Acta naturalia Islandica - 01.02.1946, Blaðsíða 64

58
TRAUSTI EINARSSON
In the same way as the youngest tuffs are produced by large bodies
of fragmental material directly erupted the somewhat older “plateau
tuffs” are at least partly built up of immense floods of such material.
There may then have been several separate plateaus of this kind.
9r SUMMARY AND GENERÁL CONCLUSIONS
For a full understanding of the Palagonite Formation the general
geological structure of Iceland is of importance. The picture here
given is partly gained through the here described studies, partly
as far as new conceptions are concerned, it had to be based on stu-
dies which are intended to form the subject of a separate paper. This
picture is as follows:
The fundament of Iceland is an extensive thick plateau which has
suffered considerable disturbances, its surface lying at different
levels in the various parts of the country. East of Bárðardalur as
far as Vopnafjörður it has an average height of 200—400 m. At
Bárðardalur there is a major dislocation, as first pointed out by
Thoroddsen, yet it is only marked in the north. West of Bárðardalur
as far as Skagafjörður the plateau lies much higher, 900—1500 m.
Yet in the southern part this area, it dips gently to the south and
runs uninterruptedly to Southern Iceland, where it terminates ab-
ruptly in Hreppar.
In Western Iceland the plateau has a height of 1100-—1200 m in
Skarðsheiði, declines gently towards the east, reaching a medium
height at the head of Hvalfjörður.
The higher parts of this plateau are built up of basaltic lavas,
almost exclusively a grey, fine-grained porphyritic rather porous
basalt, and layers of brown glassy tuffs and breccias, and an occasion-
al layer of grey conglomerate. As a further feature of the lavas must
be added that their vesicles are very often filled with a loose mass
of sideromelan fragments.
Near Akureyri, Tertiary lignite and petrified trees are embed-
ded in these higher parts of the plateau series. The lignite at Illuga-
staðir in Fnjóskadalur and at Árbær in Skagafjörður also occur in
these higher reaches and most probably a number of other known
localities for lignite are situated in the higher parts of the plateau.
Glassy tuffs and breccias with sideromelan as the main constitu-