Acta naturalia Islandica - 01.02.1946, Blaðsíða 75

ORIGIN OF THE BASIC TUFFS OF ICELAND
69
observations: “The intrusive dykes in the breccia are parti-
cularly frequent in the cliff wall behind Kirkjubæjarklaustur. These
dykes, which are very winding and irregular in their course, are coat-
ed with thick skins of sideromelan” (p. 36). But basic instrusives are
usually not coated with sideromelan and this therefore rather speaks
against ultra-rapid cooling as a vital factor, unless the ultra-rapid
cooling of these intrusives is explained by some special hypothesis.
The question of the moraines is treated as usually. A number of
cases is mentioned where the volcanic products are connected with
moraines (tillites). But the interpretation of the hard conglomerates
as moraines rests entirely on their macroscopic features, which are
misleading. In one case Noe-Nygaard reports the microscopic char-
acter of a tillite. He finds that “the finer material comprises various
more or less opaque particles and small pieces of mineral such as
olivine, pyroxene, plagioclase, and zeolites” (p. 41). Stratified clay
in the tillite “contains considerable quantities of sideromelan grains.”
This conforms well with the composition of the conglomerates which I
assume to be of volcanic origin. Another case of a tillite mentioned
in the paper is noteworthy:” Along the lower contact the matrix of
the tillite is somewhat bleached, and the globular basalt directly
adjacent to the tillite has a distinct glassy layer” (p. 14). We are
thus expected to believe that a Quaternary ice-sheet crept over the
loose “globular basalt” and deposited a moraine of some 3 m thickness
without even scratching the glassy surface of the basalt. Moreover
“besides its ordinary stones, the tillite includes large and small frag-
ments of basalt-globes, often with areas of the glass coating pre-
served” (p. 14). Thus Noe-Nygaard seems to assume that the presence
of striated stones proves the morainic nature of the structureless
conglomerates, even in the face of strong evidence to the contrary
and it is therefore certainly questionable whether he is right in his
interpretations.
The assumption of ultra-rapid chilling and of a connexion of the
eruptives with moraines represents the main basis on which the “sub-
glacial extrusion hypothesis” rests. But some weight is also laid by
Noe-Nygaard on a supposed sub-glacial eruption in Vatnajökull
in 1934, as indicated by the breaking up and subsidence of a part
of the glacier and by a great flow of water in the river Djúpá. As
to the river the best local authority, Hannes Jónsson of Núpsstaður,
confirmed to me, what is otherwise known, that the river is often