In Egypt, we are not at present acquainted with such gigantic overfolding, fracture, and pushing of whole masses of strata one over the other, as have been studied, for example, in the North-West Highlands of Scotland, but there is in Egypt itself, a vast series of gneisses, crystalline schists, marbles and dolomites whose origin is but dimly understood, and which may represent changes long anterior those exhibited by the ancient rocks of which we have been speaking. Developed in the untrodden wastes of the Sudan, in the wilder stretches of the Cataracts, in the desolate regions of the Etbai Desert of Egypt, their study is attended with difficulty and all our hypotheses must depend on the comparison of isolated specimens with similar rocks from more favoured regions. SUMMARY. 1.—Every rock originates either from the solidification of molten material or of sediments, these constituting the Igneous and Sedimentary Rocks respectively. 2.—These may undergo deformation due to movements of the earth’s crust, such movements according to their intensity determining the re-arrangement of the constituents with the development of new structures, producing the Metamorphic Rocks. 3.—These varied rocks, when exposed at the earth’s surface, are brought under the influence of the agents of transformation, such as frost, contraction and expansion due to variations of temperature, the mechanical action of rain, sand borne by wind, and running water in the form of streams. In addition to these purely physical activities are others of a chemical nature, either proceeding from changes outside the terrestrial surface (carbonic acid dissolved in rain, etc.), or from processes operating within the earth’s crust (production of petroleum, coal, mineral veins, etc.). 4.—The chemical and physical activities deep within the earth’s crust result in the formation of the plutonic rocks solidified from molten solutions, intimately associated with them being the external manifestations of the working of these hidden forces as revealed in volcanoes and their associated phenomena. 5.—By the surface changes on the consolidated igneous and other rocks, the materials are supplied for the formation of some of the members of the sedimentary series, the sands and clays, while the limestones in large measure owe their origin to the property possessed by living animals of constructing shells from the carbonate of lime brought in solution to the oceans, etc., these shells, after the death of the animals, contributing to the formation of the calcareous rocks. FOOTNOTES: [Footnote 1: In geology, the term _rock_ is applied alike to the soft and hard materials composing the earth’s crust, as sands may pass into solid sandstones, soft muds become the most tenacious of clays, and hard basalt originate as a molten lava.]