468 Notices of Memoirs—L. Hawkes—Rocks of Iceland. 
IlI.—Tur Acm Rocxs or Iceranp. By Lronarp Hawxes, M.Sc. 
N account was given of the preliminary results of an investigation 
of the Tertiary Acid series. It is known that these rocks are 
widely developed in Kast Iceland, but hitherto definite information 
as to their extent, nature, and mode of occurrence has been lacking. 
Whilst they have been stated to be partly intrusive and partly 
extrusive,' it has generally been accepted that they are dominantly 
intrusive in character,” a view which has probably been influenced by 
the general intrusive nature of the British Tertiary acid rocks.° 
The main exposures of acid rocks in East Iceland from Borganfjord 
to Bernfjord have been studied in the field. Evidence was brought 
forward to show that these rocks are in the main extrusive in 
character. In places the acid series is at least 2,000 feet in thickness. 
Tuffs and spherolitic liparites and obsidians are very common. The 
author holds that the old view, that the acid rocks are dominantly 
intrusive, being thus marked off from the basic rocks, is incorrect. 
Tertiary volcanic activity was similar to that which has obtained in 
Iceland in post-Glacial times, when acid rocks have been extruded . 
along with the basic, but in a smaller amount. Acid eruptions seem 
to have taken place almost continuously during the building up of 
the Tertiary plateau. The uneroded character of the liparite lava- 
streams shows how rapidly the successive basalts which submerge 
them were poured out, and this throws some light on the problem of 
the intrusive or extrusive origin of the Antrim rhyolites. 
- Since the close of the Tertiary volcanic period enormous denudation 
has obtained, and the varying resistance offered to erosive agents by 
acid and basic rocks has produced some remarkable effects. 
Thoroddsen has described some peculiar streams of acid rocks 
which he regards as post-Glacial lava-flows, formed by the extrusion 
of liparite blocks in a half-melted condition from the mountain sides. 
The most noteworthy of these occurs in the Lodmundarfjord. The 
rocks of the district are Tertiary bedded basalts with the exception 
of an acid series, contemporaneous with the basalts, revealed in 
a huge cirque excavation in aside valley. The valley is full ofa 
chaotic assemblage largely composed of spherolitic liparite reaching 
down from the cirque (Skimhottur) on the bottom of the main valley 
(Lodmundarfjord). 
The author holds that these blocks do not represent a lava-stream 
but a moraine. All the rocks of the stream occur in situ in the 
Skimhéttur mountain. The theory of morainic origin has been 
previously rejected, partly on account of the reported exclusive 
liparite composition, where a mixture of acid and basie rocks would 
have been expected. It was found, however, that the stream is not 
exclusively composed of acid types, though dominantly so. ‘The 
1 Th. Thoroddsen, ‘‘Island: Grundriss der Geographie und Geologie,”’ 
No. 152: Pet. Mitt., 1905, p. 269. 
2 Ibid., p. 232. H. Pjeturss, ‘‘Island’’: Handbuch der Regionalen 
Geologie, 1910, p. 5. OC. W. Schmidt, ‘‘ Der Liparite Islands in geologischer 
und petrographischer Beziehung’’: Zeit. Deutsch. geol. Ges., vol. xxxvii, 
p. 783, 1885. 
3 Sir A. Geikie, Ancient Volcanoes of Great Britain, vol. ii, p. 364, 1897. 
