- VOLUME CHANGES IN METAMORPHISM — 549 
The serpentinization of magnesian rocks is often cited as a clear 
case of expansion by replacement. Recalling the olivine crystals 
referred to above, I believe it improbable that such an expansion 
has taken place. Large serpentine masses do not usually show the 
smooth slickensides supposed to be caused by this movement but 
are rather compact and solid, the slickensided fragments being 
confined to crushed zones or near the surface where yielding was 
possible. It seems much more probable that the replacement has 
been effected with extensive removal of magnesia, and this is 
supported by the prevalence of ascending magnesian waters where 
serpentines abound as in California. 
Expansion by hydration of anhydrite I regard in the same light. 
If it takes place it is under exceptional conditions of light load. 
Many anhydrites altering to gypsum show no evidence of expan- 
sion, the veins of the latter mineral cross-cutting the crystalline 
structures without disturbance. Some gypsum evidently goes into 
solution. 
Calculation of volume changes.—In view of what has been said 
above it must be concluded that the application of Lepsius’ volume 
law" (Mot. vol. =n eet) to calculate volume changes in 
metamorphic rocks formed under uniform pressure is unwarranted 
and leads to totally erroneous conclusions, unless every detail of 
the complicated processes is known. 
Contact metamorphism under uniform pressure.—In many cases 
contact metamorphism proceeds under uniform rock pressure. 
There is, however, strong gas pressure, and if its gradient is high 
this may appreciably affect the replacement. Contact meta- 
morphism may take place with or without rock metasomatism. 
In the special case of intense metasomatism of limestone under the 
influence of extremely hot and concentrated gases one would think 
that changes of volume would occur if anywhere. And yet the 
field evidence as well as the microscopical evidence is strongly 
opposed to such a view as I have shown in the Clifton-Morenci 
« Van Hise expresses this law as follows: ‘‘The volume of the original compound is 
to the volume of the compound produced directly as their molecular weights and 
indirectly as their specific gravities” (U.S. Geol. Survey, Monograph 47, p. 209). 
