Acta naturalia Islandica - 01.02.1946, Page 77

ORIGIN OF THE BASIC TUFFS OF ICELAND
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southerly direction we find that it passes smoothly into a basalt
without globular structure, but with pronounced columnar structure
(irregularly orientated columns)” (p. 57). Then, a part of this lava
might well be of sub-aerial or intrusive origin, and then, why not
all of it? In short, the sub-glacial origin of this particular globular
basalt is not derived from observations of the case itself, it is simply
assumed on theoretical grounds, moreover, it would seem, in spite
of the local observations.
Thus, all the main assumptions on which the sub-glacial extrusion
hypothesis rests, are certainly disputable, and some are beyond
doubt untenable.
In conclusion I may add that my critique of course in no way
touches the valuable descriptions of rocks, we find in the paper. I
might, however, point out some differences of terminology and con-
ception between this paper and mine. What 1 term a lava with block
jointing Noe-Nygaard seems ahvays to call a globular basalt, and
when he speaks of a breccia with basalt-globes which are more or less
broken into pieces, I speak of a breccia with basalt blocks. By this
terminology Noe-Nygaard is, it seems to me, emphasizing his belief
that the primary form is always or mostly a globe or a pillow. Yet,
in the facies I have described, the globular structure was nowhere so
apparent as to be characteristic and there is no doubt that Noe-Ny-
gaard overemphasizes the occurrence of the globular structure.
This may be understood when it is considered that the pillow
structure is generally thought to be the structure of subaquaeous
lavas and Noe-Nygaard believes that the rocks in question were form-
ed under the influence of meltwater.
This naturally raises the question whether the pillow structure is a
weighty argument for the sub-glacial extrusion hypothesis, as ob-
viously assumed by Noe-Nygaard.
Most writers seem to assume the sub- aquaeous origin of pillow
structure, but it should nevertheless be kept in mind that other ex-
planations have been put forward and the theory of J. Volney Lewis
seems well worth considering (Origin of pillow lavas. Bull. Geol.
Soc. Amer. XXV, 1914, 591—664). According to Alb. Johannsen in
A Descriptive Petrography of the Igneous Rocks, Vol. III, p. 278,
“Lewis thought that neither the presence nor the absence of water
can be predicted as particularly favorable to the formation of pillows.
He assumed first that the lava was free-flowing and viscous, and