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.