Acta naturalia Islandica - 01.02.1946, Blaðsíða 42

36
TRAUSTI EINARSSON
simply the breaking away of a somewhat watersoaked cliff, perhaps
accompanied by loose ejecta that have accumulated on slopes, the
masses set in motion have an ability to move in their lower courses
for long distances over gentle grades in a manner that is difficult
to conceive but that is attested by abundant evidence. Landslides
of such origin may temporarily impound rivers, and the resulting
floods may effect a rounding of boulders and their transportation
for many miles. Even torrential rains may have a similar effect.”
(1. c. p. 237—239).
From these descriptions it is clear that sheets of conglomerates
and breccias as a secondary product of certain not very uncommon
types of volcanism may be formed under non-glacial conditions and
this fact should not be overlooked in a discussion of the volcanic
series of Iceland.
On the contrary, the reader will already have recognized that these
types of volcanism are of crucial interest in the study of the Pala-
gonite Formation as a whole.
A certain feature of eruptions of this type which does not appear
in the previous quotations is the filing effect of a nuée ardente.
This is of great interest with respect to the fact that the Icelandic
conglomerates in some cases rest on a filed floor which has hith-
erto been considered as an especially clear proof of their mo-
rainic nature.
To give here some impression of the filing effects of a volcanic blast
a discussion of the outburst of the Lassen Peak in California on
May 19, 1915 will be quoted20).
By this blast a dense forest was swept away over a large area
and a mudflow, caused by the melting of a considerable amount of
snow in the slopes of the volcano, ran for a distance of 7—8 miles.
The mud flow consisted of ash falling during the eruption and of
large and small blocks. “It was, however, not the mud flow but a
volcanic blast of terrific power (nuée ardente) and moderately high
temperature, heavily charged with dust and rock fragments, de-
livered at a low angle in an east-northeast direction down Lost Creek
Valley, which cleared the valley of its immense trees and indeed of
every movable object for more than 4 miles” (p 21). “Before May
19 the valley of Lost Creek is reported by the Forest Service to have
contained 5,000,000 feet of standing timber (original forest), much
of which reached the diameter of 3 to 5 feet. After that date the