Acta naturalia Islandica - 01.07.1964, Síða 31

THE ANKARAMITES OP HVAMMSMÚLI, EYJAFJÖLL, SOUTHERN ICELAND 29
On the top face of the rock these veins may sometimes be seen as honeycomb
structures: i. e. the black rock has been weathered out leaving ridges of the
intruding veins. The vein between Rocks VI and VII is of this type.
Inclusions in the rock may give some clue as to the relative ages of the
rocks. The black sub-ophitic rock of Pöst and Dysjarhóll (Type VII) contains
granular inclusions which are conceivably derived from the sparsely porphyri-
tic, hard rock of Pöst (Type VI). A light grey, fine-grained and granular in-
clusion, which might be derived from the similar rock underneath, was, further-
more, found in the black rock of Pöst. All these inclusions are fish-like in
shape and orientated parallel to each other, dipping slightly towards S or
SW. The reason for the uniform shape of the inclusions is not clear, but it
seems likely that the „raw material" was tabular jointed, such as is seen
some places in Pöst, the tabulae giving rise to the fish-shape upon resorption
or melting.
We are now in a position to try and correlate the intrusions, for it seems
clear that we are dealing with more than one magma injection. In the three
quarry outcrops there seems to be the minimum of three phases, two pene-
contemporaneous and a third later. The main mass of the quarry (Rocks I
and II) was emplaced first, and followed by the highly porphyritic rock (Rock
H3) while not yet fully solidified. Later, and showing clear contacts, is the over-
lying vesicular rock with its flow structure at the base and the slaggy top (Types
IV and V).
The three SW outcrops Kálfshamar, Pöst and Dysjarhóll, must represent
at least five phases: 1) Granular, fine-grained rock outcropping on the western-
most tips of Pöst and Kálfshamar. 2) The granular, sparsely porphyritic rock
with sigmoidal jointing (Rock VI). 3) The sub-ophitic black rock (VII) in-
truded into (2) (and (1) ?). 4) The sub-ophitic light coloured vein phase. 5) The
granular, vesicular dyke, the same as the vesicular rock above the quarry.
The overall relationship is then probably as follows, equating the similar
rock types of the various outcrops:
1) The fine-grained granular rock at the base of Pöst and Kálfshamar.
2) The sparsely porphyritic rock of Pöst and Kálfshamar.
3) The black rock of Pöst and Dysjarhóll (Type VII) and the main mass of
the quarry (I and II).
4) The vein phase. seen at the base of Dysjarhóll, as coatings and veins in
Pöst, and as the highly porphyritic rock in the quarry.
5) The granular, partly vesicular rock seen both in Pöst and in the quarry group.