A molten magma varies greatly in its behaviour according to its position, the solidification of that portion which remained deep-seated in the hidden reservoir beneath the earth’s crust taking place slowly and imperceptibly. A rock such as granite is not formed suddenly by instantaneous cooling, but step by step individual minerals are crystallized from the complex solution, and tend to develop as regular and often most beautiful crystals. It is a source of surprise to most to learn that the greater number of mineral substances, if free to develop from solution without external hindrances, tend to form solid bodies which are fashioned on geometrical principles, constituting one of the most interesting and mysterious problems with which scientific thought has to deal. The minerals, as a rule, tend to crystallize out directly in the order of their basicity, the oxides of iron with their eight-sided crystals being among the first, so that their octahedral outlines are usually well preserved. The other minerals of the rocks follow in succession; sometimes complicated intergrowths occur owing to two crystals of different composition starting their formation at the same time in the same portion of the solution. Finally, the last-formed minerals have to occupy the interspaces left by their consolidated neighbours, and in the more acid rocks this unenviable position is usually reserved for the free silica, forming _quartz_. These deep-seated masses, slowly solidified and entirely crystalline in their structure, have been called the _Plutonic_ rocks, the example familiar to all dwellers in Egypt being the red Aswan, or monumental granite. In it, the felspars and micas or hornblendes present tend towards their true crystal form, while the quartz occupies the interspaces. Intermediate between the extruded volcanic rocks, and the highly crystalline varieties of the internal reservoirs of molten material, are the dyke or hypabyssal rocks, which as narrow vertical or nearly vertical intrusions pass in all probability from the deep-seated source towards the surface of the earth, when they reach it giving rise to volcanic effects. The dykes have a tendency to be parallel to one another, and in some regions are the most conspicuous features, if harder than the surrounding rock giving rise to marked ridges separated in many cases by comparatively shallow valleys. Not a few of the main summits in the Red Sea hills and Sinai have assumed their present form and outline owing to the unequal denudation of one of these hard bands, while many an ascent, which otherwise would have been by no means free from danger, is simplified by the formation of gullies due to the wearing away of the softer and more basic members of this series.