usually of finer and purer substance than the rest of the stone. In
either case the action tends continually to the purification and
segregation of the elements of the stone. The energy of such action
depends on accidental circumstances: first, on the attractions of the
component elements among themselves; secondly, on every change of
external temperature and relation. So that mountains are at different
periods in different stages of health (so to call it) or disease. We
have mountains of a languid temperament, mountains with checked
circulations, mountains in nervous fevers, mountains in atrophy and
decline.

292. This change in the structure of existing rocks is traceable through
continuous gradations, so that a black mud or calcareous slime is
imperceptibly modified into a magnificently hard and crystalline
substance, inclosing nests of beryl, topaz, and sapphire, and veined
with gold. But it cannot be determined how far, or in what localities,
these changes are yet arrested; in the plurality of instances they are
evidently yet in progress. It appears rational to suppose that as each
rock approaches to its perfect type the change becomes slower; its
perfection being continually neared, but never reached; its change being
liable also to interruption or reversal by new geological phenomena. In
the process of this change, rocks expand or contract; and, in portions,
their multitudinous fissures give them a ductility or viscosity like
that of glacier-ice on a larger scale. So that many formations are best
to be conceived as glaciers, or frozen fields of crag, whose depth is to
be measured in miles instead of fathoms, whose crevasses are filled with
solvent flame, with vapor, with gelatinous flint, or with crystallizing
elements of mingled natures; the whole mass changing its dimensions and
flowing into new channels, though by gradations which cannot be
measured, and in periods of time of which human life forms no
appreciable unit.

293. II. _Formation._--Mountains are to be arranged, with respect to
their structure, under two great classes--those which are cut out of the
beds of which they are composed, and those which are formed by the
convolution or contortion of the beds themselves. The Savoy mountains
are chiefly of this latter class. When stratified formations are
contorted, it is usually either by pressure from below, which raises one
part of the formation above the rest, or by lateral pressure, which
reduces th