There remains another question, however, to be answered, and it is, How were the mountains formed? Mountains are very closely connected with earthquakes, for nearly all the regions of the earth where the great disturbances take place are in the neighbourhood of great mountain ranges; and many, indeed most, of the students of earthquakes believe this to be the case, because the great weight of the mountains, especially when near the deep sea, induces pressure on the rocks, and consequently slipping and "faults." But the causes of mountain formation are very difficult to show with certainty. One such explanation, that of the continual shifting of portions by weight of the earth, we have already given. There is another one which may perhaps supply an additional cause. It is that just as "faults" produce earthquakes, perhaps in some cases earthquakes produce "faults." In the illustration we gave of the bottle-neck being broken by continual pressure, or the slide of a microscope suddenly breaking with a crash, because the increasing pressure of a screw was greater than it could bear, we have considered cases where the pressure was _slowly_ applied. But a tap with a hammer or any sudden shock would also produce a breakage--and "faults," and an earthquake on a small scale--and it is possible that some of the convulsions of nature and some of their permanent effects are caused by sudden and violent causes. One such cause might be the violent and sudden formation of steam by the contact of water with rock at a very high temperature. Everybody knows what happens to the kitchen boiler after a severe frost. The frost clogs the pipes with ice, so that the kitchen boiler becomes dry because no water is reaching it; but it continues to grow hotter and hotter till the iron plates or iron lining become red-hot. Then the frost perhaps gives way and a small amount of water finds its way into the red-hot boiler. The water is converted instantly into steam; and, as a result, the boiler, if it is a weak one, is blown out into the kitchen, causing grave personal inconvenience to the cook. If the boiler is a strong one, it may merely crack. Now, apply these considerations to the instance of the earth, its oceans, its thin crust, and its hot rocks situated at a depth of not more than thirty miles, and perhaps at a good deal less depth than that. What would happen if the ocean leaked through into the strata of red-hot or molten rocks? There would be enormous quantities of steam formed; and if, owing to the vast pressure of strata and water above, these quantities of steam did not instantly produce violent explosions, yet underneath there would be imprisoned, peak, forces of tremendous power which would only await a favourable opportunity in order to manifest themselves. The hot steam until this favourable opportunity arose would be absorbed by the rocks, just as hot steel can be shown to absorb gases. [Illustration: