gradation, from perfectly loose sand to the hardest sandstone. In MICACEOUS
SANDSTONES mica is very abundant; and the thin silvery plates into which that
mineral divides are often arranged in layers parallel to the planes of
stratification, giving a slaty or laminated texture to the rock.

When sandstone is coarse-grained, it is usually called GRIT. If the grains are
rounded, and large enough to be called pebbles, it becomes a CONGLOMERATE or
PUDDING-STONE, which may consist of pieces of one or of many different kinds of
rock. A conglomerate, therefore, is simply gravel bound together by cement.

ARGILLACEOUS ROCKS.

Clay, strictly speaking, is a mixture of silex or flint with a large proportion,
usually about one fourth, of alumina, or argil; but in common language, any
earth which possesses sufficient ductility, when kneaded up with water, to be
fashioned like paste by the hand, or by the potter's lathe, is called a CLAY;
and such clays vary greatly in their composition, and are, in general, nothing
more than mud derived from the decomposition or wearing down of rocks. The
purest clay found in nature is porcelain clay, or kaolin, which results from the
decomposition of a rock composed of feldspar and quartz, and it is almost always
mixed with quartz. The kaolin of China consists of 71.15 parts of silex, 15.86
of alumine, 1.92 of lime, and 6.73 of water (W. Phillips Mineralogy page 33.);
but other porcelain clays differ materially, that of Cornwall being composed,
according to Boase, of nearly equal parts of silica and alumine, with 1 per cent
of magnesia. (Phil. Mag. volume 10 1837.) SHALE has also the property, like
clay, of becoming plastic in water: it is a more solid form of clay, or
argillaceous matter, condensed by pressure. It always divides into laminae more
or less regular.

One general character of all argillaceous rocks is to give out a peculiar,