Acta naturalia Islandica - 01.02.1946, Síða 12

6
TRAUSTI EINARSSON
he assumed to be the three main formations of the country, viz. the
Basalts, the Palagonite Formation, and the Dolerites.
Yet, Thoroddsen could not devote himself to more specialized
work, and this classification of the rocks was not parallelized by,
and founded on any extensive microscopical petrographical work.
No wonder, therefore, that he came to no clear conception of the
nature of the chaotic Palagonite Formation, and that his distinction
of Basalts and Dolerites by their general appearance in the field
resulted in some confusion as such a distinction must be based on
extensive petrographical research.
Thoroddsen built up the following system:
The oldest formation of the country is plateau basalts with a
thickness of at least 3000 m containing beds of a flora, which accord-
ing to 0. Heer is of Upper and Lower Miocene.
In the Pliocene or earlier, this plateau was heavily faulted, and
was covered, mainly through violent eruptions of ash, by tuffs and
breccias with a thickness of many hundreds of metres, especially
in an irregular zone across the country from NE to SW.
This „Palagonite Formation" was in turn covered by Plateau
Dolerites to a depth of 300 — 400 m. These are the „Older Dolerites“
of Thoroddsen.
This zonal tableland was in turn broken down and through the
following erosion the main features of the modern topography were
modelled. But still preceding the Ice Age a number of shield-volcanoes
were formed, pouring out large floods of the „Younger Dolerites“.
During the following Ice Age, which Thoroddsen considered to have
been of unbroken continuity, the topography was only slightly af-
fected.1)
This system of Thoroddsen’s was questioned very soon, as another
Icelander Helgi Pjetursson (now Dr. Helgi Pjeturss) in 1900 an-
nounced the discovery of morainqs embedded in the Palagonite
Formation.2) The conclusion then was that this formation was of
Quaternary age.
In the next decade this author collected a wealth of material on
moraines, occurring not only in the Palagonite Formation, but also
in the presumably Miocene plateau basalts of Middle Northern Ice-
land, in the plateau of Mt. Esja in Southwestern Iceland, and in
many other localities. Furthermore, Pjeturss’ discovery of arctic
Mollusca below Plateau Dolerites and palagonite tuffs in the Snæ-