METAMORPHIC ROCKS 307 
XV.—METAMORPHIC ROCKS. 
Metamorphic rocks have been described as those which 
have undergone subsequent alteration within the crust of 
the earth whereby their original characters have been 
changed. It is not proposed here to attempt any explana- 
tion of metamorphism ; for this the reader is referred to 
works on dynamical geology. 
Many rocks exhibit a peculiar fissile structure which 
causes them to split along planes—more or less oblique to 
their planes of bedding ; others appear to consist of a mass 
of thin leaves, 7.¢., show a foliated structure. Moreover, 
it sometimes happens that different kinds of foliated rocks 
are superimposed on each other in such a way as to suggest, 
if not indicate, stratification, Yet such masses are 
frequently crystalline, and in other respects differ onan 
from what are known as stratified rocks. 
Though considerable differences of opinion exist as to the 
origin of these rocks, most geologists are agreed that they 
were originally sedimentary deposits, and have been 
metamorphosed or changed to their present condition through 
the operation of mechanical movement, combined with the 
influence of heat, and heated water or vapour. 
Two kinds of metamorphism are recognised, one known 
as local metamorphism when the change has been 
effected within a limited area only round some eruptive 
mass, and the other as regional metamorphism when 
the change has taken place over a wide area. 
As examples of the former it may be mentioned that 
in certain places intrusive sheets of igneous rocks have 
baked the clays through which they pass into porcellanite 
_—a substance very like the material of a burnt brick ; also 
that other intrusive sheets forced into masses of limestone 
have converted the limestone into marble. That is, local 
