Jökull - 01.12.1963, Blaðsíða 49

respectively. Layer 2 is found at tlie surface in
southeastern Iceland where it is characterized
by large gabbroic and acid intrusions as well as
flood basalts. It is however not certain that the
nature of this layer is everywhere the same. It
is often found in the Tertiary flood basalt
areas at a relatively shallow depth, about 0.5
krn below sea level, where it is more likely to
consist of flood basalts.
It is of interest to note that Ewing and
Ewing (1959) found a velocity of 5.2 km/sec
under the ocean sediments in the Norwegian
and Greenlancl Seas northeast of Iceland. The
thickness of this layer was 2.5 to 3.0 km, which
is similar to what is found in Iceland for layer
2. They also give an average velocity of 5.15
km/sec for the upper layer of the Mid-Atlantic
Ridge in the north Atlantic ocean. Tryggvason
(1962) obtained from a study of surface wave
dispersion along the Mid-Atlantic Ridge south-
west of Iceland a P velocity of 4.7 km/sec and
a thickness of 4 to 5 krn for an upper layer
JÖKULL 1963
overlaying a substratum with P velocity 7.4
km / sec.
The nature of layer 3 is largely unknown as
this layer has not been found at the surface. It
is here founcl at depths frorn 1.2 to 4.5 km, if
the anomalous behaviour on profile 2 is ex-
cluded. Thus if the material of layer 3 is every-
where the sanie, the increase in velocity to this
layer cannot primarily be a pressure effect. The
mean P velocity of layer 3, 6.32 km/sec, is some-
what lower than that of the oceanic layer found
in the Atlantic ocean basins, which is 6.5 to
6.7 km/sec (Ewing and Ewing, 1959). It is
íurthermore not found in general on the Mid-
Atlantic Ridge under the 5.15 km/sec layer or
in the Norwegian and Greenlancl Seas under
the 5.2 km/sec layer. The absence of layer 3 on
the Mid-Atlantic Ridge both south and north
of Iceland suggests that it is associated t<ith the
Faeroes-Iceland-Greenland Ridge, as indicated
by Tryggvason (1962).
Jónsson (1963) has drawn attention to the
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