Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 45

45
we find clear traces of displacements and great planation at
a level which seems impossible at the 300 m stage (Hvamms-
gil region, Upper Víðidalur). Then there is southward dip in
Skagafjörður and finally the Young rocks seem to be identi-
cal with the Young Plateau Basalts farther south and east,
and therefore older than the Older 300 m stage. The Young
rocks in the Húnavatn Ileaths also suggest identity with the
Young Plateau Basalts. As there is no fully satisfactory solu-
tion of this situation, we will have to select the least objec-
tionable way out after having taken everything at our dis-
posal into account.
In this connection a mention of the occurrence of rhyolite
pebbles should be made. At several places we have noticed
the presence of rhyolite in fluviatile gravels at the base of
the Young basalts (Bakkabrúnir, Svartárdalur iri Húnávatns-
sýsla, Skagafjörður). It is significant that these gravels do
not usually contain material derived from the Young ba-
salts. The origin of the rhyolite is probably the following.
This rock is rather common in the deeper Tertiary Plateau,
and loose material of this kind should not have been infre-
quent on an old flat erosion surface cut into this variously
tilted and disturbed Plateau. In addition this surface repre-
sents a long volcanic quiesence and exactly in such times an
occasional rhyolite outburst could make this material more
frequent. Within the Young rocks here described rhyolites
have nowhere been met with; the former correspond to an
intensive volcanic phase with exclusively basaltic eruptions.
In still higher rocks, the socles of Hofsjökull and Langjök-
ull, rhyolites occur but they cannot be considered as a source
for the gravels here considered. Thus rhyolite pebbles do, as
far as the evidence goes, suggest the old flat erosion surface
below the Young Plateau Basalts.
Rhyolite pebbles seem not to occur at the base of the Young
rocks in northern Skagafjörður, nor have I noticed them in
Sólheimaháls or Höskuldstaðanúpur. On the other hand they
are clear in Svartárdalur, Skagafjörður and Bakkabrúnir.