Náttúrufræðingurinn - 1953, Blaðsíða 26

168
NÁTTÚRUFRÆÐINGURINN
Nr. 4. Móhellan, lagskiptur laus sandsteinn. Þunnsneið: Kornastærð um 0,2 mm,
kornin jöfn á alla kanta. Ótvírætt meira af svörtu gleri en í Hverfjalli.
Krystallabrot þó af svipaðri stærð og magni og i Hverfjalli. Glerið sjálft þó
mun minna krystallað en þar.
Nr. 5. Móhellan. 1 þessu sýnishorni eru linsulaga finmalarlög, kornastærð ^ 1 cm.
Fín leirhúð lykur um völurnar, mikil harka. Þetta virðist ótvírætt hafa
myndazt í vatni.
Þess má geta, að móhellan er viða miklu harðari en Hverfjallsmóbergið. Harkan
hendir til vatnsmyndunar og járnbindingar.
Remarks on S. Thorarinsson’s study of the origin of Hverf jall.
SUMMARY :
In the journal Náttúrufræðingurinn, 1948, p. 121, the present author concluded
that the tuff ring Hverfjall was formed by the extrusion of a highly viscous lava
in a faint eruption. Granulation of this stiff magma during extrusion created an ash-
porridge which was lifted temporarily by steam and formed the tuff ring. The
crater walls carry a cover of moraine.
In the same journal 1951, p. 169, S. Thorarinsson rejects my results. In his view
„it is quite obvious that this very regularly stratified tuff ring is the result of an
explosive eruption and cannot at all have flown as a thick porridge". No special
reason for this statement is, however, given. Thorarinsson bases his conclusions main-
ly on the existence of a 2—6 m thick layer of tuff covering a restricted area north
of Hverfjall. „No doubt this tuff was deposited by the eruption which built up
Hverfjall11, he states. The age of this tuff layer (and the eruption) is found to be
about 2500 years. Thorarinsson’s study of the structure of this tuff is, however,
so superficial that it has escaped his notice that the tuff is a layer of windblown
sand deposited in a former flat basin, partly occupied by a lake, and normally wet.
During a short survey of the tuff the present author found the following charac-
teristics:
1. Cross bedding (Fig. 1). 2. Complete penetration of the sand into every free
space in the rough surface of the underlying lava. 3. Sand grains are equidimen-
sional as a main rule, and well rounded grains are easily found. The Hverfjall tuff,
on the other hand, consists of unwora angular grains. 4. Scoriae and lava from
a crater near Bjarnarflag is under- and overlain by the tuff (Fig. 3). 5. Deposition
of undisturbed tuff over disturbed tuff (Fig. 5). 6. Tree trunks, twigs and leaves
were enclosed in the tuff at different levels. Fig. 4 shows how a trunk 15 cm in
diam. and 1,5 m long was first halfburied in sand, then disappeared (perhaps it
was lifted by water) and left a groove into which sand was deposited. 7. The
lowest layer of the tuff is of pisolitic structure. It has the same thickness, 5—6 cm,
everywhere where observed by the author. The „peas“ of this layer are found to
consist of hard clay enveloping sand grains; diameter of grain about Yv. that of
the whole pea. These peas are clearly concretions formed by the binding action
of iron in a former lake. Diatoms are found in the pisolitic layer as well as plant
stems at different levels. Some plant stems are of completely circular cross section
which proves in a simple way how extremely slowly the overlying tuff was de-
posited. 8. With the exception of that part of Námafjall where the intensive ther-