1. _Gneiss_ is the most abundant rock in this class, and is generally
found reposing on granite. Its stratification is sometimes very
distinct, but it is often so imperfect that it can scarcely be
recognized. This is more frequently the case in the vicinity of granite
on which it rests, and into which it insensibly passes. A large part of
the material used for building purposes, under the name of granite, is
obscurely marked gneiss. In all primary countries it is an abundant
rock, occupying extensive districts, and sometimes forming mountain
masses.
2. _Mica slate_ lies next above gneiss, and is a very abundant rock. As
it differs from gneiss only in the proportion of mica which it contains,
and as the quantity of mica in it is very different in different places,
it is often difficult to make the distinction between them. It also
passes by insensible degrees into the argillaceous rocks. Many of the
argillaceous rocks are found, upon close examination, to contain mica in
minute scales in such abundance as to make it doubtful whether they
ought not to be regarded as mica slates; that is, the metamorphic action
by which argillaceous slate is converted into mica slate had proceeded
so far, before it was arrested, that it becomes impossible to say
whether the argillaceous or micaceous characters predominate.
3. _Argillaceous slate._--The last rock of this series is a slaty rock,
more or less highly argillaceous. It does not differ in lithological
characters from the same rock in the higher strata. It is doubtful
whether the roofing-slates should be considered as belonging to the
metamorphic series or not. They have been subjected to a very high
degree of metamorphic action, and yet strata intimately associated with
them have, in occasional instances, contained fossils.
It is not easy to fix the exact upper limit of this series. The fossils
are few, obscure, and seldom met with in the lowest fossiliferous
series; and the transition is very gradual from the distinctly
metamorphic to the fossiliferous rocks. This renders it impossible
always to determine accurately the line of separation.
The gneiss, mica slate and argillaceous slate, have the order of
superposition in which they are here named. They differ only in the
amount of metamorphic action to which they have been subjected; and the
gneiss which is most highly metamorphic has, by being the lowest, been
most acted upon,--the mica slate less, and the argillaceous slate least.
In a particular locality, however, the lowest rock which was subjected
to these causes of change, instead of having been of such a character as
to produce gneiss, may have been a limestone, and in that case the
lowest metamorphic rock would be a saccharine marble. In another
locality the lowest rock may have been a sandstone, which would be
converted into quartz rock. Hence there may occur, in any part of the
metamorphic series, crystalline limestone, quartz rock, hornblende
slate, chlorite slate, and talcose slate; and any one of these rocks may
be as abundant in any particular region, as gneiss, mica slate or
argillaceous slate, is in another.
The metamorphic rocks occur in all countries where there has been any
considerable amount of volcanic action, and their total amount is very
great; but their stratification is so confused and contorted, their
superposition so irregular, and denudations have been so extensive, that
no estimate can be made of their thickness. They are, perhaps, equal to
all the other stratified rocks.