This seems a fitting place again to draw attention to the
fact that, according to the admirable views of modern geognosy, the
metamorphism of rocks is not a mere phenomenon of contact, limited to the
effect produced by the apposition of two rocks, since it comprehends all the
generic phenomena that have accompanied the appearance of a particular
erupted mass. Even where there is no immediate contact, the proximity of
such a mass gives rise to modifications of solidification, cohesion,
granulation, and crystallization.
All eruptive rocks penetrate, as ramifying veins either into the sedimentary
strata, or into other equally endogenous masses; but there is a special
importance to be attached to the difference manifested between 'Plutonic'
rocks* (granite, porphyry, and serpentine) and those termed 'volcanic' in
the strict sense of the word (as trachyte, basalt, and lava).
[footnote] ([Lyell, 'Principales of Geology', vol. i.i., p. 353 and 359.]
-- Tr.
The rocks produced by the activity of our present volcanoes appear as
band-like streams, but by the confluence of several of them they may form an
extended basin. Wherever it has been possible to trace basaltic eruptions,
they have generally been found to terminate in slender threads.
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