Considering the earth-movements of Egypt as a whole, the evidence shows folding to have been the more important type of displacement on its western side, and intense faulting to have been most conspicuous in the east, while between these two extremes, the relative importance of folding and faulting will remain a contested point. The discussion will only close when the borders of the Nile Valley and the Gulf of Suez have been mapped with the accuracy and precision of a Geological Survey in Europe—a pious aspiration, whose realization can scarcely be hoped for while the broad geological picture is still being filled in. Egypt, then, has passed through a long history, of which the following seems to be the record put in dogmatic form. The ancient schists and granites which form the central core of its eastern arch, or anticline, the Red Sea hills, are witnesses to a period of sedimentation, of volcanic action, and of the influence of deep-seated molten igneous masses. Step by step these were revealed by denudation, becoming part of an ancient continent. In addition to a brief marine invasion during the Carboniferous period, there was a second advance of the sea in Cretaceous times, which is represented first by the deposit of such coarse detrital materials as the sands composing the Nubian sandstones. Then, as the depression increased, only the finer clayey materials reached the Egyptian region, and finally the sea covered, if not the whole, at least the greater part of its area for a lengthened period. With the close of the Eocene epoch a reverse movement set in, by which the pure white marine limestones of the Moqattam hills were capped by the sandy limestones and clays forming the brown-tinted beds of their upper portion, the whole being, in its turn, covered by the coarse flint and quartz gravel which containing silicified trees[5] is the chief desert-former in the immediate neighbourhood of Cairo, as also of a large area in north-west Egypt. Any gain by the sea since this period has been comparatively local and limited, and the present conditions suggest that the land is gaining on the sea rather than the reverse. The great fold-movements which we have seen to determine so many important features are relatively of very late date in this series of events, on their nature and position depending the present character and visible extent of the different formations. IV.—PHYSICAL TRANSFORMATION OF ROCKS. The geological features of Egypt as presented to-day are the results of the formation through varying conditions and the subsequent deformation of the rocks composing its solid crust; we may next consider the varied agencies through whose action these are now undergoing transformation. A comparatively small portion of this country is undergoing erosion by the sea, and if elevation be still taking place, there is rather gain from the sea than destruction by it.