The great prominence which we have given in the preceding pages to earthquakes is owing to the growing belief in the influence of earthquakes on the appearance and structure of those portions of the world's crust which are known to us. There are two views which we can take of earthquakes. One is to regard the larger number of them as being caused by slipping movements of the earth's crust. Looking at things in this way we should say that whenever there was a sudden break in the earth's strata, such as might occur (in accordance with an illustration given in a previous chapter) if all the level strata were broken up like a crumpled page of type--then that an earthquake would result. So that whenever we saw what geologists call a "fault" in strata we should know that an earthquake had occurred there. And why did it occur? Well, if we had a massive column of steel or of granite five miles high, the steel or granite at the bottom of the column would have to sustain such an enormous weight of material above it that it would begin to spread. If we had a pyramid of the same materials five miles high, the tendency to spread would not be so great, but still it would be there. Consequently, wherever there are high mountains there is a tendency of the earth strata beneath them to spread, perhaps slowly, but inevitably; and if there is any weakness in the structure of the rocks near the base of the mountain, then these will give way with a crash. A great "fault" will be produced, and with it an earthquake. People living on the earth will only see the results of the earthquake on the ground just immediately below their feet; and there these results are often very destructive to life and property; yet if they were all that happened, we should expect them to be covered up in time, and the "geological record" of an earthquake would not be a very important or even discernible thing a million years after it had happened. But are these things, which the eye of man can perceive, the only things that are happening during an earthquake? Is nothing happening underneath the earth which will leave its mark thousands of years after man has left the spot where the earthquake took place? May it not be that the earthquake is the outcome of some mighty force deep down in the earth; and may not this force cause both the earthquake and the geological "fault" which remains as the witness of its occurrence? If this be the case then the earthquake may be of enormous importance in geology.