What we term
the ancient silurian strata are thus only the upper portions of the solid
crust of the earth. The erupted rocks which have broken through and
upheaved these strata have been elevated from depths that are wholly
inaccessible to our research; they must, therefore, have existed under the
silurian strata, and been composed of the same association of minerals which
we term granite, augite, and quartzose porphyry, when they are made known to
us by eruption through the surface. Basing our inquiries on analogy, we may
assume that the substances which fill up deep fissures and traverse the
sedimentary strata are merely the ramifications of a lower deposit. The
foci of active volcanoes are situated at enormous depths, and judging from
the remarkable fragments which I have found in various parts of the earth
incrusted in lava currents, I should deem it more than probable tht a
primordial granite rock forms the substratum of the whole stratified edifice
of fossil remains.*
[footnote] *See Elie de Beaumont, 'Descr. Geol. de la France', t. i., p.
65; Beaudant, 'Geologie', 1844, p. 269.
Basalt containing olivine first shows itself in the period of the chalk
trachyte still later, while eruptions of granite belong, as we learn from
the products of their metamorphic action to the epoch of the oldest
sedimentary strata of the transition formation.
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