190 STRUCTURAL AND FIELD GEOLOGY 
In such cases the granite may either be of the nature ofa 
laccolith, or thick sill or sheet, following an irregular course 
through the rocks among which it has been intruded (Fig. 64), 

or the central mass may be a true boss from which sheets 
extend outwards at different levels and in different directions 
(Fig. 65). 
The rocks for some distance around a mass of granite 

are usually more or less highly metamorphosed, and traversed 
by numerous dykes and veins (Plate XLIII. 2) proceeding 
from the eruptive rock, as will be more fully explained in the 
sequel. 
Rocks of a more basic character than granite, such as 
diorite, syenite, gabbro, dolerite, etc., not infrequently occur in 
the form of great batholiths—usually on a smaller scale, 
however, than the more typical granitic masses, Like the 
latter, they seem sometimes to occupy the place of rocks 
which may either have been absorbed or pushed up and 
blown out. In other cases they are of the nature of gigantic 
laccoliths. Mr Harker has described some in the island of 
Skye which attain a thickness of 3000 feet. Not a few basic 
batholiths are apparently of less deep-seated origin than 
sranite, and although many may never have communicated 
with the surface, yet there are good grounds for believing 
that some of them, at least, are the roots or cores of old 
volcanoes, the effusive products of which are grouped 
immediately around them. As examples of the kind, we 
may cite the bosses of diabase, etc., which appear in the 
