II.—ORIGIN OR FORMATION OF ROCKS.
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?