STRUCTURE OF ERUPTIVE ROCKS 187 
Tertiary times. By far the larger number, however, date 
back to Paleozoic and Archean ages—very few, indeed, 
being referable to Mesozoic and Cainozoic horizons. From 
this we are not justified in concluding that intrusions of granite 
were of more common occurrence in the earlier than in the 
later stages of the world’s history. Granite, being of plutonic 
origin, can only appear at the surface as the result of long- 
continued and profound denudation, and its relative age is 
fixed by that of the rocks it traverses. If the surrounding 
rocks be of Palzozoic age, all we can say is that the intrusion 
is of later date than these—but how much later we cannot 
tell. For, obviously, much denudation must have taken 
place before the granite could become exposed at the 
surface. It may originally have risen to a much _ higher 
eeological horizon—all evidence of this having been 
destroyed by the complete removal of the overlying 
younger rocks, and those portions of the batholith itself by 
which these may have been penetrated. As every granite 
intrusion must in this way have traversed older rocks before it 
could reach superincumbent younger rocks, we might have 
expected to find batholiths most frequently associated with 
the former, although many may really belong to a much 
later date. 
Collateral evidence sometimes enables the geologist to fix 
the approximate age of a batholith. When, for example, the 
rock is seen traversing Carboniferous strata, while fragments 
of it are enclosed in beds of early Permian age, we may infer 
that the intrusion probably took place towards the close of 
Carboniferous times. 
Along its junction with adjacent rocks, granite is often 
finer grained than elsewhere in the same mass, as if the 
molten magma had become chilled by contact with its 
surrounding walls, and cooled too rapidly to allow the 
constituent minerals to attain fuller development (sce Plate 
XLIII. 1). Not infrequently, however, the rock is as coarsely 
crystalline along its margin as towards the centre of the 
mass. In such cases we may suppose that the surrounding 
rocks were so highly heated as to have no chilling influence. 
Although the junction between granite and the rocks invaded 
by it is usually so clearly defined that a knife-edge may be 

