Rit (Vísindafélag Íslendinga) - 01.07.1962, Side 65

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suggests rather late-Pleistocene age of the forms. In some
cases the scarps have been formed in the post-glacial time
(Thingvellir area).
The area which I have selected for detailed study of the
Basic Tuffs is a part only of the total area of these rocks.
It was selected with the aim of finding out the structure of
the fragmental masses, their genesis and relative ages and
finally their age relation to the Plateau Basalts and their ero-
sional forms. Some of the tuff mountains are younger than
the valleys cutting the Plateau Basalts (Hvalfell, Thverfell,
Mosfell in Mosfellsdalur) while others are older than these
valleys (Súlur massif). We will find also that they contain
several magnetic groups; parts of the Tuffs were formed dur-
ing the last period of normal magnetic polarity but in addi-
tion 3 more magnetic periods can be discemed, two of reverse
and one of normal polarity. It is thus found that the Tuffs
extend over a long time and their older parts must prohably
be considered as Tertiary. At one level lignite is found which
is probably Tertiary.
Opinions of the nature of the “chaotic” Palagonite Forma-
tion have varied greatly in the past.
In the middle of the last century v. Waltershausen con-
sidered these rocks as the result of submarine volcanism and
alteration. Th. Thoroddsen (1905—06) on the basis of his
extensive travels considered them as formed by Pliocene sub-
aerial explosive eruptions on a very large scale. H. Pjeturss
thought that the entire Palagonite Formation was of Pleisto-
cene age and after this the hypothesis has been widely held
that these fragmental masses are due to sub-glacial emptions.
The table mountains — steepsided heaps of tuff-breccias with
a cap of basalt lavas — have been explained as unitary sub-
glacial structures: during the earlier stages of an eruption
a cavity and a hole is melted into the ice sheet and filled
with fragmental volcanic masses. Only when the surface of
this pile has bceome higher than the glacier are regular lava
flows formed.
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