escape, and the breaking-up of the tissues of the vegetation goes on
rapidly.

The chemical changes which have taken place in the beds of vegetation of
the carboniferous epoch, and which have transformed it into coal, are
even now but imperfectly understood. All we know is that, under certain
circumstances, one kind of coal is formed, whilst under other conditions,
other kinds have resulted; whilst in some cases the processes have
resulted in the preparation of large quantities of mineral oils, such as
naphtha and petroleum. Oils are also artificially produced from the
so-called waste-products of the gas-works, but in some parts of the world
the process of their manufacture has gone on naturally, and a yearly
increasing quantity is being utilised. In England oil has been pumped up
from the carboniferous strata of Coalbrook Dale, whilst in Sussex it has
been found in smaller quantities, where, in all probability, it has had
its origin in the lignitic beds of the Wealden strata. Immense quantities
are used for fuel by the Russian steamers on the Caspian Sea, the Baku
petroleum wells being a most valuable possession. In Sicily, Persia, and,
far more important, in the United States, mineral oils are found in great
quantity.

In all probability coniferous trees, similar to the living firs, pines,
larches, &c., gave rise for the most part to the mineral oils. The class
of living _coniferae_ is well known for the various oils which it
furnishes naturally, and for others which its representatives yield on
being subjected to distillation. The gradually increasing amount of heat
which we meet the deeper we go beneath the surface, has been the cause of
a slow and continuous distillation, whilst the oil so distilled has found
its way to the surface in the shape of mineral-oil springs, or has
accumulated in troughs in the strata, ready for use, to be drawn up when