Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 188

186
trast, sideromelane has not been produced in the ordinary
post-glacial eruptions in Iceland. Peacock suggested that
sideromelane was the result of “ultra-rapid chilling” that pre-
vented the formation of magnetite in the glass, and this fur-
ther supported the idea of sub-glacial extrusion. But this ex-
planation of the origin of sideromelane is untenahle.
In my early study of the Fragmental Rocks (1946) is given
a stratigraphic picture which is a step towards the conclu-
sions arrived at in the present paper: 1) The upper part of
the Basalt Plateau contains alternating layers of lavas and
fragmental rocks and was termed Composite Rocks. These
are nearly identical with our present Young Plateau Basalts
and constitute a considerable part of the old “Palagonite
Formation”. 2) Over this plateau was spread a separate thick
layer of fragmental rocks, called “The Younger Tuffs”. These
incorporated what is here considered as remnants of a rather
general thick layer of fragmental rocks (e.g. the main Súlur
tuff-breccia) as well as the younger elements. Lacking in
this early picture was the peneplanation between the Gene-
ral layer and the Young Plateau Basalts. But it was stated
that the present topography in the Plateau Basalt regions was
the result of uplifts younger than the rocks corresponding to
our General layer.
My early studies were mainly concemed with the frag-
mental rocks of the Young Plateau Basalts, and for these I
found the hypothesis of sub-glacial extmsion unacceptable.
My main conclusion may be said to have heen that chilling
did not play such an exclusive role as was mostly assumed.
My views on the origin of sideromelane were further deve-
loped in my studies of the emption of Hekla 1947—48. Here
I found that the translucent (in thin section) type of glass
closely similar to sideromelane (a little more acid) was form-
ed by slow cooling in cores of lava flows. These studies led
to the conclusion that crystallization depends in part on
“stirring”, in the form of shearing movement, and where this
is at minimum, as in the core of a lava flow, glass may be