Let us look at the next theory. We all know that water is continually soaking into the ground, and is dissolving out all matter which it can. Limestone is one of these substances, and it is to this property that we owe the great caverns of Kentucky. Water not only works on or near the surface, but it even penetrates thousands of feet into the earth, and at this depth possesses even greater power of solution. Indeed, we have every reason to believe that deep down in the earth's crust vast caverns have been excavated by this erosion of water, and that in some places, especially limestone regions, these are numerous. Granting, then, that there are such great holes in the ground, and supposing that one of them should suddenly collapse, we can readily imagine a shock which would cause wide-spread devastation. But this theory also will apply merely to certain limited localities, and we have not yet found a general theory—one for all regions.
Let us travel for a moment in a mountainous region, and we will see thousands of feet of rock folded, twisted, and bent in every conceivable manner. Now, it is a well-proved fact that these rocks were originally deposited in a horizontal layer, and that they have since undergone contortion. It can be conceived, then, that such rocks must be in a constant state of strain ready to relieve themselves at any favorable moment. Let us suppose that we have a long board held down on each end by a bank of earth. Remove a part of one bank, and a point will be reached when the board will spring up with considerable force, striking anything above it a severe blow. This is the condition of our mountain-rocks. Material is constantly being eroded from some and piled upon others. The time finally comes when they snap or spring, and, striking the rocks above, send out an earthquake shock. Quarrymen frequently feel a slight shock, and immediately after hear a report, showing that the strain in some rock has been relieved. A most curious instance is reported where a very long column of granite rock was being split from a quarry by the use of wedges. At last the mass was split from the bed, and instantly it expanded itself with such suddenness as to produce a very perceptible jar; and when the rock was returned to the place where it originally lay it no longer fitted, and the drill-holes did not coincide as before. Here, then, we have an earthquake on a small scale started by artificial means. Probably many of the slight shocks result from similar causes, or from the action of frost, or by contraction and expansion caused by the daily change in temperature.
These, then, are the theories for earthquakes, but neither of the three can be called general. We can usually tell when volcanic action is the cause, and are thus limited in our choice to two. In some cases the cavern theory may apply, but these are rare, and can usually be determined, so that in any region not volcanic the third theory is the most probable. This accounts for the fact that earthquakes are much
