STRUCTURE OF ERUPTIVE ROCKS 195 
a case in point—for volcanic action was manifested in that country again 
and again during Carboniferous times. It is probable, therefore, that 
_ most of the sills referred to were contemporaneous in origin with the 
lavas and tuffs of that period. Some of them may have been intruded 
before the eruptive forces had succeeded in establishing any communica- 
tion with the surface; others may well be synchronous with the full 
development of volcanic activity ; while yet others may mark the dying 
out of that action, when the eruptive energy was insufficient to pump 
lava to the surface. The out-cropping of these sills is, of course, the 
result of the general folding and denudation of the strata. But no one 
who compares the phenomena they present with those exhibited by the 
well-preserved laccoliths of North America, can doubt that the older and 
younger structures have much in common, and may well have had the 
same origin. Sills which crop out at the surface so as to form lofty 
mural escarpments have been proved in many cases to wedge out down- 
wards, and now and again their “feeders” have been recognised. In 
such cases it is not hard to reconstruct the original condition of the 
intrusion (Fig. 68). Indeed, it may be said that most of the salient 
“t-te 

Fic. 68.— DIAGRAM OF A SILL, SHOWING ITS FORMER EXTENSION AS A 
LACCOLITH. 
features of the American laccoliths are reproduced by the sills of our own 
country. The latter occur singly or in groups just as the former do. A 
laccolith may divide, as it were, into two or more wedge-shaped and 
approximately parallel sheets, and many Scottish sills behave in the same 
way. So again from laccoliths and sills alike veins and dykes are pro- 
truded into the contiguous strata. There is no evidence, however, that 
would lead us to infer that the Scottish sills affected the configuration of 
the surface, forming dome-shaped elevations in the same way as the 
laccoliths of the Henry Mountains. 
From the foregoing account of batholiths and laccoliths it 
is obvious that no hard-and-fast line can be drawn between 
the two: for many batholiths assume the laccolitic habit. 
Batholiths, however, usually occur on a much larger scale 
than laccoliths, and are, upon the whole, of more deep- 
seated origin, while now and again they seem to occur 
as bosses occupying enormous vertical pipes or funnels. 

