192 STRUCTURAL AND FIELD GEOLOGY 
and thus produced a dome-like elevation at the surface. 
Proceeding from such a laccolith are more or less numerous 
intrusions—some of which have been injected along the 
bedding-planes, while others cut across the fissured strata at 
all angles. While laccoliths sometimes occur singly, they more 
usually appear in clusters—the presence of each cluster being 
indicated by a dome-shaped mountain. The number of 
individual laccoliths in a cluster is variable—sometimes there 
are no more than two, in other cases there may be a score, 
the largest number recognised in one group being thirty. 
Let us now see what light this American type of intrusive 
rock throws upon the phenomena of the sills or intrusive 
sheets which are of such common occurrence in our own country. 
Sills are eruptive masses which have usually been intruded 
along planes of stratification, and hence they tend to assume 
a more or less regularly bedded aspect. The plane along 
which intrusion has taken place is not necessarily, however, a 
plane of bedding. Some sills have followed planes of slaty 
cleavage and foliation, while others continue for longer or 
shorter distances along lines of fracture. But certainly the 
most typical examples are met with amongst stratified rocks, 
with which they have the appearance of being interbedded. 
Almost any kind of eruptive rock may assume the form of 
a sill, although the deeper-seated granitoid rocks, such as 
granite, syenite, diorite, etc., appear less frequently in sheet- 
like masses than the hypabyssal dolerites, basalts, andesites, 
etc. Perhaps the most typical examples of the true sill are 
those which occur so frequently among the Paleozoic strata of 
these islands—the sills of the Carboniferous areas being 
particularly well known. It may suffice, therefore, to give a 
short account of the latter. 
We may note, then, that a sill, although it may seem to 
be interbedded as a member of one consecutive series of 
strata, does not exactly conform to the immediately overlying 
and underlying beds (Fig. 67). Followed along the outcrop, 
it is found now and again to leave the plane upon which it 
first appeared—either rising to a slightly higher or descending 
to a slightly lower level. Or it may suddenly break across 
a considerable thickness of strata and proceed thereafter along 
a totally different horizon. Not infrequently it contains 
