Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 32

30
magnetic polarity, i.e. on truely classical fundamental prin-
ciples of stratigraphy, whereas the guesswork of picking out
geopolarity epochs and events from a most questionable Geo-
polarity Time Scale, can only lead to confusion in the geology of
the country.
The group (R3n) extends from Heljarkinn in Hreppar (17)
somewhat northwards, where it covers directly the extensive
group (N4n). The higher parts of the latter form brecciaceous
ridges, as Fitjaásar, Geldingafell, and Digraalda, which stretch
towards the tuff and breccia mountains of the Kerlingarfjöll
group and the socle of Hofsjökull, the connection with which
we cannot consider here.
But on the north side of Langjökull and the Kjölur pass,
we find again an uppermost normal group which is flatlying
and brecciaceous towards the top. The extensive doleritic lavas
farther north were first recognized and studied by J. Líndal,
but more extensively and also paleomagnetically divided by the
present author (17). There are three untilted magnetic groups,
and finally a reverse group with slight tilt from Blanda river
west towards the axis of the Median Active Zone. This lowest
group is very fresh as the others, and is separated by a great
unconformity from the Lower Tertiary lavas underneath, which
contain lignite, a sign of Lower Tertiary age, as we shall
see below. For this reason, I included in (17) the fresh
Blanda group and (N4n) into a general class of „young“ rocks.
We should now obviously be very cautious in our strati-
graphic placing and magnetic denotation of the youngest tilted
groups. (N4n) in the south and apparently (R5n) in the north,
as the youngest tilted groups suggest in any case that the
geomagnetic polarity succession is in either area not register-
ed completely. But the age difference between (N4n) in the
south and the Blanda reverse group in the north is probably
small.
The tilting should be taken to herald the first formation (as
far as we know) of active zones in Iceland. The horizontal
younger groups then are due to zonál activity. The zonal rocks
are, as a rule, untilted, with very small-scale exceptions, cf.