176 
THE TERTIARY VOLCANOES 
BOOK VIII 
the following remarks : — “ Scotland is renowned for the number of its basaltic 
veins, which gave Hutton his ideas regarding the injection of lava from 
below ; but, as the greatest genius is not infallible, and as volcanic countries 
present us with examples of such veins arising evidently from accidental 
fissures that were filled up by currents of lava which moved over them, and 
as the Scottish instances are of the same kind, we regard it as infinitely 
probable that all these veins have been formed in the same way notwith- 
standing the enormous denudation which this supposition involves ; and 
that only rarely do cases occur where they have been filled laterally or in 
some other irregular manner.” 1 I need not say that this view, which, 
except among Wernerians, had never many supporters, has long ago been 
abandoned and forgotten. There is no further cpiestion that the molten 
material came from below. 
1. In discussing the history of the dykes, we are first confronted with 
the problem of the formation of the fissures up which the molten material 
rose. From what lias been said above regarding the usual want of relation 
between dykes and the nature and arrangements of the rocks which they 
traverse, it is, I think, manifest that the fissures could not have been caused 
by any superficial action, such as that which produces cracks of the ground 
during earthquake-shocks. The fact that they traverse rocks of the most 
extreme diversities of elasticity, structure, and resistance, and yet maintain 
the same persistent trend through them all, shows that they originated far 
below the limits to which the known rocks of the surface descend. We 
have seen that in the case of the Cleveland dyke, the fissure can be proved 
to be at least some three miles deep. But the seat of the origin of the rents 
no doubt lay much deeper down within the earth’s crust. 
It is also evident that the cause which gave rise to these abundant 
fissures must have been quite distinct from the movements that produced 
the prevalent strike and the main faults of this country. From early 
geological time, as is well known, the movements of the earth s crust 
beneath the area of Britain, have been directed in such a manner as to give 
the different stratified formations a general north-east and south-west 
strike, and to dislocate them by great faults with the same average trend. 
But the fissures of the Tertiary dykes run obliquely and even at a right 
angle across this prevalent older series of lines and are distinct from any 
other architectonic feature in the geology of the country. They did not 
arise therefore by a. mere renewal of some previous order of disturbances, 
but were brought about by a new set of movements to which it is difficult 
to find any parallel in the earlier records of the region. 2 
We have further to remember that the fissures were not produced 
merely by one great disturbance. The evidence of the dykes proves beyond 
question that some of them are earlier than others, and hence that the cause 
to which the fissures owed their origin came into operation repeatedly during 
1 Essai Gcoloyiquc sur I’Ecosse, p. 272. 
2 The only other known example of such a dyke-structure in Britain is that of the Pre- 
Cambrian series of dykes in the Lewisian gneiss of Sutherland, described in Chapter viii. 
