Tertiary Volcanic Plateau. 393 
could twelve basalt surfaces be weathered down whilst the acid rock 
remained unattacked ?? 
Duration oF THE Tertiary Votcanic PERIOD. 
The investigation of the red partings themselves, the analogous 
deposits of the modern lava deserts, and the Seydisfjord section, all 
go to show that no great time elapsed between the outpourings of the 
majority of the basalts. Is it possible to form any idea of the absolute 
average period of rest ? 
We have the evidence of the voleanic activity in Iceland in historic 
times, but the record only stretches over a thousand years, and much 
of it isuncertain in character. Also the relative intensity of vulcanism 
in Tertiary times and now must be taken into account. When we 
conceive of the magnitude of the Tertiary outpourings which probably 
obtained right across the North Atlantic, forming a land-bridge 
connecting the Old and New Worlds, and then consider the modern 
outflows covering what is relatively a very small area, we appreciate 
the great diminution of activity. It is quite reasonable to suppose 
that vuleanism so stupendous should, at any particular place, have 
been manifested more frequently than in the restricted area which 
has witnessed volcanic activity in historic times. At the last fissure 
eruption in the Myvatnsorcef{, in 1875, lava was poured out five times 
from various parts of the fissure (10, p. 139). In August of that year 
Watts observed the flooding of the stream of the preceding April 
(6, pp. 154-5). 
Taking all the evidence into consideration, it seems that a thousand 
years would not be an improbable estimate for the average duration 
of the inter-eruptive periods represented by the red partings. That 
this is a liberal allowance will probably be conceded on consideration 
of what it means in the case of the Seydisfjord section described 
above, i.e. that in 12,000 years the acid flow suffered no marked 
amount of denudation. Leaving out of count the longer rest intervals, 
with the known thickness of the Tertiary formation 11,000 feet, and 
taking 26 feet as an average basalt thickness, we arrive at a period of 
approximately half a million years for the building up of the series. 
It is impossible to say what interval of time is represented by the 
1 These observations are of interest with respect to former discussions on the 
difficult problem of the age of the Antrim rhyolites. Considering the question 
of their intrusive or extrusive origin Sir A. Geikie writes: ‘‘ The rhyolite has 
been supposed to form the summit of an ancient volcanic dome, perhaps of 
Eocene age, which had been worn down before the outflow of the plateau- 
basalts under which it was eventually entombed. Had this been the true 
history of the locality it is inconceivable that of a rock which decays so rapidly 
as this rhyolite, and strews its slopes with such abundance of detritus, not 
a single fragment should occur between the successive beds of basalt which are 
supposed to have surrounded and buried it. . . . Yet it is clear from the upper 
surfaces of some of these lavas that a considerable interval of time separated 
their successive overflows, so that there was opportunity enough for the 
scattering of rhyolite-debris had any hill of that rock existed in the vicinity ’’ 
(1, p. 428). The relationship of the basalts to the rhyolites in Antrim is very 
obscure, but in the Seydisfjord section the extrusive nature of the acid rock is 
quite clear, and arguing from its undenuded surface we conclude that relatively 
no great interval of time elapsed during the outpouring of the basalts. 
