Rit (Vísindafélag Íslendinga) - 01.07.1962, Page 106

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raine-like conglomerate at this level in Vörðufell, on which
is the level of the Vörðufell lignite.
In the sections I to V lavas of reverse polarity occur above
mx; they are considered a separate group, r2. Then comes a
thin normal group n1? covered by the moraine m2, and final-
ly a thick mass of tuff-breccias, n2, which for convenience
is kept separated from nx.
At a first glance very little structure is seen in the more
than 200 m thick tuff-breccia mantle n2 of Ingólfsfjall. Yet,
on the east slope near the farm Alviðra clear structure emer-
ges, Fig. 55, and from this place it can he traced in much of
the cap.
We see here a succession of breccia layers dipping 20—30°
south. In front of a steep hreccia slope has heen deposited
a succession of horizontal lavas. Farther north is unstratified
breccia in front of which the stratified hreccia has heen de-
posited. The whole mass is cut by a number of vertical dykes,
also of normal polarity, and these, like the dipping breccia
layers, are cut off at a largely horizontal surface.
The breccia is of the bomb fragments type. The local hori-
zontal lavas seem to follow immediately after deposition of
the breccia, there is no sign of any erosion or sedimentation
in hetween. Both being of the same fine-porphyritic hasalt
type they must belong to the same volcanic phase although
not necessarily from the same volcano; the lavas have flowed
on the relatively low ground at the foot of the breccia slope.
The lavas have piled up rapidly, only thin tuff layers of
primary glass fragment being found in between. Vertical steam
pipes are common in the lavas. The lava series has been cut
off by a fault on the south side, hut no trace of the displace-
ment is left on the surface.
Tracing the structure south and west along the slope, the
south-dipping breccia layers are seen at several places. In the
southern part of the mountain there is a general cover of
horizontal lavas, reaching partly a thickness of 100 m and
very likely this lava cap covered originally most of the moun-