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? III.—DEFORMATION OF ROCKS. Examination of the rock exposures at many localities in the neighbourhood of Cairo bears witness to the fact that the strata of limestone which were laid down horizontally upon the bed of the sea are no longer in the position they once occupied, but are now inclined to the horizontal plane at angles which are easily perceptible. This is especially noticeable in the two shallow cuttings under the Great Pyramid, where the strata are inclined 5 degrees to the south, revealing the reason why the whole Pyramid slope descends so rapidly in the southward direction. Again, if standing at the base of the same mighty structure, the gaze be allowed to wander over the broad expanse of the Nile Valley to the scarp of the Moqattam hills behind the Citadel, it will be seen that the white limestone of their lower slopes forms, not a long horizontal wall, but an arch, sloping strongly both north and south of the Citadel. There has been obviously change of form, but how has it taken place?