VOLUME CHANGES IN METAMORPHISM 543 
ical reactions proceed accompanied by increase or decrease of 
volume. Here hydration will result in expansion and strong solvent 
action may be followed by contraction of volume. 
Metamorphism and metasomatism.—Metamorphism is_ here 
defined as the sum total of the chemical and mechanical changes 
which take place in solid rocks, below the zone of oxidation. The 
agents are heat, pressure, and chemical energy. Metamorphism 
by pressure is probably always accompanied by some chemical 
change. Chemical energy works by means of solutions, gaseous 
or liquid, which penetrate the rocks on capillary and supercapillary 
openings. 
Metasomatism is here defined broadly as any change in com- 
position of a mineral when exposed to conditions under which it is 
unstable. Solutions, gaseous or liquid, effect the change. A more 
restricted definition is that metasomatism comprises any change 
in composition of a mineral in a solid rock induced by a change in 
the physical conditions and resulting in its space being occupied by 
another mineral stable under the prevailing conditions. 
Metamorphism is in most cases accompanied by metasomatism 
of individual minerals and usually also by metasomatism of the 
rock as a whole, though the latter change may proceed very slowly. 
If the supply of new material is rapid the composition of the rock 
may be greatly changed within a short time. Replacement is used 
as equivalent to metasomatism. 
_ Thesis of this paper.—In a previous paper," principally devoted 
to metasomatism in mineral deposits, I have advanced the view 
that replacement normally occurs without change of volume of 
individual minerals or rocks. I would now like to broaden this 
theory by the thesis that metamorphism by replacement does not 
normally involve changes of volume. 
Replacement.—Replacement in solid rocks consists in solution 
of the host mineral, followed immediately by deposition of an equal 
volume of the guest mineral. This is an empirical observation 
based on the microscopic examination of rocks. In other words 
the volume of the replacing mineral equals the volume of the mineral 
replaced. Deposition follows so closely upon solution that at no 
1“ The Nature of Replacement,” Econ. Geology, VII (1912), 521-35. 
