Jökull - 01.12.1982, Page 96

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famme ífpfl, eg §»ilfen Jigget jufl uabet famme ffiiertbian, og tffo mere enb
Fig. 1. A page from P. Löven-
örn’s paper: ‘Observations of the
compass’ declinadon at diíferent
locations during two marine voy-
ages, with explanatory notes, as
well as some observatíons on the
confusion of the compass needle
in Icelandic ports, its daily varia-
tion there, and a few observations
of the magnetic inclination". Nye
Samiing af det Kongelige Danske
Videnskabernes Selskabs Skrift-
er, Femte Deel, Copenhagen
1799: 299-326 (in Danish).
Mynd 1. Utskýring Lövenörns á
kompásskekkjum við Island.
by J. Graham and collaborators in the U. S., and
detailed description of inversely magnetized
igneous rock outcrops from Europe by A. Roche
and others.
However, the origin of the main magnetic íield
was still not understood, as may be seen from the
1952 publication of P. M. S. Blackett’s work regard-
ing his hypothesis that a magnetic dipole field might
be an intrinsic property of all rotating objects. In
the first edition of T. Nagata’s book on Rock Magne-
tism in 1953, the reality of geomagnetic field rever-
sals is doubted, partly in view of the discovery of a
repeatably self-reversing rock type in Japan; how-
ever, this rock has tumed out to be quite unique in
its properties, and numerous subsequent studies
have also shown that even irrepeatable self-reversal
must be a relatively rare phenomenon.
It must have been known to many travellers and
navigators that Icelandic rocks can be quite
strongly magnetic. Thus, it is mentioned in the
Ferðabók (Travelogue) of E. Ólafsson and B. Páls-
son, first published in 1772, that during their
famous ascent of Snaefellsjökull in 1753 their com-
pass had behaved very erratically. In a 1799 paper,
P. Lövenörn (Fig. 1) reported that during his care-
ful declination measurements in Icelandic harbors
in 1786 he had obtained confusing results which he
attributed to the magnetic effects of iron-rich miner-
als in nearby basalts.
Early in this century R. Chevallier and P. Mer-
canton measured the magnetization (N. R. M.) ofa
number of oriented and semi-oriented basalt
samples collected during French and Swiss Arctic
expeditions. Basalts from Mull, Disko, the Faeroes
and Spitzbergen turned out to be generally in-
versely magnetized, but samples from Jan Mayen
were normally magnetized. Mercanton (1931, 1932)
reported laboratory measurements on altogether 11
basalt samples which he collected, during the 1929
and 1931 cruises of Pourquoi Pas?, at Eskifjörður,
Eyjafjörður, Vatneyri, Þingvellir, and Hvammsey
in Hvalfjörður. The magnetization of all of these
was normal except in one sample from Esktíjörður
which gave an inclination of — 7°. However, as these
92 JÖKULL 32. ÁR