CHAP. VI 
DYKES AND VEINS 
8i 
or tortuously irregular walls of intruded lava. Occasionally a radiate 
arrangement may be observed in sucli eases, like that noticeable at some 
modern volcanoes, the dykes diverging from the eruptive centre. 
Many dykes exist regarding w-hich there is no evidence to connect 
them with any actual volcanic rocks. They have been injected into fissures, 
but whether this took place during volcanic paroxysms, or owing to some 
subterranean movements which never culminated in any eruption, cannot be 
decided. 
The cprestion of the age of dykes, like that of intrusive masses of all 
kinds, is often difficult or impossible to decide. A dyke must of course be 
younger than the rocks which it traverses, and a limit to its antiquity is thus 
easily li.xed. But we cannot always affirm that because a dyke stops short 
of a particular rock, or series of rocks, it is older than these. The Hett 
Dyke, in the north of England, rises through the Coal-measures, but stops 
at the Magnesian Limestone ; yet this cessation does not necessarily imply 
that the dyke wms in place hefore the deposition of tliat limestone. The 
structure may have arisen from the dylce-fissure having ended at the bottom 
of the limestone. Where dykes rise up to the base of an uuconformable 
formation without in any single case entering it, and where fragments of 
them are enclosed in that formation, they must be of higher antiquity, and 
must have been laid bare by extensive denudation before the uuconformable 
strata were deposited upon them. The great system of dykes in the Lewisian 
Griieiss of the north-west of Scotland is in this way proved to be much more 
ancient than the Torridon Sandstones under which it passes (Figs. 35, 30). 
Where two dykes cross each other, it is sometimes not difficult to decide 
irpon their relative antiquity. In intrusive rocks, the finest-grained parts 
are those which lie nearest the outer margin, where the molten material was 
rapidly chilled by coming in contact with cool surfaces of rock. Such 
“ chilled margins ” of closer grain are common characteristics of dykes. 
Wherever a dyke carries its chilled margin across another dyke, it must be 
the younger of the two, and wherever such a margin is interrupted by 
another dyke, it must belong to the older. 
As a rule, the uprise of molten material in a fissure has so effectually 
sealed it up that in the subsequent disturbances of the terrestrial crust the 
fissure has not been reopened, though others inay have been produced near 
it, or across it. Sometimes, however, the enormous tension to which the 
crust was exposed opened the fissui-e once more, sometimes even splitting 
a dyke along its centre, and a new ascent of molten rock took place within 
the rent. Hence double or treble or compound dykes have been produced. 
The second or later infillings are generally somewhat difierent from the 
original dyke. Occasionally, indeed, they present a strong contrast to it. 
Thus, among the dykes of Skye examples occur where the centre is occupied 
by an acid granophyre, while the sides are occupied by dykes of basalt. 
Instances of this compound type of dyke will be given in the account of 
the Tertiary volcanic rocks of Britain. 
It is obvious that in a wide fissure the central portion may remain 
VOL. I O 
