Acta naturalia Islandica - 01.02.1946, Síða 37

ORIGIN OF THE BASIC TUFFS OF ICELAND
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the conglomerates is not more naturally explained by some other
agency than glaciers.
It must be borne in mind that a glaciation has a great many effects
besides striation. A sheet of conglomerate occurring in the midst of
strata of a different material, or which do not point to the presence
of glaciers at all, can hardly be accepted as a proof of a glacial condi-
tion or as indicating a general glaciation of a whole country, simply
because of the presence of occasional, striated, or what is possibly
just as correct, scratched stones.
In such a case the possibility of another explanation of the scratches
should be considered. The composition of the conglomerates I have
studied in the area considered and their relation to other strata
have suggested to me that an entirely volcanic origin is the most
natural explanation.
Eruptions of such material as the conglomerates consist of, are
not unknown. A quite similar material is known in theTertiary
volcanic series of e. g. the Faroes, Scotland and Ireland.
In these series we find, according to A. Geikie and others, beds
of volcanic conglomerates and breccias, interbedded between the
basaltic layers. Sometimes these conglomerates consist mostly of
coarse blocks, “more commonly however, the dirty-green or dark
brown granular matrix exceeds in bulk the stones embeclded in it”16)
Of still more interest, on account of their unequivocal meaning
are, however, the infillings of volcanic vents, the so-called necks.
The neck of Maclean’s Nose, with a cross section of 1000 x 300
yards, contains blocks of all sizes up to eight or ten feet in diameter.
“By far the largest” number of the blocks are “varieties of basalt,
slaggy and vesicular structures being especially conspicuous. There
are also large blocks of different porphyries and felsitic rocks”.
“All the stones are more or less rounded and are wrapped up in a
dull-green compact matrix of basalt-debris. There is no stratifica-
tion or structure of any kind in the mass.” (l.c. 106)
The largest neck mentionecl by Geikie is the one in the valley of
Strath in Skye.
There seems no reason to cloubt that the mass occupies part of
the site of a single volcanic funnel, which was ahnost two miles in dia-
meter. “This agglomerate is a coarse tumultuous assemblage of
blocks and bombs, embedcled in the usual dull clirty-green matrix.
Among the stones, scoriaceous, vesicular, and amydaloidal basalts
are specially abundant.” “Towards the end if not from the beginning