544 WALDEMAR LINDGREN 
time can any open spaces be discerned under the microscope. It 
is evident, however, that capillary spaces are necessary for the 
movement of the fluid phases which in most cases are aqueous solu- 
tions. In fact, replacement usually begins from such capillary 
cracks or from a series of minute fluid inclusions along such a 
fissure. Replacement is primarily caused by solution or decomposi- 
tion of the host mineral in solute films separating grains and filling 
capillary fissures. The solution of the host mineral produces 
supersaturation in the liquid, and an amount of secondary material 
is precipitated equal to the volume of the host mineral removed. 
Replacement proceeds particle by particle but not ‘‘molecule 
for molecule.” In other words there is no simple molecular 
equivalence between the material dissolved and the material 
precipitated. Replacement proceeds independently of specific 
gravity and molecular volume. Chemical reactions may take 
place in the contact film, but the sum total of the change is not 
expressible by the chemical formulas usually supposed to represent 
the process. 
In some cases there is simply solution and deposition without 
chemical reaction, as when pyrite replaces calcite or limestone or 
shale, but whether or not the exchange is accompanied by chemical 
reaction does not influence the law of equal volumes. Replace- 
ment is dependent upon the velocity of the solution movement and 
naturally also upon composition, pressure, and temperature of the 
replacing solution. If the speed and chemical activity of the solu- 
tions be great the empirical volume law may fail to hold, deposition 
will lag behind solution, and a drusy texture will result. This 
happens occasionally in some metasomatic processes in the forma- 
tion of mineral deposits, but in metamorphism of rocks the solutions 
move slowly and only a small amount of new material is introduced, 
so that the resulting textures are always compact. Even in the 
case of drusy texture the bulk volume remains the same and sub- 
sequent rock pressure fails to close the cavities, even under the 
weight of a rock column of 10,000 t6 15,000 feet. Ultimately, at 
extreme depths these cavities would most probably be closed with 
permanent reduction of volume if they have not previously been 
filled by cementing solutions. 
