The vast fissures which were
formerly open in the solid crust of the earth have since been filled up or
closed by the protrusion of elevated mountain chains, or by the penetration
of veins of rocks of eruption (granite, porphyry, basalt, and melaphyre);
and while, scarcely more than four volcanoes remaining through which fire
and stones are erupted, the thinner, more fissured, and unstable crust of
the earth was anciently almost every where covered by channels of
communication between the fused interior and the external atmosphere.
Gaseous emanations rising from very unequal depths, and therefore conveying
substances differing in their chemical nature, imparted greater activity to
the Plutonic processes of formation and transformation. The sedimentary
formations, the deposits of liquid fluids from cold and hot springs, which
we daily see producing the travertine strata near Rome, and near Hobart Town
in Van Diemen's Land, afford but a faint idea of the flotz formation. In
our seas, small banks of limestone, almost equal in hardness at some parts
to Carrara marble,* are in the course of formation, by gradual
precipitation, accumulation, and cementation -- processes whose mode of
action has not been sufficiently well investigated.
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