Jökull - 01.12.1967, Page 51

particular sample are represented horizontally,
while vertically the samples are ranked accord-
ing to depth. In front of the samples is a
column listing all the tephra layers that could
be determined. Behind the samples their grad-
ing into gravel, sand and silt according to
weight percentages is shown. In the figure the
stratigraphy in the hole should be discernible.
It is most clearly perceivable where the samples
Fig. 3. The sieved samples from the drillhole
glued to a sheet of pasteboard and arranged
according to decreasing grain size from left to
right. The first and the last columns show
depth in meters. The 2nd column shows classi-
fication of ash layers. The 12th column shows
histograms for the grain size groups gravel, sand
and silt.
Mynd, 3. Sigtuð sýnishorn úr borholunni límd
á pappspjald. Sýnd er skipting sýnishornanna
i liornastcerðarjlokka og greining á öskulögum.
contain white pumice. A shade in colour can
also be perceived in the most fine-grained
group, but there the light colour is due to
diatoms in the samples from the 9—16 m depth.
The classification of the tephra layers was
done by determination of the refraction of
plagioclase phenocrysts. The classification of
the light pumice layers was performed with the
assistance of J. Tómasson geologist, who has
investigated the mineralogy of light pumice lay-
ers (Tómasson 1967). The classification of the
uppermost tephra layer as Laki 1783 was also
made with the aid of Tómasson’s examination
of the refraction in plagioclase phenocrysts frorn
the Laki lava flow.
The classification of the three lowest tephra
layers is also based on the refraction of plagio-
clase phenocrysts. All these layers have very
basic phenocrysts of the same type as the plagio-
clase phenocrysts in Tungnaá ancl Veidivötn
lava flows, i.e. 85—95% anorthite, and the tephra
layers can be asserted to originate from the
volcanic zone west of the Snjóöldufjallgardur
mountain range. They cannot be traced back
to definite eruptions within the area, but their
age relation to the light tephra layers indicates
their being contemporary to lava flows origin-
ating in the area. The samples are almost ex-
clusively pumice and glass. Singular móberg
and basalt grains can be found intermingled.
Most of them are of the same nature as the
móberg and basalt in Snjóöldufjallgardur (i.e.
having big phenocrysts of plagioclase).
3) In the samples from the 9—16 m depth
diatoms were also found. J. Jónsson geologist
investigated the diatoms of three samples, from
9, 12 and 15 m depth, and some remarks in
his report will now be quoted: “— The samples
from 9 m and 15 m depth are very rich in
diatoms, but the sample from 12 m depth much
poorer. The number of species is in itself not
great but none the less noteworthy considering
the location of the sampling place . . . It may
also be pointed out that the individuals in
these samples are on the whole remarkably
large and mature.” Jónsson’s conclusions are
the following: “Juclging from the general re-
view, which is now available, I would conclude
that it was deposited in a lake, probably rather
big, containing comparatively great quantities
of nutritive substances. On the other hand the
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