'Rocks of eruption', which have issued from the interior of the earth either
in a state of fusion from volcanic action, or in a more or less soft,
viscous condition, from Plutonic action.
'Sedimentary rocks', which have been precipitated and deposited on the
earth's surface from a fluid, in which the most minute particles were either
dissolved or held in suspension constituting the greater part of the
secondary (or flotz) and tertiary groups.
'Transformed or metamorphic rocks',* in which the internal texture and the
mode of stratification have been changed, either
p 249
by contact or proximity with a Plutonic or volcanic endogenous rock of
eruption,** or, what is more frequently the case, by a gaseous sublimation
of substances*** which accompany certain masses erupted in a hot, fluid
condition.
[footnote] *[As the doctrine of mineral metamorphism is now exciting very
general attention, we subjoin a few explanatory observations by the 'New
Philos. Journ.', Jan., 1848: "In its widest sense, mineral metamorphism
means every change of aggregation, structure, or chemical condition which
rocks have undergone subsequently to their deposition and stratification, or
the effects which have been produced by other forces than gravity and
cohesion.
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