Rit (Vísindafélag Íslendinga) - 01.07.1962, Side 192

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sive, resting on the peneplaned Young Plateau Basalts. Thus,
after a time of peneplanation when the land must have been
a low plain, and after a general glaciation of this plain, condi-
tions for the formation of fragmental rocks were especially
favourable.
Here one might be tempted to postulate submergence of
the land, and the previous glaciation might have led to this;
later the weight of the new rocks would also depress the land.
In one case I have found breccia overlying directly fossili-
ferous marine clay and consider that breccia (Hvammsheiði
breccia near Húsavík) to be of sub-marine origin. But no
other direct indications of submergence are known in the ex-
tensive tuff-breccia masses of the General layer. For lack of
such evidence one must assume an essentially subaerial ori-
gin.
As to sub-glacial environment the wide extension of the
General layer seems incompatible with such an origin. Fur-
thermore, the structure of the layer is in many cases quite
incompatible with this hypothesis. Thus, the main Súlur tuff-
breccia is made up of nearly horizontal layers, altemating
between fine tuff and coarse breccia. Especially the tuff lay-
ers are an indication of wide air transport from explosive cen-
tres and subglacial conditions cannot be assumed. The Gene-
ral layer is very largely the result of explosive volcanism,
and more quiet flow of volcanic breccias seems also to have
played a considerable role. The question then remains why
this period should have been characterized in this way. To
explain this it is suggested that explosivity in our basaltic
eruptions is mainly caused by access of water at considerable
depths into the volcanic funnels or fissures. This is in keep-
ing with conclusions regarding the explosive activity in the
Hekla eruption of 1947—48 (Einarsson 1951 b).
Now the volcanism in Iceland is characterized by fissure
eruptions. Suppose that the newly deglaciated peneplane was
densely set with lakes (cf. Finland of to-day). Then there
was a great likelyhood that volcanic fissures would strike a