Similarly, going westward and crossing the great limestone desert, on reaching the edge of the scarp which bounds the oases, clays appear from under the limestones, and in their turn overlie sandstones forming the floors of the oasis depressions. Closer examination reveals the fact that in the upper part of the sandstone series seams of clay alternate with the sandy layers, and in that part of the cliff where clay bands predominate, beds of limestone alternate with them. In broad outline, it may be stated that in southern Egypt, limestone rests on clays alternating with limestone, these on sandstones which in their upper part alternate with clay, and the sandstone on granite and metamorphic rocks (slates, schists, etc.). The inclination or dip of the various beds is such that should a boring be made through the limestone near Cairo, one might expect to reach first a succession of beds where clay was predominant, followed by beds of sandstone, and finally the series of which the granite of Aswan is a conspicuous member.[2] In northern Egypt, the conditions are reversed, sandstones and clays _overlying_ the limestones near Cairo, but never attaining the extent and importance of those exposed to view in the south. These facts require an explanation—the one furnished by the science of Geology, being briefly as follows: Each of the rocks observed has a definite origin or formation; the nature of their present distribution is due to subsequent movement, or deformation; the various meteorological agencies at work lead to the transformation of the original structures, resulting in the formation of a new series of rocks composed of materials derived from the wearing away of the older deposits. II.—ORIGIN OR FORMATION OF ROCKS.