on goes on.
They are principally finely divided quartz, epidote, zoisite, rutile,
limonite, calcite, pyrites, and very small particles of these are rarely
absent from natural clays. These fine-grained materials are at first
mixed with broken and more or less weathered rock fragments and coarser
mineral particles in the soil and subsoil, but by the action of wind and
rain they are swept away and deposited in distant situations. "Loess" is
a fine calcareous clay, which has been wind-borne, and subsequently laid
down on the margins of dry steppes and deserts. Most clays are
water-borne, having been carried from the surface of the land by rain
and transported by the brooks and rivers into lakes or the sea. In this
state the fine particles are known as "mud." They are deposited where
the currents are checked and the water becomes very still. If
temporarily laid down in other situations they are ultimately lifted
again and removed. A little clay, stirred up with water in a glass
vessel, takes hours to settle, and even after two or three days some
remains in suspension; in fact, it has been suggested that in such cases
the clay forms a sort of "colloidal solution" in the water. Traces of
dissolved salts, such as common salt, gypsum or alum, greatly accelerate
deposition. For these reasons the principal gathering places of fine
pure clays are deep, still lakes, and the sea bottom at considerable
distances from the shore. The coarser materials settle nearer the land,
and the shallower portions of the sea floor are strewn with gravel and
sand, except in occasional depressions and near the mouths of rivers
where mud may gather. Farther out the great mud deposits begin,
extending from 50 to 200 m. from the land, according to the amount of
sediment brought in, and the rate at which the water deepens. A girdle
of mud accumulations encircles all the continents. These sediments are
fine and tenacious; their principal components, in addition to clay,
being small grains of quartz, zircon, tourmaline, hornblende, felspar
and iron compounds. Their typical colour is blackish-blue, owing to the
abundance of sulphuretted hydrogen; when fresh they have a sulphurous
odour, when weathered they are brown, as their iron is present as
hydrous oxides (limonite, &c). These deposits are tenanted by numerous
forms of marine life, and the sulphur they contain is derived from
decomposing organic matter. Occasionally water-logged plant debris is
mingled wi