Jökull - 01.12.1975, Síða 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.
REFERENCES
Carslaw, H. S. and ]. C. Jaeger. 1959: Conduc-
tion of Fleat in Solids. 2nd ed. Clarendon
Press, Oxford.
Dalrymple, G. B. a?id M. A. Lanphere. 1969:
Potassium-Argon Dating. Freeman & Co.,
San Francisco.
— and J. G. Moore. 1968: Argon-40 Excess in
Submarine Pillow Basalts from Kilauea
Volcano, Hawaii. Science, 161, 1132—1135.
Einarsson, Tr. 1949: The Flowing Lava. Studies
of its main Physical and Chemical Pro-
perties. Soc. Sci. Isl.: The Eruption of
Hekla 1947-48, IV, 3, p. 1-70.
— 1950: Chemical Analyses and Differentia-
tion of Hekla’s Magma. Soc. Sci. Isl.: The
Eruption of Hekla 1947—48, IV, 4, Table
ÍV..
— 1971: Magnetic Polarity Groups in the
Fljótsdalsheidi Area, including Gilsá. Tök-
ull, 21, 53-58.
Friedrich, W. 1966: Zur Geologie von Brjáns-
laekur (Nordwest-Island) unter besonderer
Beriicksichtigung der fossilen Flora. Spec.
Publications of the Geological Institute,
Univ. Cologne, No. 10.
Harland, W. B. 1975: The two geological time
scales. Nature, 253, 505—507.
Lanphere, M. A. and G. B. Dalrymple. 1971:
A Test of the 40Ar/39Ar Age Spectrum
Technique On Some Terrestrial Materials.
Lee, R. R., D. A. Leich, T. A. Lombrello,
J. E. Ericson, and I. Friedman. 1974: Ob-
sidian hydration profile measurements us-
ing a nuclear reaction technique. Nature,
250, 44-47.
Musset, A. E., G. C. Brown, M. Eckford, and
S. R. Charlton. 1972: The British Tertiary
Igneous Province: K-Ar ages of some Dykes
and Lavas, from Mull, Scotland. Geophys.
J. R. astr. Soc. 30, 405-414.
Pflug, H. D. 1959: Sporenbilder aus Island und
ihre stratigraphische Deutung. Neues Jb.
Geol. Palaont. Abh. 107, 141-172.
Verhoogen, J., Fr. J. Turner, L. E. Weiss, C.
Wahrhaftig, and W. S. Fyfe. 1970: The
Earth; an Introduction to Physical Geo-
logy. Holt, Rinehart and Winston.
Walker, G. L. P. 1960: Zeolite zones and dike
distribution in relation to the structure of
the basalts of eastern Iceland. J. Geol., 68,
515-528.
Winkler-Hermaden, A. 1957: Geologisches
Kráftespiel und Landformung. Vienna.
28 JÖKULL 25. ÁR