Acta naturalia Islandica - 01.02.1946, Síða 39

ORIGIN OF THE BASIC TUFFS OF ICELAND
33
An interesting feature is the different clegree of roundness of the
blocks. Those derived from the younger formations of the area are
angular while the older rocks such as granite are more rounded. “Es
ist das sehr erklárlich; denn letztere hatten einen viel lángeren Weg
im Ausbruchskanale zuruckzulegen als erstere” (1. c. p. 504) .In a long
explosion pipe we may thus expect an advanced rounding of the
blocks, a conclusion which seems well worth considering in con-
nexion with the Icelandic conglomerates. To make the analogy with
the Icelanclic conglomerate even more complete the blocks of granite
sometimes show polished surfaces. The explanation of Deffner is
this: “Entweder konnten die Stucke dadurch abgeschliffen werden,
dass sie, in die Tuffmassen der Kraterausfiillung eingebettet, mit
diesen im Kraterkanal auf- uncl abstiegen und sich dabei an einem
hárteren Gestein abrieben, bis sie endlich einmal umkanteten und eine
neue Seite zum Abreiben darboten. Oder konnten sich auch die
Stucke in den Kraterwandungen festklemmen und hier durch die
vorbeipassierenden Auswurflinge in gewissen Richtungen glatt ge-
schliffen werden” (1. c. p. 506).
As a further possible analogy to our conglomerates we shall now
consider the Early Basic Breccias in Yellowstone Park, which are
thought to be of Miocene or Oligocene age. A discussion of these
masses by C. N. Fenner will be quoted here at some length19).
“In places they form deposits several thousand feet thick, some
of which are composed chiefly of fragmental material, whereas
others have many basaltic lava-flows intercalated.
On first acquaintance with these great deposits two features are
likely to attract attention. In the exposures revealed on the preci-
pitous mountain sides the strata have a nearly horizontal attitude
for mile after mile . . . and many of the beds are made up largely
of rounded boulders of great size. What was the means by which
transportation and distribution of this material were effected? To
one who has chiefly in mind the ordinary text-book illustrations of
volcanoes, showing the inclination of beds and the general structure
of such typical volcanoes as Vesuvius and Etna, and recalls de-
scriptions of the conditions of extrusion that gave rise to these
structures, these two features seem difficult to explain.
Although the structures of Vesuvius and Etna are typical of cer-
tain forms of volcanic extrusion, geologists have had impressed
upon them forcibly within recent years that other volcanoes that
Origin of the hasic tuffs of Iceland 3