THE AQUEOUS ROCKS.

The Aqueous Rocks may be divided into two great sections, the
Mechanically-formed and the Chemically-formed, including under
the last head all rocks which owe their origin to vital action,
as well as those produced by ordinary chemical agencies.

[Illustration: Fig. 6.--Microscopic section of a calcareous breccia
in the Lower Silurian (Coniston Limestone) of Shap Wells,
Westmoreland. The fragments are all of small size, and consist of
angular pieces of transparent quartz, volcanic ashes, and limestone
embedded in a matrix of crystalline limestone. (Original.)]

A. MECHANICALLY-FORMED ROCKS.--These are all those Aqueous Rocks
of which we can obtain proofs that their particles have been
mechanically transported to their present situation. Thus, if
we examine a piece of _conglomerate_ or puddingstone, we find
it to be composed of a number of rounded pebbles embedded in an
enveloping matrix or paste, which is usually of a sandy nature,
but may be composed of carbonate of lime (when the rock is said to
be a "calcareous conglomerate"). The pebbles in all conglomerates
are worn and rounded by the action of water in motion, and thus
show that they have been subjected to much mechanical attrition,
whilst they have been mechanically transported for a greater
or less distance from the rock of which they originally formed
part. The analogue of the old conglomerates at the present day
is to be found in the great beds of shingle and gravel which
are formed by the action of the sea on every coast-line, and
which are composed of water-worn and well-rounded pebbles of
different sizes. A _breccia_ is a mechanically-formed rock, very
similar to a conglomerate, and consisting of larger or smaller
fragments of rock embedded in a common matrix. The fragments,
however, are in this case all more or less angular, and are not
worn or rounded. The fragments in breccias may be of large size,
or they may be comparatively small (fig. 6); and the matrix may
be composed of sand (arenaceous) or of carbonate of lime
(calcareous). In the case of an ordinary sandstone, again, we
have a rock which may be regarded as simply a very fine-grained
conglomerate or breccia, being composed of small grains of sand
(silica), sometimes rounded, sometimes more or less angular,
cemented together by some such substance as oxide of iron, silicate
of iron, or carbonate of lime. A sandstone, therefore, like a
conglomerate is a mechanically-formed