394: L. Hawkes—WN. Atlantic Tertiary Voleanre Plateaw. 
rare long periods of quiescence, but there is no evidence that that 
would be of a different order of magnitude from that required for the 
rest of the series, and reckoning this interval as as much again we 
obtain a figure of one million years for the building up of the whole 
volcanic formation. 
What impresses the student of Icelandic geology is not so much 
the great time which elapsed during the accumulation of the Tertiary 
series, but rather the long period necessary to allow of the enormous 
denudation which has obtained. Sir A. Geikie has emphasized this 
latter point with reference to the British and Ferode Islands: ‘‘ Among 
the more impressive lessons which the basalt-plateaux of North- 
Western Europe teach the geologist, the enormous erosion of the 
surface of this part of the continental area since older Tertiary time 
takes a foremost place” (1, p. 455), and from observations on Mull 
this interval is estimated at twelve million years. For the present 
purpose this may be taken as a general figure for the whole North 
Atlantic plateau, although the application to Iceland of the methods 
by which it was obtained would show a greater interval. 
Whilst in absolute amount we can have little confidence in the 
figure of one million years given for the building up of the plateau, 
and that of twelve million years estimated for its degradation since 
the Tertiary volcanoes became extinct, the relative proportion shown 
by these figures would seem to be correct in kind, if not in degree. 
When we investigate the Tertiary series of Iceland with respect to 
the conditions under which it was built up and see how it has been 
tilted, faulted, and denuded, one fact is clear, and that is, the relative 
smallness of the time interval which was consumed in building 
compared with that which has elapsed subsequently—this conclusion 
doubtless applies to the whole of the North Atlantic plateau. It 
must be remembered, however, that the dislocation and denudation 
of the plateau began long before the close of Tertiary times—indeed, 
the major part was completed before the advent of the Glacial Period. 
One of the hindrances in the way of a better understanding of 
the old Icelandic series is the lack of ‘horizons’. It is certainly 
remarkable that definite horizons should be wanting in such an 
enormous thickness of rock. The Tertiary volcanic formation is 
probably unique in this respect amongst all the post-Archzan rocks 
of the Atlantic countries. The explanation is to be found in the 
rapidity of building. Per unit of thickness the Tertiary volcanic 
series represents the smallest time of accumulation of all the post- 
Archean formations. 
In conclusion, I wish to express my great indebtedness to Professor 
G. A. Lebour for assistance and encouragement in the carrying out 
of this investigation, and also to Professor V. M. Goldschmidt, of 
Christiania, for guidance in the petrographic work. 
REFERENCES. 
1. Sir ARCHIBALD GEIKIE, The Ancient Volcanoes of Great Britain, vol. ii, 
1897. 
2. The Interbasaltic Rocks of North-Hast Ireland (Memoirs of the Geological 
Survey of Ireland), 1912. 
8. TH. THORODDSEN, ‘‘Island. Grundriss der Geographie und Geologie ”’ : 
Erginzungsheft No. 152 zu Petermanns Mitteilungen, 1905. 
