lxviii., p. 128.
It would not be very surprising, therefore, as is well observed by the
distinguished geognosist, Von Dechen, if we were to meet with a fragment of
gneiss formed on the walls of a smelting furnace which was built of
argillaceous slate and graywacke.
After having taken this general view of the three classes of erupted,
sedimentary, and metamorphic rocks of the earth's crust, it still remains
for us to consider the fourth class, comprising 'conglomerates', or 'rocks
of detrius'. The very term recalls the destruction which the earth's crust
has suffered, and likewise, perhaps reminds us of the process of
cementation, which has connected together, by means of oxyd of iron, or of
some argillaceous and calcareous substances, the sometimes rounded and
sometimes angular portions of fragments. Conglomerates and rocks of
detritus, when considered in the widest sense of the term, manifest
characters of a double origin. The substances which enter into their
mechanical composition have not been alone accumulated by the action of the
waves of the sea or currents of fresh water, for there are some of these
rocks the formation of which can not be attributed to the action of water.
"When basaltic islands and trachytic rocks rise on fissures, friction of the
elevated rock against the walls of the fissures causes the elevated rock to
be inclosed by conglomerates composed of its own matter.
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