

186 STRUCTURAL AND FIELD GEOLOGY 
absorbing certain rocks, such as coal and limestone, and even 
much less readily reduced materials. 
It is obvious, therefore, that molten masses which have 
cooled and consolidated within the earth’s crust must vary 
in shape according to the form of the passages and cavities 
which have been opened for them. Consequently, from the 
point of view of tectonic geology, intrusive eruptive rocks are 
grouped under certain more or less definite structural types. 
It must not be supposed, however, that each individual 
intrusive mass necessarily belongs wholly to one or other of 
these typical structures. Not infrequently, as we shall learn, 
several different types may be represented by one and the 
same eruptive rock-mass. The more important structures 
recognised by geologists are as follows :—Batholiths ; Lacco- 
Liths and Sills or Sheets; Necks or Plugs; and Dykes and 
Veins. 
I. BATHOLITHS 
The term Batholith is applied to an intrusive mass, of 
deep-seated origin, which seems to occupy an amorphous or 
irregular shaped cavity, usually of large dimensions, often, 
indeed, measuring several miles in diameter (Fig. 63). Batho- 

Fic. 63.—DIAGRAMMATIC SECTION ACROSS A BATHOLITH. 
liths consist usually of some granitoid holocrystalline rock, 
as granite, syenite, diorite, gabbro, dolerite, etc. Occasionally, 
however, quartz-porphyry occurs in similar masses. But the 
most characteristic batholiths are unquestionably the deep- 
seated plutonic masses of granitoid holocrystalline rocks, of 
which granite itself may be taken as the type. 
The petrographical character of granite, not less than the 
phenomena presented by the rocks it traverses, are sufficient 
proof of its deep-seated origin. Granitic intrusions range in 
age from the oldest period recognised by geologists down to 



