Acta naturalia Islandica - 01.07.1964, Síða 11

THE ANKARAMITES OF HVAMMSMÚLI, EYJAFJÖLL, SOUTHERN ICELAND 9
Pig. 3. Geological cross-section of Hvammsmúli from the east. Ankaramite outcrops,
Dysjarhóll and the quarry group. The dashed line represents the old travellers’ route
(cf. text). Heights in metres.
3 quarry outcrops to the north east. They have no proper names and are con-
veniently decribed as the quarry, SW of the quarry, and NE of the quarry.
Four or more phases of intrusion can be distinguished, some of which
at least are divided by only a short time span.
METHODS AND MINERALOGY
The rocks are composed of 4 primary minerals: plagioclase, clinopyroxene,
olivine and iron ore. The pyroxene and olivine occur abundantly both as pheno-
crysts and as constituents of the groundmass, whereas the feldspar occurs
mainly, and the magnetite exclusively, in the groundmass. Deuteric alteration
of olivine to serpentine and iddingsite is common, but on the whole, the pheno-
crysts are extremely fresh.
A representative specimen of the rocks might contain 21% feldspar, 37%
pyroxene, 33% olivine, and 9% ore. The phenocrysts often constitute about
30% of the rock, of whieh 60% may be olivine, 40% pyroxene. The rock is
therefore ankaramite or ankaramite type of picritic basalt (Johannsen vol.
III; Macdonald 1949).