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Vulkansk Sand og Sandler.
Øen Jan Mayen er som bekjendt af vulkansk
Oprindelse. Den stærke lokale Hævning, der bar foraarsaget
dens Dannelse, giver sig tydeligst tilkjende paa Nordkysten,
hvor det egentlige Hovedkrater — den 6000 Fod bøie
-Beerenberg" — lindes. Her træffer vi et Braadyb af
1000 Favne i en Afstand af omtrent 2 Mile fra Øens
nordligste Punkt. Paa Øst- og Vestsiden af Øen skraaner
Kysten mindre bråt ned mod Dybet. Ved de talrige
Dybde-maalinger. som Expeditionen her bar foretaget, befandtes
alle Bundprøver, der var optagne paa mindre Dyb end 600
Favne, at bestaa af et graasort fint Sand eller Sandler, der
indeholdt talrige Brudstykker af den basaltiske Lavas
Mineraler: Olivin, Augit, Hornblende. Disse forekom ofte med
vel uddannede og vel bevarede Krystalflader.
Det vulkanske Sandler viser sig under Mikroskopet
at indeholde en Mængde forskjelligfarvede krystallinske Korn,
der væsentlig bestaar af de ovennævnte Mineraler, især er
den grønne Olivin meget fremtrædende. Forøvrigt ser man
ogsaa en Del sorte metalglindsende Korn, der ved Hjælp
af Magneten lader sig udtrække af Bundprøverne. Disse
synes i det Hele taget at indeholde de samme Mineraler —
i fint fordelt Tilstand — som dem der forekommer i de rige
Sandleier langs Jan Mayens Kyster. Dette sorte Sand er
dannet af temmelig grove Korn af Lava, Tuf, Olivin,
Feld-spath, Augit, Hornblende og Magnetjern.
Af det sidstnævnte Mineral fandt jeg i Sandet ved to
-Bestemmelser 26 pCt. og 29 pCt.1
Fra disse Sanddynger, der ligger ubeskyttede for
Bølgerne langs Øens aabne Kyster, maa der stadig kunne føres
nyt Materiale ud til den nærliggende Havbund.
I Syd for Jan Mayen synes der ifølge tidligere
Lodskud at være grundt Vand (100 Favne) indtil en Afstand
af omtrent 15 Mile fra Øen.
Her har den norsk© Expedition imidlertid ikke
foretaget nogen Dybdemaaling, og jeg tør derfor ikke indestaa
for Rigtigheden af de Grændser, jeg paa Kartet har
optrukket for det vulkanske Lers Udbredelse söndenfor Jan
Mayen.
I Bundprøverne fra Kysten af denne 0 findes næsten
ingen Dyrelevninger, og Leret indeholder kun Spor af
kulsur Kalk.
1 Professor Carl Vogt, der i 1*63 gjæstede Jan Mayen, har ogsaa
underkastet dette Sand en Undersøgelse, ved hvilken han fandt 21.6
pCt. Magnetjern. i Nord-Fahrt entlang der Norwegisehen Küste, nach
dem Nordcap, den Inseln Jan Mayen und Island, unternommen von
Dr. Georg Berna i 863).
Volcanic Sand and Sabulous Clay.
The island of Jan Mayen is, as well known, of
volcanic origin. More especially on the north coast, have the
prodigous forces whereby the ocean-bed was upheaved in
this locality of the North Atlantic, left evidence of their
bygone action; there lies Mount Beerenberg, the
principal crater — 6000 feet above the level of the sea. About
2 geographical miles from the most northerly extremity of
the island we meet with a depth of 1000 fathoms. Off
the eastern and western shores, the bottom is found to
shelve less rapidly down to the depths. All of the
numerous samples collected on the Expedition throughout this
tract from depths of less than 600 fathoms, consisted
exclusively of a dark-grey sand or sabulous clay, containing
fragments of basaltic lava, as olivine, augite, hornblende.
Many of these had well developed and well preserved
crystal faces.
The volcanic sabulous clay, when examined under the
microscope, is found to contain a great many differently
coloured crystalline particles, consisting chiefly of the
above-mentioned minerals, in particular green olivine. For the
rest, numerous black granules of metallic lustre are also
observed, which, with the aid of a magnet, may be extracted
from the clay. They would appear to consist in greater
part of the same minerals — in a state of minute
subdivision — that occur in the sand forming extensive banks
on the coast of Jan Mayen. This black sand is composed
of comparatively coarse particles of lava, tuf, olivine,
feldspar, augite, hornblende, and magnetite.
The last of these minerals I found, from two
determinations, to constitute respectively 26 per cent and 29
per cent of the sand.1
These sand-hills, stretching as they do along the
exposed shores of the island, must obviously at all times
contribute to the distribution of deposit over the adjacent
parts of the sea-bed.
South of Jan Mayen — as shown by the results of
former soundings — comparatively shallow water (100
fathoms) extends about 15 geographical miles from the coast.
On the Norwegian Expedition, the depth was not
measured in this locality, and I cannot therefore answer
for the accuracy of the limits I have traced on the map
to mark the distribution of the volcanic clay south of the
island.
Very few, if any, animal remains are found in samples
of the bottom from the coast of Jan Mayen, and the deposit
contains traces only of carbonate of lime.
1 Professor Carl Vogt, who visited Jan Mayen in 1863, has also
submitted this sand to analysis: he found 21.6 percent of magnetite.
(Nord-Fahrt entlang der Norwegisehen Küste, nach dem Nordkap, den
Inseln Jan Mayen und Island, unternommen von Dr. Georg Berna
1863.)
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