That veins of carbonate of lime should be present in limestone districts is, in view of the above statements, not surprising, but it does appear somewhat startling at first sight, to find marked deposits of carbonate of lime lining the floors and sides of torrent-beds in districts entirely composed of igneous or volcanic rocks of complicated mineral structure. Experience has shown, however, that the lime silicates, so abundant in the more basic members of the igneous series, such as diabases and diorites, are liable to the attack of the rain-waters containing carbonic acid, carbonate of lime being produced by the reaction. (4) Of the results of Hydration, the most striking examples in Egypt are the formation of kaolin near Aswan, due to the absorption of water by the felspars of the granitic and gneissose rocks, and the thick zone of decomposition (kaolinic) products, which was cut through in excavating the navigation canal in the syenite which forms the main rock at that locality. The total effect of all the above-mentioned meteorological influences results in the weathering of the rock-surface, involving the softening and crumbling of the harder materials, but sometimes leading to the solidification of materials previously loosely aggregated by substances left as cementing agents when the water containing them in solution has evaporated. In addition to the various direct results of the meteorological activities upon the earth’s surface, there are others which indicate more subtle changes. Perhaps amongst the most interesting of these is the formation of concretions—bodies composed of one material aggregated in more or less rounded or irregular form in a rock of another composition. Among the most interesting and abundant of these are the layers of flint, which form bands of strikingly parallel character in the limestones of Upper Egypt. These have not yet been submitted to the detailed study which similar concretions have received in Europe, but there is little doubt that they, in large measure, represent the aggregation of gelatinous silica round decomposing organic materials, the shells of organisms and the framework of siliceous sponges often forming their centre. In some cases, as in the fossil trees, the replacement appears to have taken place molecule by molecule, as the outlines of every cell of the once woody fibre are now replaced in silica. By a well-known transition, this once gelatinous material has now become one of the most solid of substances. Ferruginous concretions, composed of oxide of iron, are present in many of the Egyptian sandy clays, some of the beautifully-tinted purple, yellow and red ochres being found in this form; and the natives collect them for the use of the women as ornamental coloration.