which
they have been upheaved in the course of geological changes. The
process of their formation may be watched at the present day at the
mouths of all great rivers, where a delta composed of the suspended
matters carried down by the waters is slowly formed. The nature of these
rocks must therefore depend entirely on that of the country through
which the river flows. If its course runs through a country in which
lime is abundant, calcareous rocks will be deposited, and if it passes
through districts of different geological characters the deposit must
necessarily consist of a mixture of the disintegrated particles of the
different rocks the river has encountered. For this reason it is
impossible to enter upon a detailed account of their composition. It is
to be observed, however, that the particles of which they are composed,
though originally derived from the crystalline rocks, have generally
undergone a complex series of changes, geology teaching that, after
deposition, they may in their turn undergo disintegration and be carried
away by water, to be again deposited. Their composition must therefore
vary not merely according to the nature of the rock from which they have
been formed, but also according to the extent to which the decomposition
has gone, and the successive changes to which they have been exposed.
They may be reduced to the three great classes of clays, including the
different kinds of clay slates, shales, etc., sandstone and limestone.
It must be added also, that many of them contain carbonaceous matters
produced by the decomposition of early races of plants and animals, and
that mixtures of two or more of the different classes are frequent.

The purest clays are produced by the decomposition of felspar, but
almost all the crystalline rocks may produce them by the removal of
their alkalies, iron, lime, etc. Where circumstances have been
favourable, the whole of these substances are removed, and the clay
which remains consists almost entirely of silica and alumina, and yields
a soil which is almost barren, not merely on account of the deficiency
of many of the necessary elements of plants, but because it is so stiff
and impenetrable that the roots find their way into it with difficulty.
It rarely happens, however, that decomposition has advanced so far as to
remove the whole of the alkalies, which is exemplified by the following
analyses of the fire clay of the coal formation, and of transition clay
slat