It has, however, been suggested by Mr. Hopkins,
that if one part of the area, after rising and before being denuded,
subsided, the deposit formed during the rising movement, though not thick,
might afterwards become protected by fresh accumulations, and thus be
preserved for a long period.
Mr. Hopkins also expresses his belief that sedimentary beds of considerable
horizontal extent have rarely been completely destroyed. But all
geologists, excepting the few who believe that our present metamorphic
schists and plutonic rocks once formed the primordial nucleus of the globe,
will admit that these latter rocks have been stripped of their covering to
an enormous extent. For it is scarcely possible that such rocks could have
been solidified and crystallised while uncovered; but if the metamorphic
action occurred at profound depths of the ocean, the former protecting
mantle of rock may not have been very thick. Admitting then that gneiss,
mica-schist, granite, diorite, etc., were once necessarily covered up, how
can we account for the naked and extensive areas of such rocks in many
parts of the world, except on the belief that they have subsequently been
completely denuded of all overlying strata? That such extensive areas do
exist cannot be doubted: the granitic region of Parime is described by
Humboldt as being at least nineteen times as large as Switzerland. South
of the Amazon, Boue colours an area composed of rocks of this nature as
equal to that of Spain, France, Italy, part of Germany, and the British
Islands, all conjoined.
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