METAMORPHISM 3TT 
the action of water, may cause an enlargement of the 
area occupied by any given mass of rock; and from 
this source also, pressure will be applied to the strata 
on all sides. The folding of mountains is a potent 
cause of pressure, — perhaps the most important of all. 
Given pressure and heat, water becomes a powerful 
Fig. 228. 
A crumpled gneiss, showing presence of pressure during formation. 
agent of change. With a temperature of hundreds of 
degrees, it takes many substances into solution, and 
working with these, causes changes of the utmost 
consequence. It has not been possible to imitate these 
variations of rock condition by actual experiment; but 
the evidence of the operation of these agents in the 
crust of the earth is thoroughly convincing. 
