14 
THE TERTIARY VOLCANOES 
BOOK VIII 
tions, as the earlier geologists had supposed, hut were rather the result of 
the subairial outpouring of lava at successive intervals, during which terres- 
trial vegetation sprang up upon the older outflows. 
While Forbes brought forward paheontological proofs of the Tertiary 
age of the volcanic rocks of the south-west of Mull, he at the same time 
laid before the Geological Society a paper on the Estuary Beds and the 
Oxford Clay of Loch Staffin, in Skye, wherein, while admitting the 
existence of appearances which might be regarded as favourable to the 
view that the intercalated basalts of that region were of much later date 
than the Oolitic strata between which they might have been intrusively 
injected, he stated his own belief that they were really contemporaneous 
with the associated stratified rocks, and thus marked an outbreak of volcanic 
energy at the close of the Middle Oolitic period. 1 The Duke of Argyll, in 
the paper which he on the same occasion communicated to the Geological 
Society, adopted this view of the probable age of most of the basalts of the 
Western Islands. He looked upon the Tertiary volcanic rocks of Mull as 
occupying a restricted area, the great mass of the basalt of that island, like 
that of Skye, being regarded by him as probably not later than some part 
of the Secondary period. 
It must be granted that the appearances of contemporaneous inter- 
calation of the basalt among the Secondary strata are singularly deceptive. 
When, several years after the announcement of the Tertiary age of the 
basalts of Ardtun, I began my geological work in the Inner Hebrides, I was 
led to the same conclusion as Edward Forbes, and expressed it in an early 
paper. 2 All over the north of Skye I traced what appeared to be evidence 
of the contemporaneous interstratification of basalts with the J urassic rocks 
and I concluded (though with some reservation) that the whole of the vast 
basaltic plateaux of that island were not younger than some late part of the 
Jurassic period. In that same paper the attention of geologists was called 
to the probable connection of the great system of east-and-west dykes 
traversing Scotland and the North of England, with the basalt-plateaux of 
the Inner Hebrides, and as I believed the latter to be probably of the age 
of the Oolitic rocks, I assigned the dykes to the same period in geological 
history. But subsequent explorations enabled me to correct the mistake into 
which, with other geologists, I had fallen regarding the age of the volcanic 
phenomena of the Western Islands. In 1867 I showed that instead of 
being confined to a mere corner of Mull, the Tertiary basalts, with younger 
associated trachytic or granitic rocks, covered nearly the whole of that 
island, and that in all likelihood the long chain of basaltic masses, extend- 
ing from the North of Ireland along the west coast of Scotland to the Faroe 
Islands, and beyond these to Iceland, was all erupted during the Tertiary 
period. At the same time I drew special attention to the system of east- 
and-west dykes as proofs of the vigour of volcanic action at that period, and 
1 Quart. Journ. Geol. Soc. vol. vii. (1851), p. 104. 
2 “On the Chronology of the Trap-rocks of Scotland,” Trans. Roy. Soc. Edin. xxii. (1861), 
p. 649. 
