Thus we see how immensely important the same causes that give rise to earthquakes may be in moulding the outlines and contours of the rocks and the "everlasting hills." In the present state of geological knowledge we cannot say that these steam explosions are the sole causes of mountain building, but it is evident that they must play a great part in them. The action of the submarine explosions may be compared to a man digging out a trench. As he digs along the trench, the earth that he excavates he throws on to either side of the trench, so that a ridge appears on each side of the excavation. The result is the same in the case of the continuous lava explosions in deep seas, especially in those deep seas like the waters off the west coast of South America, where a great range of mountains runs parallel to an ocean that is of great depth only a short distance from the land. A trough or trench is cut downwards by successive explosions and expulsions of lava. As the trough is arched downwards like a broad letter =U=, the steam pressure from beneath cannot easily force it upwards. What will therefore happen? Imagine what would happen in a steam saucepan or kettle if the vapour could not get out at the top or lid. It would tend to blow out at the sides. Or if you think of a slab of dough rising under the effect of yeast. Suppose the baker presses a flat board on the top of the rising dough, and presses down on it so that it cannot force its way upwards. It will then naturally spread out to the sides. Similarly the rising yeasty lava under the curved ocean bed has to force its way sideways under the crust. It forces its way partly towards the land--where the mountains run along the coast as in the case of the Andes--or farther out underneath the ocean. Generally the movement of the lava will be towards the mountains till the trough gets broad and deep and the mountains very far away, and so high that their weight offers unexpectedly great resistance to the underground stream of lava. Then the release will at length become easier towards the ocean by the forcing up of ridges or volcanoes along the other margin of the trough. Ridges with peaks in them will usually result, and this is the beginning of a new range of mountains in the sea, which are destined to rise slowly from it parallel to the great range of mountains on the shore. There may thus be two parallel ridges, perhaps hundreds of miles apart, with a valley between them. This valley may be drained in the course of ages, or filled in by the processes of erosion which we have described in the earliest chapters of this volume. It will be of interest to quote at this point what Pliny nearly two thousand years ago said in his _Natural History_ (Book II) on islands which have been uplifted from the Mediterranean, evidently as the result of volcanic causes:--