21. _Limestone_ consists of carbonate of lime, but usually contains some
impurities. The varieties of this rock are numerous; some of them are as
follows: _Chalk_; _oolite_, a rock built up of little spheroidal
concretions, whence its name, _egg_ or _roe stone_ (the coarser oolites
are called _pisolite_, or _pea-stone_); _lacustrine limestone_, &c. When
much silica is diffused through the rock, we have a _siliceous
limestone_; the presence of clay and of carbonaceous matter gives us
_argillaceous_ and _carbonaceous limestones_. _Cornstone_ is a limestone
containing a large quantity of arenaceous matter or sand. Many
limestones are distinguished by the different kinds of organic remains
which they yield. Thus, we have _muschelkalk_ or _shell-limestone_,
_nummulitic_, _crinoidal_, &c. limestone. The crystalline limestones,
such as _statuary marble_, are metamorphosed limestones. Not a few
limestones are chemically formed rocks, and many, also, are partly of
chemical and partly of organic origin, so that no hard and fast line can
be drawn between these two classes of rock.
_Dolomite_, or _magnesian limestone_.--This is a compound of carbonate
of lime and carbonate of magnesia. Its colour is usually yellow, or
yellowish brown, but gray and black varieties are sometimes met with. It
is generally fine-grained, with a crystalline texture, and pearly
lustre. It effervesces less freely with acids than pure limestone. In
many cases dolomite is merely a metamorphosed limestone.
22. _Coal_ is composed of vegetable matter, but usually contains a
greater or less percentage of impurities. The varieties of this
substance are very numerous, and differ from each other principally in
regard to their bituminous or non-bituminous character. Coal is
bituminous or non-bituminous according as it is less or more highly
mineralised. Bitumen results from the decomposition of vegetable matter;
but, when the mineralising process (to which the formation of coal is
due) has proceeded far enough, the vegetable matter gradually loses its
bituminous character, and the result is a non-bituminous coal. Varieties
of coal are the following: _Lignite_ or _brown coal_; _caking coal_;
_cannel_, _parrot_, or _gas coal_; _splint coal_; _cherry_ or _soft
coal_; _anthracite_ or _blind coal_, so called because it burns with no
flame. _Peat_ may be mentioned as another natural fuel. It is composed
of vegetable matter. In some kinds it is so far decomposed, or
mineralised, that the eye does not detect vegetable fibres; when
thoroughly dried, such peat breaks like a good lignite, and forms an
excellent fuel.