The simplest illustration of the formation of a rock is one which
may be observed by the dweller in Egypt every year during the Nile
flood. The “red water” of the Nile, if collected during this
period, and evaporated to dryness, leaves behind it the fine-grained
sediment, or Nile mud, forming the soil to which this land owes its
fertility; while in the central portions of the river, the coarser
material, consisting largely of sand-grains, is being transported by
the stronger current. As a result, the river flows for the main part
over a sandy bed, the clays being restricted to the sides where the
water is moving less swiftly, or to the fields on which the finer
sediment is deposited. This “red water” has been traced step by
step to its parent source, and has been proved to be derived from
the wearing away of the widely-spread volcanic rocks of Abyssinia,
disintegrated by differences of temperature, etc., and denuded
by the destructive rainstorms which break over that region in the
early summer. It is equally a matter of experience that on drying,
this sediment passes from the condition of a soft and sticky mud
to a hard and resistant clay, which, drying during the heat of the
summer, cracks in every direction. The fluviatile character of these
clay deposits is often revealed by the presence of the river shells
enclosed in them, and in each succeeding year slight differences
in composition in the material brought down are indicated by the
layers being sharply marked off from one another, and so presenting
the familiar stratified appearance. Again, much sand and clay is
being carried seaward and deposited, the former, in general, nearer
the land on account of its greater specific gravity and less finely
divided character.
These clays and sands are forming both on the land and in the sea,
a point which does not need elaboration, but when considering the
origin of the limestones, and how it is that they seem at times to
be built up of fossil shells, as a rule the general student would
be at a loss for an answer. The researches in the great oceans,
which have been carried on with such assiduity during recent years,
have shown that the upper layers of their waters are crowded with a
vast number of living organisms, apparently simple in structure, but
having the power of extracting the carbonate of lime in solution and
constructing shells of complicated and beautiful form. As the animals
die, these tiny shells rain down to the bed of the ocean, slowly
forming a muddy white calcareous paste which encloses the sea-urchins
and other marine animals living in the depths of the sea. Thus, step
by step, muds (which on drying are as genuine limestones as any now
forming the cliffs and scarps of Egypt) are laid down on one another,
separated into strata whenever some external change, such as the
addition of some clayey matter transported from a river in flood,
slightly alters the composition. The alteration is subsequently
indicated either by variation in tint or by differing resistance
to the wearing influences of the meteorological agencies. But how
are these argillaceous muds transformed into the solid clays, the
sands into sandstones, and the calcareous muds into fossiliferous
limestones? How have these loose materials become consolidated so
as to form the compact rock-masses with which we become acquainted
in the most casual study of the physical structure of Egypt?