Acta naturalia Islandica - 01.02.1946, Blaðsíða 74

68
TRAUSTI EINARSSON
of glass. Many of the horizontal layers in these mountains as well as
in those of Eyjafjörður may have resulted more or less directly
from such eruptions. In the dyke of Klambrasel (p. 53) on the other
hand the percentage of foreign blocks is high, which is also often the
case with the sheets of conglomerates.
This type is clearly characterized by a very low temperature of the
magma and is therefore closely related to the Peléean type of vol-
canism which may also be expected to have occurred and may reason-
ably have had such effects as the striation sometimes observed in
connection with the grey conglomerates.
6. A separate type is represented by the layer (1) of Southern
Ljósavatnsskarð (p. 50) and a similar layer of Leirhafnarfjöll (not
described in this paper) and (257) of Vörðufell (p. 18). The melt
extruded contained a great amount of fragments of porphyritic
sideromelan of an earlier generation. Here the temperature most
probably was very low and fluidity was maintained by a high content
of water.
POSTSCRIPT
After finishing the manuscript I had access to the detailed report
of a part of the studies of Dr. N. Nielsen and A. Noe-Nygaard, re-
ferred to in this paper: Sub-glacial Volcanic Activity in Ancient and
Recent Times (Studies in the Palagonite-System of Iceland. No. 1).
By Arne Noe-Nygaard. Köbenhavn 1940. (Folia Geographica Danica.
Tom. I. No. 2). The subject of this paper is “the form-complex within
the system which we assume to be the result of sub-glacial volcanic
activity” and which “plays a vital role quantitatively”. The paper
thus treats of the same main question as the present one but with
a different outcome as its author firmly believes in the “sub-glacial
extrusion hypothesis”.
In the main text I have pointed out the flaws in this hypothesis
and as far as I can see they are still inherent in the new paper. Yet
it may be useful to comment specially on this paper.
Noe-Nygaard believes that to explain the formation of sideromelan
the assumption of ultra-rapid chilling is inevitable. We have seen
that this is not so and that the assumption does not conform to cei'-
tain observations. Noe-Nygaard’s paper, in my view, contains such