Acta naturalia Islandica - 01.02.1946, Blaðsíða 63

ORIGIN OF THE BASIC TUFFS OF ICELAND
57
sideromelan. But the comparison with (71) makes it very likely.
It is probable, therefore, that sideromelan was found among the
oldest rocks of the country.
I have described the younger tuffs of Hvalfell and Botnssúlur as
remnants of a wide cover. In the same way plateaus as Gagnheiði,
Ármannsfell, Laugarvatnsfjall, Miðdalsfjall and the base of the ice-
cap Langjökull are clearly parts of one or more larger plateaus.
The same is true of the separate plateau mountains of NE-Iceland.
It may be tempting prima facie to assume that there was
originally a continuous cover of tuffs over large parts of the country.
Yet, such a plateau cannot at present be reconstructed with any
certainty and the possibility remains that there were originally
several separate plateaus of tuffs. In that connexion the youngest
group of tuffs and breccias is of interest.
This group occurs as very young heaps of fragmental material
which have been piled up on volcanic vents or as narrow ridges built
up along volcanic lines. Of the former type are the tuff heaps in the
valley Hítardalur in Snæfellsnes, of the latter type are Jarlhettur,
and Fagradalsfjall, at Langjökull. Along the same volcanic line occur
the more isolated heaps Kálfstindur, Högnhöfði and Rauðafell. Of
the same nature are Klukkutindar, and probably also Kálfstindar,
and many smaller heaps along the western main fracture line of
Hengill,such as Reykjafell. A small flow of such material some 3 km.
south of Kolviðarhóll is of post-glacial age. The central part of Vífils-
fell is a large body of fragmental material thrust up in a half-fluid
state. Eldey and most of the Westman Isles are also of this origin.
In the examples mentioned the internal flow structure and often
the original smooth streamlined surface of the heaps are seen and
it is clear that these masses were thrust up to the surface as a very
viscous body of fragmental material.
Sometimes the heaps have very steep and even overhanging sides
and it seems difficult to account for this except by the aid of an
ice-wall. The elevated structures, on the other hand, seem hardly
compatible with subglacial conditions.In the tuffs heaps near the
farm Hítardalur I found glacially striated stones which are clearly
foreign to the tuffs, and the tuffs rest on a perfectly fresh glacially
striated floor. These facts indicate that the tuffs in this case were
extruded beneath an ice-sheet.