Igneous action and movements of the earth’s crust stand in intimate relation to one another, a point which has been clearly stated by Dr. Harker[9] as follows:—“Setting aside operations conducted in hypothetical intercrustal magma-basins, or generally in the unknown depths of the earth’s crust, we recognize the actual manifestations of igneous action chiefly in the forcing outward of molten magmas from a lower to a higher level within the crust or through the crust to the surface.” This study has led directly to the recognition of three phases of igneous action, of which two are intrusive, being phases in which the molten materials are raised from a lower to a higher level within the earth’s crust, and an extrusive phase, in which these materials are raised to the surface and poured out there as lavas. Study in all parts of the world has further shown that these events follow a definite sequence or cycle of igneous activity, volcanic phenomena marking its commencement, followed by the movement of deep-seated igneous molten magmas (representing the plutonic phase), and closing with a number of minor intrusions, which may seam both the volcanic, plutonic, and sedimentary strata. Egypt clearly illustrates this remarkable succession, and thus gives additional grounds for believing that it is of fundamental importance. Volcanic activity was developed on a gigantic scale when the most ancient sediments of Egypt, now forming the folded and altered slates and schists of the Red Sea hills, were being laid down. To this part of the cycle belong some of the most interesting rocks of the country, the imperial porphyry of Dokhan, the dark andesites that crown the highest summits in Sinai, and the country-rock in which some of the most ancient of Egypt’s gold-mines are situated. Here, too, belong the banded and columnar lavas of the Sixth Cataract, and the fragments of volcanic rocks which play an important part in the characteristic conglomerates of the Eastern Desert. Still more conspicuous is the phase of plutonic activity, vast masses of granite, diorite, and other highly crystalline rocks as molten magmas having been in contact with or intruded into the overlying sediments and volcanic materials. From Sinai to the Sudan there is a geographical complex due to the intermingling of these deep-seated igneous rocks with the older and metamorphosed volcanic and sedimentary members. Desolate volcanic hills of dull-green shade, whose sides are covered with weathered debris of irregular outline, alternate with broad plain, out of which rise rounded masses of granite, or with mountain ranges, whose precipitous sides and serrated outlines constitute some of the most striking features of the Red Sea hill scenery.