Acta naturalia Islandica - 01.02.1946, Síða 18

12
TRAUSTI BINARSSON
by Al. Penck that the Icelandic Palagonite tuffs consist of fragments
of isotropic yellow glass, sideromelan, which were superficially and
sometimes wholly altered by hydration into a brown anisotropic or
isotropic substance, palagonite.1!)
According to Penck, sideromelan and the opaque tachylyte cannot
be distinguished chemically, and his conelusion was that they are
essentially identical. Therefore, Penck considered the Palagonite tuffs
to be simply a common loose product of volcanism, altered by hydra-
tion. (“Sideromelan, ein vulkanischer Auswilrfling”).
Thoroddsen rejected v. Waltershausen’s reasons for a submarine
origin of the formation. In his view, based on extensive field work,
there is “keine Veranlassung vorhanden, etwas anderes anzunehmen,
als dass die Bildung von Tuffen und Breccien auf trockenem Lande
bei Ausbriichen íiber dem Meere vor sich gegangen ist.” (Island,
p. 291).
On the other hand Thoroddsen had no clear conception of the
mechanism of these eruptions. The difficulty of some of the pro-
blems facing him is seen from the following passage:
“In langeren Profilen werden bisweilen zahlreiche grosse, eckige
Felsblöcke iiber grosse Areale unregelmassig ausgestreut, angetroff-
en, und es ist nicht leicht zu begreifen auf welche Weise das Aus-
streuen dieser grossen Basaltblöcke stattgefunden hat; vorausge-
setzt dass dieselben von Vulkanen ausgeworfen sind, liegt die Vermut-
ung nahe, dass die Explosionen der Vorzeit ungleich kráftiger als
die gegenwártigen waren. An einzelnen Stellen können diese Blöcke
durch Schuttströme und in den neueren Tuffen durch Gletscher und
Gletscherláufe ausgetreut sein, aber im grossen ganzen sind die Ver-
háltnisse noch recht rátselhaft”. (Island, p. 290).
In the main Thoroddsen, however, believes clearly in an explosive
origin of these masses, although he is very well aware of the hope-
less confusion the formation presents on closer observation and the
many problems it creates.
The conception of the origin of the formation was, however, again
to be greatly altered.
In 1899 H. Pjeturss brought forward the view that certain peculiar
grey conglomerates embedded in the formation were of glacial origin,
and in the following decade this author discovered such conglomer-
ates containing striated boulders and sometimes resting on a striated
floor in a considerable number of localities in the formation. He