Náttúrufræðingurinn - 1952, Blaðsíða 45

MEIRA UM RAUÐHÓL
89
interrupta, Hyalodiscus scoticus, Mastogloia pumila, Melosira Juergensii, M. numifau-
loides, Pleurosigma sp. og Synedra pulchella.
Der er forctaget Tælling paa et Præparat, og paa Grundlag heraf kan fplgendc
Halobiespektrum opstilles:
Antal Arter % af talte Skaller
halophobe ............................... 4 0.2
indifferente ........................... 73 96.6
halophile ............................... 6 1.2
mesohalobe.............................. 15 1.4
euhalobe ................................ 3 0.2
? 4 0.4
Aflejringen af forelagte Prpve maa f0lgelig antages at have fundet Sted i lavvandet
(mange Epifyter) S0 med roligt Vand, der utvivlsomt har været kalkholdigt og svagt
alkalisk, sandsynligvis eutrof. Om Alderen kan intet siges paa Grundlag af den fore-
tagne Analyse.
August 1949.
Niels Foged
Odense
SUMMARY
In the field of postglacial lavas south of Hafnarfjörður near Reykjavík a little cone-
shaped hill consisting mostly of red volcanic scoria and named Rauðhóll has been
quarried for road material. In a previous article, Náttúrufræðingurinn XIX, p. 9—19,
I discussed the (ptestion whether this hill be a genuine volcano or a secondary crater
only, i. e., in the latter case, formed by steam explosions from the swampy land surface,
when this was overflowed by the lava. No decisive conclusion was arrived at. —
Now a further excavation has revealed a layer of soil and some charred stems of a
heather vegetation lying above the scoria and under the lava. As the formation of such
a soily layer takes at least many years this excludes the possibility of the scoria hill
and the lavaflow being formed simultaneously. Hence Rauðhóll must be a genuine
volcano, older than the surrounding lava.
Furthermore the continued quarrying has provided a section into the sedimentary
beds upon which tlie volcanic material had been piled. As shown by Fig. II. and by
the idealised section in Fig. III. these beds prove the occurrence of two transgressions
with an intervening regression in late- and post-glacial times. — The highest reach of
the first transgression is marked by a shore-line at 32 metres a. s. 1. on a near-by liill-
side. — During the following regression fresh-water sediments, mostly diatomaceous
earth, were formed at only 10 metres a. s. 1. — In the second transgrcssion these beds
became covered with sand, which contains a rich fauna of marine shclls indicating a
climate at least as mild as the present one. The reach of this transgression lies between
15 and 32 inetres a. s. 1. — At the time of the eruption that gave birth to Rauðhóll
the sea had again retreated below the 10 metres height curve, and at the outpouring
of the lava (Hvaleyrarhraun) it had sunk to or below the present sea level, which it
has never passed since.