CHAP. LI 
SUMMARY AND GENERAL DEDUCTIONS 
473 
from the internal magma to the surface was effected by successive explo- 
sions which finally blew out an orifice at the surface with no visible relation 
to any fissures or dykes. Of course, beneath the formations that now form 
the surface, and through which the necks rise, there may be lines ot fault 
or weakness in older rocks which we cannot see. But, in what can be 
actually examined, vents have commonly been drilled through rocks inde- 
pendently of faults. , , 
The discharge of explosive vapours was sometimes the first and on y 
effort of volcanic energy. Generally, however, fragmentary volcanic 
materials were ejected, or, if the eruption was more vigorous lava was 
poured out. In a vast number of cases, especially m the later ages ot 
Pakeozoic time, only ashes were projected, and cones of tuff were formed. 
In the earlier ages, on the other hand, there was a much larger proportion 
of lava expelled. Towards the close of a volcanic period, the vents were 
gradually choked up with the fragmentary materials that were ejected from 
and fell back into them. Occasionally, during the process of extinction, 
an explosion might still occur and clear the chimney, so as to allow ot the 
uprise of a column of molten rock which solidified there ; or the sides ot 
the crater, as well as of the cavernous funnel underneath, fell m and filled 
up the passage. Heated vapours sometimes continued to ascend through 
the debris in’ the vent, and to produce on it a marked metamorphism. 
There seems to have been commonly a contraction and subsidence o 
the materials in the vents, with a consequent dragging down or sagging ot 
the rocks immediately outside, which are thus made to plunge steeply 
towards the necks. . „ , 
When the vents were plugged up by the consolidation of fragmen ary 
matter or the uprise of lava in them, the final efforts of the volcanoes led 
to the intrusion of sills and dykes, not only into the rocks beneath the 
volcanic sheets, but also, in many instances, into at least the older parts 
of the sheets themselves. These subterranean manifestations of volcanic 
action may be recognized in almost every district. They vary greatly in 
the degree to which they are developed. Sometimes, as m the Bader 
Idris, Arenig and Snowdon regions, they attain considerable importance, 
alike as regards the number and thickness of the sheets. In ot ic i cases, 
they are exhibited on so small a scale that they might be overlooked, as m 
the tract of Carboniferous puy-eruptions in the north of Ayrs ire. ^ 
they are so generally present as to form a remarkably characteristic feat 
of the volcanic activity of each geological period from the easiest time 
the latest. The basic sheets in the Dalradian series of Scotland display 
early and colossal examples. All through the successive eruptive _ periods 
of Palaeozoic time, sills are found as accompaniments of superficial ejectio . 
The Tertiary basalt-plateaux supply numerous and gigantic examples o 
intruded sheets. Tertiary cones of Vesuvian type are not found m Bn ton but 
where on the continent they have been sufficiently laid open by denudation, 
sent sometimes n„ sstooislrmg series ot sills. As s stnkmg Jkstea. 
tion of this structure reference may be made to the sheets of trachyte that 
