phosphate of lime,
vivianite (phosphate of iron), oxides of manganese, copper ores (e.g.
_Kupferschiefer_), wavellite and amber. As the impurities increase in
amount the clay rocks pass gradually into argillaceous sands and
sandstones, argillaceous limestones and dolomites, shaly coals and clay
ironstones.

Natural clays, even when most pure, show a considerable range of
composition, and hence cannot be regarded as consisting of a single
mineral; clay is a _rock_, and has that variability which characterizes
all rocks. Of the essential properties of clay some are merely physical,
and depend on the minute size of the particles. If any rock be taken
(even a piece of pure quartz) and crushed to a very fine powder, it will
show some of the peculiarities of clays; for example, it will be
plastic, retentive of moisture, impermeable to water, and will shrink to
some extent if the moist mass be kneaded, and then allowed to dry. It
happens, however, that many rocks are not disintegrated to this extreme
degree by natural processes, and weathering invariably accompanies
disintegration. Quartz, for example, has little or no cleavage, and is
not attacked by the atmosphere. It breaks up into fragments, which
become rounded by attrition, but after they reach a certain minuteness
are borne along by currents of water or air in a state of suspension,
and are not further reduced in size. Hence sands are more coarse grained
than clays. A great number of rock-forming minerals, however, possess a
good cleavage, so that when bruised they split into thin fragments; many
of these minerals decompose somewhat readily, yielding secondary
minerals, which are comparatively soft and have a scaly character, with
eminently perfect cleavages, which facilitate splitting into exceedingly
thin plates. The principal substances of this description are kaolin,
muscovite and chlorite. Kaolin and muscovite are formed principally
after felspar (and the felspars are the commonest minerals of all
crystalline rocks); also from nepheline, leucite, scapolite and a
variety of other rock-forming minerals. Chlorite arises from biotite,
augite and hornblende. Serpentine, which may be fibrous or scaly, is a
secondary product of olivine and certain pyroxenes. Clays consist
essentially of the above ingredients (although serpentine is not known
to take part in them to any extent, it is closely allied to chlorite).
At the same time other substances are produced as decompositi