On land the bases of the high mountains are indicated, and certain lower hills, of sandstone, which rise in the midst of the maritime plain. A striking fact is visible on first inspection of this map, viz. that not only is the Red Sea a nearly parallel-sided trough but that the constituents of the sides are themselves placed in lines parallel to the coast. The Archean hills[65], the lesser sandstone ranges, the coral bounding the maritime plain, and the barrier reefs, are all four roughly parallel to the main axis of the sea. We will consider each feature in more detail. For the Archean hills consult the extremely interesting memoirs of the Egyptian Geological Survey[66]; for our purposes it is enough to note that they are all of ancient igneous and metamorphic rocks, that they rise to heights of from four to eight thousand feet, and the valley bottoms are generally flat and filled in with gravel. The maritime plain is from five to ten miles wide, sloping up regularly from the sea towards the bases of the hills, where it may attain a level of several hundred feet. Except at its seaward edge, it is composed of black gravel, the product of the decay of the hills carried down by the torrents resulting from the rare but furious rain-storms, and spread out to form the plain. Sand-hills occur, but not very commonly, though the gravel is mingled with sand throughout, and in sections of the plain exposed by wells, layers of gravel alternate with sand, fine or coarse, as far as the deepest borings have been carried[67].