VOLUME CHANGES IN METAMORPHISM 545 
While, therefore, in extreme cases of metasomatism there may 
be diminution of actual volume expressed in porosity, the opposite, 
i.e., increase in volume, it is believed, will not take place by replace- 
ment in rigid rocks. 
Form and texture of guest mineral.—The guest mineral may form 
aggregates with their irregular outlines simply determined by the 
ways open for the replacing solutions. In this case, as in replace- 
ment of feldspar by sericite, there are capillary openings between 
the individuals of sericite, and the space is thus filled less con- 
tinuously than in the host mineral. In other words the porosity 
may increase by several per cent. The altered rock is then 
more accessible to later solutions and may easily suffer further 
alteration if the solutions change so as to make the new mineral 
unstable. 
In other cases the guest mineral is compact and may assume its 
own crystal form, owing, it is believed, to differential pressure and 
corresponding difference in rapidity of growth in certain directions. 
At times the replacement may proceed so rapidly that parts of the 
host mineral or host minerals become inclosed by the guest mineral, 
as frequently seen in quartz crystals in limestone or garnets and 
ottrelites in crystalline schists. 
Movement of solutions on capillary fisswres.—I am aware that 
attempts have been made to show that circulation of solutions on 
capillary fissures is so slow as to be negligible.t While not able to 
refute these calculations, I can only say that the whole process of 
metamorphism appears to be opposed to such a conclusion. 
Metasomatic shells.—It is frequently observed that metasomatic 
crystals, for instance, cubes of pyrite in feldspar, are surrounded 
by a thin covering of another mineral, i.e., quartz or chlorite 
following the outlines of the crystal. This is believed to mean that 
at a certain stage the iron solutions failed. The replacement pro- 
ceeded, but instead of pyrite the next combinations available in the 
replacing solutions were precipitated. 
Complex replacement.—One guest mineral may replace two or 
several host minerals without change of its crystallographic form. 
One pyrite crystal may extend across the contact of a quartz and a 
tJ. Johnston and L: A. Adams, Centralblatt fiir Mineralogie, 1914, p. 171. 
