122 
THE TERTIARY VOLCANOES 
BOOK VIII 
geology of the British Isles. I have made use of this detailed information, 
and besides the data accumulated in my own note-books, I have availed 
myself of those of my colleagues in the Survey, for which due acknowledg- 
ment is made where they are cited. 
The Tertiary basalt-plateaux of Britain have their counterpart in the 
Faroe Islands and in Iceland, and whether or not the lava-fields stretched 
throughout North-western Europe from Antrim to the farthest headlands of 
Ultima Thule, there can hardly be any doubt that, if not continuous, these 
volcanic areas were at least geologically contemporaneous in their activity. 
Their characteristic scenery and structure are prolonged throughout the 
whole region, reappearing with all their familiar aspects alike in Faroe and 
in Iceland. I have not seen the latter island, but in the Faroe archipelago 
I have found the dykes to be sufficiently common, and to cut the basalt- 
plateaux there in the same way as they do those of the Inner Hebrides. 
On the whole, however, dykes do not play, in these northern isles, the im- 
portant part which they take in the geology and scenery of the West of 
Scotland. I have not had sufficient opportunity to ascertain whether there 
is a general direction or system among the Faroe dykes. In the fjords 
north of Thorsliaven, and again along the west side of Stromo, many of them 
show an E. and W. strike or one from E.N.E. to W.S.W. 
2. TWO TYPES OF PKOTKUSION 
The dykes are far from being equally distributed over the wide region 
within which they occur. In certain limited areas they are crowded 
together, sometimes touching each other to the almost entire exclusion of 
the rocks through which they ascend, while elsewhere they appear only 
at intervals of several miles. Viewed in a broad way, they may be 
conveniently grouped in two types, which, though no hard line can be 
drawn between them, nevertheless probably point to two more or less 
distinct phases of volcanic action and to more than one period of in- 
trusion. In the first, which for the sake of distinction we may term the 
Solitary type, there is either a single dyke separated from its nearest 
neighbours by miles of intervening and entirely dykeless ground, or a group 
of two or more running parallel to each other, but sometimes a mile or more 
apart. The rock of which they consist is, on the whole, less basic than in 
the second type ; it includes the andesitic varieties. It is to this type that 
the great dykes of the north of England and the south and centre of Scot- 
land belong. The Cleveland dyke, for example, at its eastern end has no 
known dyke near it for many miles. The coal-field of Scotland is traversed 
by five main dykes, which run in a general sense parallel to each other, with 
intervals of from half a mile to nearly five miles between them. Dykes of 
this type display most conspicuously the essential characters of the dyke- 
structure, in particular the vertical marginal walls, the parallelism of their 
sides, their great length, and their persistence in the same line. 
