It will be seen from many of the foregoing instances that whatever are the principal causes of earthquakes, they must have played a great part in the shaping of events in the geological past; and the only limitation which we can place in the importance of the part they played will depend on whether we regard the earthquake as having been caused by a movement of the underlying strata, or whether we believe that the same cause which produces earthquakes may produce alterations in the lie of the strata themselves. In the next chapter we shall describe some of the effects produced on land by earthquakes. But impressive as some of these effects are, it is by no means certain that the greatest earthquakes take place on land at all. They may take place at sea, deep underneath the ocean. Our opportunities for observing such quakes, however, are much smaller than are afforded by land earthquakes. The instruments which have been devised for observing earth tremors will measure the smallest of such disturbances, and will record earthquakes the centres of which are thousands of miles away. These delicate instruments often record distant earthquakes, the exact locality of which is never determined. No doubt, some of these distant tremors originate in the ocean bed; but the seaquake can only be localised when the water is put into a state of vibration sufficiently energetic to rock the ship and its loose objects and thus affect the senses. Vast waves are sometimes rolled in on the shores of continents, and are undoubtedly caused by some great disturbance beneath the ocean. Such waves have been known through a long period of history in the Eastern Mediterranean, where they have ravaged the shores of Syria and Asia Minor; and it is sometimes supposed that the great deluge on which the Ark of Noah floated was accompanied by a mighty sea-wave, rolled in upon the lands of Chaldea from the Persian Gulf. Off the Pacific coast of South America these waves arise most often and most mightily. They have been especially formidable in the angle where the coast of Peru meets that of Chili, and the harbours of Pisco, Arica, Tacna, Iquique, and Pisagua have been repeatedly subjected to these destructive invasions. Usually they are foreshadowed by a violent earthquake, and the inhabitants, taking warning, fly to the hills. The sea-wave does not, however, always follow the earthquake, but it appears often enough to arouse serious fear that it may come whenever the ground is strongly shaken. The first sign of the coming disaster is the withdrawal of the sea from the shore, leaving bare the bed of the harbour. A few minutes later the sea returns in a high, irresistible wave, which overflows the adjoining lands. Again it withdraws and again returns, and these oscillations may last for many hours, slowly diminishing in the amount of rise and fall till they die out.