39 it was inferred, from their crystalline character, that the
metamorphic rocks of Anglesea were more ancient than any rocks of the
adjacent main land; but it has since been shown that they are of the
same age with the slates and grits of Carnarvon and Merioneth. Again,
slaty cleavage having been first found only in the lowest rocks, was
taken as an indication of the highest antiquity: whence resulted serious
mistakes; for this mineral characteristic is now known to occur in the
Carboniferous system. Once more, certain red conglomerates and grits on
the north-west coast of Scotland, long supposed from their lithological
aspect to belong to the Old Red Sandstone, are now identified with the
Lower Silurians. These are a few instances of the small trust to be
placed in mineral qualities, as evidence of the ages or relative
positions of strata. From the recently-published third edition of
_Siluria_, may be culled numerous facts of like implication. Sir R.
Murchison considers it ascertained, that the siliceous Stiper stones of
Shropshire are the equivalents of the Tremadock slates of North Wales.
Judged by their fossils, Bala slate and limestone are of the same age as
the Caradoc sandstone, lying forty miles off. In Radnorshire, the
formation classed as upper Llandovery rock, is described at different
spots, as "sandstone or conglomerate," "impure limestone," "hard coarse
grits," "siliceous grit"--a considerable variation for so small an area
as that of a county. Certain sandy beds on the left bank of the Towy,
which Sir R. Murchison had, in his _Silurian System_, classed as Caradoc
sandstone (evidently from their mineral characters), he now finds, from
their fossils, belong to the Llandeilo formation. Nevertheless,
inferences from mineral characters are still habitually drawn and
received. Though _Siluria_, in common with other geological works,
supplies numerous proofs that rocks of the same age are often of
widely-different composition a few miles off, while rocks of
widely-different ages are often of similar composition; and though Sir
R. Murchison shows us, as in the case just cited, that he has himself in
past times been misled by trusting to lithological evidence; yet his
reasoning all through _Siluria_, shows that he still thinks it natural
to expect formations of the same age to be chemically similar, even in
remote regions. For example, in treating of the Silurian rocks of South
Scotland, he says:--"When traversing