Jökull - 01.12.1963, Síða 50

Fig. 4. Schematic structure section A-B through the basalt lavas in northern Iceland.
existence of gabbro zenoliths in lavas of late
Tertiary age and younger in various locadons
in Iceland. He considers these as probably
deriving frorn a zone in the earth’s crust sorne-
what higher up than the peridotite shell.
Bodvarsson and Walker (1964) have discussed
the nature of layer 3 in relation to the problem
of crustal drift in Iceland. They find the most
likely hypothesis that it consists of a heterogene-
ous mixture of basaldc lava and intrusives. Such
a mixture is found at the surface in southeast-
ern Iceland and has there, according to the
present study, a P velocity corresponding to
layer 2, with layer 3 occurring at 1.2 to 1.8
km depth.
An indication of the nature of layer 3 may be
given by the value of Poisson’s ratio which is
essentially the same as for the two upper layers.
Lyakhovitsky (1961) has reviewed the data on
Poisson’s ratio for rocks and concludes that it.
depends on quartz content, increasing with de-
creasing quartz content. For granite, for instance,
a typical value seems to be about 0.24. As far
as this property is concerned it appears likely
that layer 3 is similár to layers 1 and 2.
Tryggvason and Báth (1961) reported a layer
with P velocity about 5.6 km/sec on three un-
reversed profiles in northern and central Ice-
land. Ewing and Ewing (1959) also give a similar
velocity on the Mid-Atlandc Ridge southwest
of Iceland. A layer with this velocity has not
been identified with certainty on any of the
profiles described here. Also Tryggvason (1962)
finds a somewhat lower P velocity of about 4.7
km/sec on the Mid-Atlantic Ridge southwest
of Iceland. The existence of a layer with P
velocity about 5.6 km/sec in the Iceland region
must therefore be considered somewhat un-
certain.
REFERENCES:
Áskelsson, J., G. Bodvarsson, Tr. Einarsson, G.
Ivjartansson and S. Thorarinsson: On the
geology and geophysics of Iceland, Intern.
Geol. Congr., Guide to excursion A 2,
Reykjavik, 1960.
Báth, M.: Crustal structure of Iceland, J. Geo-
phys. Research, 65, 1793—1807, 1960.
Bodvarsson, G. and G. P. L. Walker: Crustal
drift in Iceland, Geophys. J. Roy. Astron.
Soc., 8, 285-300, 1964.
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JOKULL 1963