s of Sir R.
Murchison's inference? At page 189 of _Siluria_, a still more conclusive
fact will be found. The "Glengariff grits," and other accompanying
strata there described as 13,500 feet thick, contain no signs of
contemporaneous life. Yet Sir R. Murchison refers them to the Devonian
period--a period which had a large and varied marine Fauna. How then,
from the absence of fossils in the Longmynd beds and their equivalents,
can we conclude that the Earth was "azoic" when they were formed?

"But," it may be asked, "if living creatures then existed, why do we not
find fossiliferous strata of that age, or an earlier age?" One reply is,
that the non-existence of such strata is but a negative fact--we have
not found them. And considering how little we know even of the
two-fifths of the Earth's surface now above the sea, and how absolutely
ignorant we are of the three-fifths below the sea, it is rash to say
that no such strata exist. But the chief reply is, that these records of
the Earth's earlier history have been in great part destroyed, by
agencies which are ever tending to destroy such records.

It is an established geological doctrine, that sedimentary strata are
liable to be changed, more or less profoundly, by igneous action. The
rocks originally classed as "transition," because they were
intermediate in character between the igneous rocks found below them,
and the sedimentary strata found above them, are now known to be nothing
else than sedimentary strata altered in texture and appearance by the
intense heat of adjacent molten matter; and hence are renamed
"metamorphic rocks." Modern researches have shown, too, that these
metamorphic rocks are not, as was once supposed, all of the same age.
Besides primary and secondary strata which have been transformed by
igneous action, there are similarly-changed deposits of tertiary
origin--deposits changed, even as far as a quarter of a mile from the
point of contact with neighbouring granite. By this process fossils are
of course destroyed. "In some cases," says Sir Charles Lyell, "dark
limestones, replete with shells and corals, have been turned into white
statuary marble, and hard clays, containing vegetable or other remains,
into slates called mica-schist or hornblende-schist; every vestige of
the organic bodies having been obliterated." Again, it is fast becoming
an acknowledged truth that igneous rock, of whatever kind, is the
product of sedimentary strata which have