A continent with all the irregularities of mountain, plain, and valley sinks beneath the level of the sea, and over the once dry land sediments are laid down, filling in all the hollows and reducing the surface to a more or less featureless plain. The junction between the old land surface and the newer sediments is called an unconformity. When the land rises again the action of rain and rivers again cuts into sediments, and produces once more mountains, plains, and valleys. In all continents the process has been repeated many times, each land period being marked by an unconformity between the older and newer sediments. The central idea of a continent is, therefore, a block of sediments compacted into rock which has been carved into the characteristic features by rivers and other agencies. The older the continent the more prominent the features, the more irregular the coast line, the more diverse the scenery. South Africa, land of straight seaboard and wide stretches of monotonously similar landscape, is by this criterion at once recognized as a new continent.
The rocks forming the land surfaces are mostly marine sediments. These are exposed on the flanks of mountains, arid the once horizontal layers of sediment have been tilted by earth movements, so that by gathering information from various places we can estimate the total thickness of sediments. These may be set down at a maximum of 20 ml., but it must be understood that at no one place can we find a complete succession. Under the section dealing with earthquakes we shall learn that shocks may be recorded from distant places, and on studying these vibrations it is found that as long as the straight path between the origin of the earthquake and recording- station does not pass deeper than 30 ml. in the earth's
