Earthquakes bring about many changes on the surface of the earth. For example, on mountain-slopes forests are shattered, and large masses of soil and débris are shaken loose from the rock on which they rested, and hurled into the valleys; streams are thus choked up, and sometimes lakes formed, either by the damming up of a river or by the subsidence of the ground. It is frequently found after an earthquake that the level of the ground has been permanently altered; and this effect of earthquakes is important in connection with the subject we are now considering,--namely, how mountains are upheaved. Sometimes, it is true, the movement is a downward one; but more generally it takes place in an upward direction. As an example of this, we may mention the Chilian earthquake of 1835, which was very violent, and destroyed several towns on that coast, from Copiapo to Chile. It was afterwards found that the land in the Bay of Conception had been raised four or five feet. At the island of Santa Maria, to the southwest of this bay, the land was raised eight feet, and in one part ten feet; for beds of dead mussels were seen at that height above high water, and a considerable rocky flat that formerly was covered by the sea now became dry land. It was also proved by means of soundings that the sea round the island was shallower by about nine feet. Now the question arises, "How are earthquakes caused?" Various suggestions have been made; but it is pretty clear that all earthquakes are not produced in the same way. For instance, volcanic eruptions are frequently attended by earthquakes. Violent shocks of this nature generally precede and accompany a great eruption, as is frequently the case before an eruption of Mount Vesuvius. Steam plays a very important part in all volcanic eruptions; and these earthquakes are probably caused by great quantities of pent-up steam at a high pressure struggling to escape. It is also possible that when molten rock is forcibly injected into the crevices and joints of overlying rocks earthquake shocks may be produced by the concussion. The old Roman poet and philosopher, Lucretius, endeavoured to solve this problem, and concluded that "the shakings of the surface of the globe are occasioned by the falling in of enormous caverns which time has succeeded in destroying." But though the explanation might possibly apply to a few cases of small earthquakes, it is not a satisfactory one, for it is not at all likely that many large cavities exist below the earth's surface, because the great weight of the overlying rock would inevitably crush them in.