17. (_A._) SEDIMENTARY CLASS.--Three of the most commonly occurring
rocks of this class have already been described, but a few details are
added here.
_Conglomerate._--This is a consolidated mass of more or less water-worn
and rounded stones. These stones may be of any size. When they are very
large, the rock is called a _coarse conglomerate_; the finer varieties,
in which the stones are small, are known as _pebbly conglomerates_. The
ingredients of a conglomerate may consist of any kind of rock, or of a
mixture of many different kinds. When they consist entirely of quartz,
the rock becomes _quartzose_. The finer-grained conglomerates usually
shew lines of deposition or bedding, but in some of the coarser sorts it
is often difficult to detect any kind of arrangement. The stones are
usually imbedded in a matrix of quartzose grit and sand, but sometimes
this is very scanty. When the nature of the material which binds the
stones together is very well marked, the rock becomes _ferruginous_,
_calcareous_, _arenaceous_, or _argillaceous_, according as the binding
or cementing material is _iron_, _lime_, _sand_, or _clay_. _Breccia_ is
a rock in which the included fragments are _angular_.
18. _Sandstone_ is, as already remarked, merely consolidated sand. The
coarser varieties, in which the grains are as large and larger than
turnip-seeds, are termed _grit_. From these coarse varieties the rock
passes insensibly, in one direction, into a fine or pebbly conglomerate,
and in another into a rock, so fine-grained that a lens is needed to
distinguish the component particles. Quartz is the prevailing
ingredient--sometimes clear, at other times white. Frequently, however,
the grains are coated with an oxide of iron, which gives the resulting
rock a red colour. The other colours assumed by sandstone--such as
yellow, brown, green, &c.--are also in like manner due to the presence
of some compound of iron. When mica or felspar occurs plentifully, we
have, in the one case, _micaceous sandstone_, and in the other
_felspathic sandstone_. A sandstone in which the grains are cemented by
carbonate of lime is said to be _calcareous_. _Freestone_ is a sandstone
which can be worked freely in any direction. In most sandstones, the
lines of bedding are distinct; when they are so numerous as to render
the rock fissile, the sandstone is said to be _shaly_.
_Shale_ is a more or less indurated fissile or laminated clay. When the
rock becomes coarse by the admixture of sand, it gradually passes into a
_shaly sandstone_. There are many other varieties of clay-rocks--such as
_fire-clay_, _pipe-clay_, _marl_, _loam_, &c.--which are sufficiently
familiar.
19. (_B._) EOLIAN or AERIAL CLASS.--_Blown-sand_ is found at many places
on sea-coasts. It generally forms smooth rounded hummocks, which are
sometimes arranged in long lines parallel to the trend of the coast, as,
for example, in the Tents Moor, near St Andrews. The _sand-hills_ of
deserts also belong to this class.
_Debris_ is the loose angular rubbish which collects at the base of
cliffs, on hill-tops, and hill-slopes. Immense accumulations of it occur
in lofty mountainous districts and in arctic regions. In Nova Zembla,
for example, the solid rock of the country is almost concealed beneath a
thick covering of debris. But the various kinds of debris will be more
particularly described further on.
_Soil._--An account of this can hardly be given without entering into
the theory of its origin, and therefore we reserve its consideration for
the present.