Acta naturalia Islandica - 01.02.1946, Síða 49

ORIGIN OF THE BASIC TUFFS OF ICELAND
43
and a few beds of
conglomerates of vol-
canic origin. Hvalfell
and Botnssúlur are
two immense piles of
brown tuffs or brec-
cias resting on this
composite series. The
thickness of these
piles is well over 300
m both in Hvalfell and in Botnssúlur. Especially in Hvalfell there is
a clear stratification, with the same general dip as that of the
composite series below. It is clear, that both mountains are only
remnants of an extensive cover of pyroclastic material and it seems
also clear, that this cover was laid down before the tectonic period
which is evidenced by the general dip. In both mountains these tuffs
are covered with lavas of ophitic dolerite which is entirely different
from the lower basalts. These lavas of dolerite probably covered ori-
ginally much larger parts of the main layer of brown tuffs.
Above the edge on the western side of Botnssúlur at a height of
about 900 m there is what appears to be a relatively recent flow of
fragmentary brown glass, flowing out over the present edge of the
mountain. The tuff is of very fresh appearance and clearly distinct
from the tuffs of the mountain itself. In a thin section (380) this tuff
consists of fresh angular fragments of pale yellow sideromelan of
1—2 mm diameter. These fragments are highly charged with
phenocrysts of plagioclase and some olivine (20 — 30 % crystals).
The fragments are embedded in a rather sparse groundmass of
comminuted sideromelan and its phenocrysts. There is no indication
of porosity of the glass. (379) is a grey tuff below (380) from the
main mass of the mountain. It is similar to (380), the main dif-
ference being a slight porosity of the glass and a few fragments of
porous dark glass.
A brown tuff (378) lower than (379) is perhaps more typical of
the main mass of the mountain. It differs from (380) by the fact
that every fragment of sideromelan is lined with a narrow brown
rim of palagonite and the fragments are completely cemented by this
palagonite and an appreciable amount of zeolites. Phenocrysts are
fewer than in (380) and there is some porosity as in (379).
Gtc/snur
SSírtÍÍtiítnA—' •' ’
Fig. 7. — Section of Hvalfell from the south, show-
ing- the mountain as a remnant of a thick cover of
stratified tuffs.