METASOMATISM IN CONNECTION WITH VEIN FORMATION. 
185 
Chemically the process when carried to completion involves an almost entire 
loss of soda and a corresponding gain of potash attending the formation of sericite 
and adularia. Small amounts of sulphur, carbon dioxide, and fluorine are intro¬ 
duced. The percentage of silica is usually slightly reduced and a leaching of 
magnesia and lime is sometimes evident. After all, the most important process 
is the replacement of soda by potash, so commonly resulting elsewhere from the 
action of thermal waters at moderate temperature and pressure on ordinary feld- 
spathic rocks. 
METASOMATIC MINERALS. 
Among the metallic minerals pyrite is easily the most common, and develops 
abundantly by replacement as small crystals in the groundmass of the porphyritic 
rocks or in the cementing mass of the breccia, as well as in phenocrysts or in the 
minerals of granular, rocks. The form is usually that of the pyritohedron combined 
with the cube; some larger masses have irregular outlines. Although the mineral 
develops throughout the rock, it is most abundant near replaced phenocrysts of 
pyroxene, amphibole, or biotite; it also forms directly from magnetite, and on the 
whole the iron seems to be supplied by the original minerals of the rock, and the 
transformation is effected simply by the addition of sulphur derived from hydrogen 
sulphide or from sodium sulphide. 
Zinc blende appears occasionally in the altered rocks, but is, on the whole, 
very rare; it is associated with pyrite as anhedral grains of dark-brown color. 
Other sulphides are very seldom observed. 
Tellurides, chiefly calaverite, appear as metasomatic minerals in granite, schist, 
breccia, or phonolite, and also in the basic dikes. It forms anhedral grains, rarely 
prisms, and is associated with pyrite. Another form of its occurrence is as minute 
crystals coating small cavities of dissolution in various rocks, such as granite and 
phonolite. Compared to its abundant development as crusts in fissures, its meta¬ 
somatic distribution is very limited. 
Magnetite, specularite, pyroxenes, and amphiboles do not appear as metaso¬ 
matic minerals in these deposits, and are, in fact, unstable under the influence of 
these vein-forming agencies. Epidote is very rare, and probably also unstable 
under these conditions. 
Sericite is common, although the extensive sericitization seen in the wall rocks 
of many classes of veins is here absent. The mineral forms, to some extent, as 
small foils and fibers in orthoclase (PL XVIII, B ), though wholly sericitized crystals 
are very seldom encountered. Much more commonly it forms as minute fibers in 
the sodic silicates, such as nepheline, sodalite, and analcite, also in soda-lime feld¬ 
spars, and in pyroxenes, amphiboles, and brown micas. The yellowish-green vana¬ 
dium mica, roscoelite, is seen occasionally in granite (Ajax mine) or in breccia 
(Mary McKinney mine). 
As a first stage in common hydrometamorphism, as wqjl as in hydrothermal 
metamorphism, of biotite, amphibole, and pyroxene in gneisses, latite-phonolites, 
and similar rocks, a variety of mica is often noted which does not seem to correspond 
to any known variety. Its aggregates of small shreds and foils of deep grass-green 
to yellowish-green and brownish-green color have a noticeable though not very 
strong pleochroism and vivid colors of polarization. 
