Greinar (Vísindafélag Íslendinga) - 01.01.1976, Síða 143

141
relation to Hengill than to try to understand the flow forms, cf. the
explanation below.
The denudation plain on Sleggja is covered by a thin layer of
rhyolitic pumice but at the same time the plain cuts the intrusive
rhyolite found here and described in (16). The rhyolite is the oldest
rock seen in Sleggja, and on its denuded surface rests the strati-
fied layer described above, enclosing rhyolite pebbles in the lowest
part. Finally, directly on the wom surface of the western dolerite,
at the N-corner of Húsmúli at a height of about 270 m, rests a
patch of stratified tuff which almost certainly corresponds to the
one on the top of Hengill and Sleggja. This would mean a maximal
uplift of about 500 m relative to the dolerite, as was already claimed
by the present author in 1951 (19) after a short survey. This up-
lift was late in the case of Hengill, Sleggja, Skardsmýrarfjall,
Reykjafell, Meitill and more blocks along this line, and it belongs
most reasonably to the same phase of uplifts along the Reykjanes
peninsula as was described in Section A.
Some of these uplifted blocks, such as Reykjafell, are partly
covered with stratified tuffs which are bent into quite clear flow
stmctures, cf. (19), Figs. 9-11). These, as those on Hengill, are
no doubt real and are due to the flow of still little-consolidated, hut
coherent and plastic upper members of the blocks during and
immediately after their uplift. In sharpening bends (19, Fig. 11),
the plastic flow goes gradually into crushing (The interpretation
of the top of Vífilsfell, given in (19, Fig, 4-7 is in some ways
faulty).
Finally, a little more should be said about the topmost tuff series
on Hengill. In (16 p. 55-56) it is termed “stratified hyaloclastite,
in which the stratification is due to alternation of relatively coarser
layers, richer in lapilli grains, and finer layer with sideromelane
grains or -powder. Inversely graded hedding is common, and on
the whole one gets the impression of deposition in water”.
This description is, however, insufficient. The main point is that
the material is very slightly, but unmistakably worn. This sign of
transport is further supported by occasional alien lmnps of silt or fine
tuff, around which fine material gathered into tilted layers, which
finally covered the lumps in the form of small stratified mounds.
Further, there are individual layers of 1-3 cm coarseness, which