Acta naturalia Islandica - 01.02.1946, Blaðsíða 60

54
ORIGIN OF THE BASIC TUFFS OF ICELAND
8. THE YOUNGER BASIC TUFFS
In the Tinclafjöll complex we observed a pile of brown tuffs and
breccias covering the main plateau of alternating lavas and layers
of fragmental material. Obviously this pile is a remnant of a more
extensive thick cover. At Hvalf jörður we similarly found that Hval-
fell and Botnssúlur represent remnants of a very thick pile of brown
fragmental material, directly covering the series of composite com-
position.
Similar masses of palagonite tuffs are of great extent in other
parts of Iceland and, in fact, form the main mass of the Palagonite
Formation. The whole mountain range from Cape Reykjanes to Lang-
jökull is mostly built up of this material. Furthermore Hofsjökull
is a similar pile resting on the composite plateau which we described
as running from Southern to Middle Northern Iceland. The same
may be said of all the separate mountains to the north of Vatna-
jökull: Dyngjufjöll, Herðubreið, Herðubreiðarfjöll, Bláfjall, Sellanda-
fjall, Gæsafjöll, Lambafjöll, and last but not least the vast pile of
Hólsfjöll covering an area of about 3500 km2. At many places it is
seen that this thick cover rests on a series of alternating lavas of
grey, mostly fine-grained and porous porphyritic basalt, and layers
of brown glassy material.
On a great many of these remnants there is, as on Botnssúlur, a
relatively thin cover of ophitic olivine-dolerite. In many cases also
the craters which produced this dolerite are still preserved, even in
a fresh condition, and in one case, on Skriða, I have found that the
last flow from such a crater consists entirely of glass, mainly sidero-
melan.
It is clear that at least a part of these fragmental masses are very
young and large parts of them are demonstrably séparated from the
composite series by a long interval, as the tuffs of Hólsfjöll and the
breccia masses of Ingólfsfjall among others which rest discordantly
on the lower series. We may therefore distinguish this thick cover
as the Younger Tuffs of relatively late origin. In Snæfellsnes a thick
pile of tuffs overlies beds with arctic molluscs as found by Pjeturss
and we may assume that the Younger Tuffs are mostly or wholly
of Quaternary age.
In spite of this their origin need not be directly connected with