the out-
side of the crater on the now disappeared island Jólnir SW of Surtsey (Fig. 5).
III. Granite boulders found on the beach near Búlandshöfdi.
The author describes a big granite boulder (weight about 600 pounds) found
on the beach near the farm Búlandshöfdi on the north shore of Snæfellsnes,
West Iceland (Fig. 6). The boulder was more likely brought to this place by
drift ice tlian by boat.
The origin and transport of a flint boulder (Fig. 8), brought up from the
sea bottom of Breiðafjörður, 7 km N of Rif, is also discussed. Possibly that
boulder was carried by drift ice too, althougli other possibilities are not ex-
cluded. Only twice since the settlement of Iceland 1100 years ago is the arctic
drift ice reported to have reached the Breidafjördur area. Conditions may,
however, liave been more favourable for ice drift into that area during cold
periods in prehistoric time, such as the Dryas periods or the first centuries of
the Subatlantic time. A systematic study of íoreign rocks on Icelandic beaches
and on the slielf is desireable.