There fall under this definition, the discoloration of the
surface of black limestone by the loss of carbon; the formation of
brownish-red crusts on rocks of limestone, sandstone, many slate structures,
serpentine, granite, etc., by the decomposition of iton pyrites, or magnetic
iron, finely disseminated in the mass of the rock; the conversion of
anhydrite into gypsum, in consequence of the absorption of water; the
crumbling of many granites and porphyries into gravel, occasioned by the
decomposition of the mica and feldspar. In its more limited sense, the term
metamorphic is confined to those changes of the rock which are produced, not
by the effect of the atmosphere or of water on the exposed surfaces, but
which are produced, directly or indirectly, by agencies seated in the
interior of the earth. In many cases the mode of change may be explained by
our physical or chemical theories, and may be viewed as the effect of
temperature or of electro-chemical actions. Adjoining rocks, or connecting
communications with the interior of the earth, also distinctly point out the
seat from which the change proceeds. In many other cases the metamorphic
process itself remains a mystery, and from the nature of the products alone
do we conclude that such a metamorphic action has taken place.
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