PLUTONIC ROCKS. 
569 
Great upheaving movements have been experienced in the 
region of the Andes, during the Post-tertiary period. In 
some part, therefore, of this chain, we may expect to discover 
tertiary plutonic rocks laid open to view; and Mr. Darwin’s 
account of the Chilian Andes, to which the reader may refer, 
fully realizes this expectation: for he shows that we have 
strong ground to presume that plutonic rocks there exposed 
on a large scale are of later date than certain Secondary and 
Tertiary formations. 
But the theory adopted in this work of the subterranean 
origin of the hypogene formations would be untenable, if the 
supposed fact here alluded to, of the appearance of tertiary 
granite at the surface, was not a rare exception to the general 
rule. A considerable lapse of time must intervene between 
the formation of plutonic and metamorphic rocks in the 
nether regions and their emergence at the surface. For a 
long series of subterranean movements must occur before 
such rocks can be uplifted into the atmosphere or the ocean; 
and, before they can be rendered visible to man, some strata 
which previously covered them must have been stripped off 
by denudation. 
We know that in the Bay of Baise in 1538, in Cutch in 1819, 
and on several occasions in Peru and Chili, since the com¬ 
mencement of the present century, the permanent upheaval 
or subsidence of land has been accompanied by the simulta¬ 
neous emission of lava at one or more points in the same vol¬ 
canic region. From these and other examples it may be in¬ 
ferred that the rising or sinking of the earth’s crust, opera¬ 
tions by which sea is converted into land, and land into sea, 
are a part only of the consequences of subterranean igneous 
action. It can scarcely be doubted that this action consists, 
in a great degree, of the baking, and occasionally the lique¬ 
faction, of rocks, causing them to assume, in some cases a 
larger, in others a smaller volume than before the aj)plicg;tion 
of heat. It consists also in the generation of gases, and their 
expansion by heat, and the injection of liquid matter into 
rents formed in superincumbent rocks. The prodigious scale 
on which these subterranean causes have operated in Sicily 
since the deposition of the Newer Pliocene strata will be ap¬ 
preciated when we remember that throughout half the sur¬ 
face of that island such strata are met with, raised to the 
height of from 50 to that of 2000 and even 3000 feet above 
the level of the sea. In the same island also the older rocks 
which are contiguous to these marine tertiary strata must 
have undergone, within the same period, a similar amount of 
upheaval. 
