Acta naturalia Islandica - 01.02.1946, Blaðsíða 19

ORIGIN OF THE BASIC TUFFS OF ICELAND
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furthermore discovered that the palagonite tuffs of Snæfellsnes rest
on sediments containing arctic molluscs.
The remarkable idea was now conceived that the Palagonite tuffs
were the result of sub-glacial eruptions during the Quaternary
period, whereas Thoroddsen had put them in the Pliocene.
A petrographic study of the basic tuffs in Iceland was published
by M. A. Peacock in 1926.12) According to Peacock a clear distinction
between tachylyte and sideromelan should be made, the latter most
probably being the result of an ultra-rapid chilling which was “suffi-
ciently drastic to inhibit the facile separation of opaque ores and
thus to produce translucent sideromelan”. (1. c. p. 67). In Iceland
this chilling was, in Peacock’s opinion, caused by the Quaternary ice
beneath which the eruptions were to have taken place.
These conclusions of the Peacock’s are, however, erroneous and this
must be made quite clear as they have not only been made the basis
of later work in Iceland but are also widely quoted and used in the
literature on basic tuffs elsewhere. As a matter of fact it is a theore-
tically unsound hypothesis that the absence of opaque ores in the
basic glass is indicative of ultra-rapid chilling. And it is a demon-
strably wrong assumption, as the observations, to be described in
this paper, show beyond doubt that sideromelan may be formed
by slow cooling.
And this should a priori have been expected. It is well-known
from the glass industry that glass may easily be formed by relatively
slow cooling and the same must be expected for natural glasses. An
interesting verification of this are the well-known post-glacial
streams of obsidian in Iceland. The emphasis laid by Peacock and later
authors on the factor of rapidity in the formation of glasses is cer-
tainly misleading.
It is, in other words, erroneous to believe that Peacock’s micro-
scopic studies of the Basic tuffs in Iceland indicate a sub-glacial ori-
gin of the glasses. These studies are a most valuable investigation
of the alteration of the glass but they leave in reality the question
of its origin unanswered. The only argument decisively in favour
of the sub-glacial origin is Pjeturss’ interpretation of the old con-
glomerates, but these are still unsatisfactorily investigated and even
though we accept Pjeturss’ view we are still far from proving the
sub-glacial extrusion of the tuffs.
The last phase in the study of our basic tuffs centered around the