324 
THE TERTIARY VOLCANOES 
BOOK VIII 
detecting a single example of them, but if the escarpment is cut down to the 
base we seldom need to search far to find them. 
That the efforts of the internal magma to establish an outlet towards 
the surface were accompanied by powerful disturbances of the terrestrial 
crust is shown by the abundant dykes which 
traverse all the volcanic districts from Antrim 
to Iceland, and some of which ascend even to 
the very highest remaining lavas of the basalt- 
plateaux. The parallel fissures filled by these 
dykes prove that even after the accumulation of 
more than 3000 feet of basalt-sheets, the move- 
nients continued to be so powerful as to disrupt 
js these vast piles of volcanic material. But un- 
~ doubtedly the highest parts of the plateau-basalts 
1 are less cut by dykes than the lower parts. 
1 There would no doubt come a time when the dis- 
■g locations would more seldom reach the surface, 
y when dykes would not be formed so abundantly 
g or up to such a high level, and when the volcanic 
•3 energies would more and more sparingly result in 
1 the opening of new vents or in the discharge of 
“ fresh eruptions from old ones. 
- It appears to me most probable that the in- 
,| jection of the sills was connected with the same 
| terrestrial disturbances that produced the dykes 
~ which traverse the plateaux. Besides being 
* dislocated by parallel fissures, the earth’s crust 
| in North-Western Europe seems to have been 
J ruptured internally along lines more or less at 
o right angles to the vertical fissures. The deep 
.§ accumulation of bedded basalts presented an 
| increasing obstacle to the ascent of the magma to 
fi the surface. Unable to gain ample enough egress 
g through such vertical fissures as might be formed 
^ in the volcanic pile, the molten rock would find 
its lines of least resistance along the planes of 
the strata and the lower basalt-beds, either by 
the aid of terrestrial ruptures there, or in virtue 
of its own energy. On these horizons, accordingly, 
the sills occur in extraordinary profusion through- 
out the volcanic regions. They are no doubt 
of all ages in the progress of the building up of 
the volcanic plateaux, but I am disposed to believe that a large number of 
them may belong to the very latest period of the uprise of basalt within the 
area of Britain. 
One of the most suggestive features of the abundant Tertiary sills lies in 
