358 THE POPULAR SCIENCE MONTHLY. 



Let us look at the next theory. We all know that water is con- 

 tinually 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 devas- 

 tation. 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 con- 

 ceivable 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 con- 

 siderable 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 col- 

 umn 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 artifi- 

 cial 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 



