Jökull - 01.12.1975, Page 30

sures by about 2 km of rock, is inferred (Musset
et al., 1972). The most straightforward inter-
pretation is that the British basalts were dis-
sected and cooled as early as the dates indicate.
And in the dykes, cutting these plateau lavas, we
seem to find a support of this interpretation.
With only one exception (the case mentioned
in Section 2) the dykes show the same age
level as the lavas, instead of indicating various
amounts of excess argon as one could suggest
on the basis of Section 2. Loss of argon in
dykes and lavas during the formation of the
zeolites, but retention after dissection of the
area, seems here an acceptable interpretation.
Dykes may then, generally, provide a test of
argon loss up to certain age levels.
For the Phanerozoic, Harland (1975) has sug-
gested that the two time scales, the chrono-
stratigraphic one and the geochronometric one
be used ancl developed separately, as otherwise
“we are liable to use a hybrid scale” (1. c., p.
505). We agree, especially as to pre-Neogene
time, and suggest the establishment of such a
lst-order system, in which the two scales are
only linked to each other at such fixpoints
which in both scales surpass all tests of reli-
ability. Besides this, and clearly distinguished
from the 1 st-order system, a hybrid scale may
be convenient, although it is liable to repeated
changes.
Addition in Proof to lst section. The test by
Lanphere and Dalrymple (1971) of the 40Ar/
30Ar-Dating method suggests to the author that
for terrestrial rocks n'ot only heating but also
groundwater effects must be taken into account,
which must further complicate the interpreta-
tion of the age spectra. Excess argon in dykes,
cf. section 2, seems to be very loosely bound,
i.e. probably interstitial.
ACKNOWLEDGEMENT
The author is indepted to Sveinbjörn Björns-
son, and Dr. Leó Kristjánsson for their interest
in this study, indications of literature, and dis-
cussions.
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