Acta naturalia Islandica - 01.02.1946, Blaðsíða 52

46
TRAUSTI EINARSSON
lavas (of basalt) but we fincl also two layers of brown stratified
tuff (when ascending from the farm Uppsalir). The lower at 680 m
above sea level is 5—6 m thick. A thin section (400a) is nearly identi-
cal with (592), described above, but the alteration is still more ad-
vanced in (400a). (400b) from another substratum shows an abun-
dance of large worn crystals of plagioclase (diam. 1—2 mm), a few
crystals of olivine, several grains of dark glass enclosing large pheno-
crysts of plagioclase — all in a yellow faintly birefringent altered
groundmass. Judging from their size the separate crystals are no
doubt phenocrysts from the glass, either from the dark variety or
the sideromelan which must have been the mother substance of the
groundmass.
Somewhat higher, at 700 m, there is the second layer, 12—15 m
thich. There are many separate substrata of different coarseness, and
some enclose worn fragments of scoriae, but in other respects the
material is the same as in the lower layer. Slide (401a) is very sim-
ilar to (400a) but unaltered sideromelan occurs in addition. (401b) is
also very similar to (400a) and (401c) must be described as identical
with (592).
Kirkjufell at Öxnadalur. (Ascending a large gully near the farm
Engimýri). A composite lava of columnar and block jointing and
breccia of dark basalt cubes in a brown glassy matrix of the usual
kind we have often described occurs at an elevation of 500 m, i. e.
below a pile of 800 m. From an elevation of 450 m to about 800
m the section consists of fragmental material, either tuffs or breccias
and conglomerates of a very heterogeneous appearance. It is likely that
they are mostly of purely volcanic origin and that they may be caused
by a type of volcanism represented by the British necks. But here
we cannot enter into a discussion of these large and complicated
masses. Only a few features may be mentioned. The dark-brown
matrix (416) of the above- mentioned breccia consists of a granular
mass of faintly polarizing translucent glass with phenocrysts of
plagioclase, augite, and olivine. The larger glass fragments show
flow structure but the smaller ones usually do not.
A thin vein (418) of brown matter traverses the fragmental mass-
es below this breccia. It consists of a compact mass of palagonite en-
closing broken crystals of plagioclase and augite and scattered frag-
ments of porphyritic opaque glass.
At an elevation of 600 m there is an apophysis (423) of a compact