548 _ WALDEMAR LINDGREN 
molecule. The last example also shows that alumina is by no means 
an immobile constituent during the metamorphic processes. 
Preservation of texture and structure.—The best proof that no 
volume change takes place during metamorphism is furnished by 
the frequent preservation of texture and structure. We find the 
sharp outlines of an olivine crystal preserved, though it may be 
wholly converted into serpentine. We find calcareous odlites 
metasomatized by fine-grained quartz; silicified dolomites simi- 
larly preserve the outlines of their individual crystals. Fossils are 
preserved in exact outlines after silicification or pyritization. 
Basic rocks are chloritized with perfect preservation of outlines of 
ferromagnesian silicates. Zinc carbonate sometimes reproduces 
faithfully the texture and structure of limestone. 
Replacement under uniform pressure.—It will be necessary to 
consider replacement under uniform pressure separately from the 
more complicated conditions of stress. Uniform pressure results in 
increase of solubility, but the effects are slight compared to a rela- 
tively small change in temperature. Uniform pressure obtains 
in static metamorphism, which, for instance, is active in the upper 
part of the crust, where hydration is an important process and the 
temperature not high. Rocks penetrated by solutions under 
static metamorphism develop many minerals by replacement, for 
instance, sericite, chlorite, calcite, epidote, serpentine, pyrite, etc. 
In many cases there would be a tendency toward increase of volume 
because heavier minerals are replaced by hydrated products. It 
is believed, however, that the law of equal volumes holds strictly 
in case of this form of metamorphism and that changes of volume 
have been more frequently assumed to exist than actually proved. 
Consider an ordinary diabase altered to greenstone. Such a rock 
is a veritable laboratory of metasomatism: sericite after feldspar, 
chlorite after augite, epidote, calcite, and quartz after any of the 
primary minerals. All these replacements are constantly going on 
in such a rock, and I think that in no case has any change of volume 
been actually proved. It would not seem impossible to do so, for 
instance, in case of a thick dike in sedimentary rocks. 
«J. Johnston and Paul Niggli, “‘The General Principles Underlying Metamorphic 
Processes,”’ Jour. Geol., XXI (1913), 504. 
