Acta naturalia Islandica - 01.07.1964, Page 23

THE ANKARAMITES OF HVAMMSMÚLI, EYJAFJÖLL, SOUTHERN ICELAND 21
northern block of Pöst (VI), (Fig. 4). It is intruded by a black sub-ophitic rock
(VII). The lowest and westernmost outcrop is a dense, very fine-grained, light-
coloured and porphyritic rock, similar to that seen in the lowest levels of the
puarry (See later). These relationships are repeated in Kálfshamar. In Dysjarhóll
the black sub-ophitic rock (VII) is underlain and intruded by light coloured rock.
The same relationship is sean in the SE cliffs of Pöst. The „dyke“ of Type V in
Pöst is thought to be the same rock as that seen in the quarry-group.
The tops of Pöst and Kálfshamar are covered by a layer of slaggy, weathered
product of the underlying rocks. As stated earlier, the main mass of the quarry
rock consists of two types which are closely intercalated, one granular (I),
the other sub-ophitic (II).
The various types will now be briefly described, mainly with reference to
thin sections. The modes are tabulated in Table I.
TABLE I.
Rock Minerals Groundmass Phenocrysts Overall
% % %
plagioclase 30.2 — 20.9
I pyroxene 45.1 20 37.4
olivine 12.5 80 33.3
ore 12.2 — 8.4
plagioclase 55.4 6.8 38.0
II pyroxene 21.1 41.7 28.5
olivine 13.3 51.5 27.0
ore 10.2 — 6.5
plagioclase 76.6 0.7 29.4
III pyroxene 6.2 56.6 37.6
olivine 8.6 42.7 29.8
ore 8.6 — 3.2
plagioclase 36.7 — 26.8
IV pyroxene 40.3 30 37.2
olivine 15.4 70 30.5
ore 7.6 — 5.5
v* plagioelase 28.0 23.5
VI pyroxene 49.9 45 50.7
olivine 14.1 55 19.0
ore 8.0 — 6.8
plagioclase 50.3 6 42.8
VII pyroxene 26.6 57 31.7
olivine 15.6 37 19.2
ore 7.5 — 6.3
* Type V is too fine grained for modal analysis. Distribution of phenocrysts is similar
to that of Type IV.