altered strata, a term derived from meta, trans, and morphe, forma.

Hence there are four great classes of rocks considered in reference to their
origin-- the aqueous, the volcanic, the Plutonic, and the metamorphic. In the
course of this work it will be shown that portions of each of these four
distinct classes have originated at many successive periods. They have all been
produced contemporaneously, and may even now be in the progress of formation on
a large scale. It is not true, as was formerly supposed, that all granites,
together with the crystalline or metamorphic strata, were first formed, and
therefore entitled to be called "primitive," and that the aqueous and volcanic
rocks were afterwards superimposed, and should, therefore, rank as secondary in
the order of time. This idea was adopted in the infancy of the science, when all
formations, whether stratified or unstratified, earthy or crystalline, with or
without fossils, were alike regarded as of aqueous origin. At that period it was
naturally argued that the foundation must be older than the superstructure; but
it was afterwards discovered that this opinion was by no means in every instance
a legitimate deduction from facts; for the inferior parts of the earth's crust
have often been modified, and even entirely changed, by the influence of
volcanic and other subterranean causes, while superimposed formations have not
been in the slightest degree altered. In other words, the destroying and
renovating processes have given birth to new rocks below, while those above,
whether crystalline or fossiliferous, have remained in their ancient condition.
Even in cities, such as Venice and Amsterdam, it cannot be laid down as
universally true that the upper parts of each edifice, whether of brick or
marble, are more modern than the foundations on which they rest, for these often
consist of wooden piles, which may have rotted and been replaced one after the
other, without the least injury to the buildings above; meanwhile, these may
have required scarcely any repair, and may have been constantly inhabited. So it
is with the habitable surface of our globe, in its relation to large masses of
rock immediately below; it may continue the same for ages, while subjacent