312 THE POPULAR SCIENCE MONTHLY. 



center of the earth. If we neglect the slight bulging of the earth 

 at the equator, and suppose it a perfect sphere, the shock of the 

 earthquake wave would reach every point of the earth's surface 

 at the same time. It would tend to throw objects vertically up- 

 ward. Moreover, the intensity of violence would be equal at 

 every point. Now, we can see from the diagram (Fig. 2) that the 

 nearer the surface the source, the smaller will be the area of prac- 

 tically simultaneous first arrival (A, B), the smaller will be the 

 area of vertical shock, and the more rapidly will the intensity de- 

 crease from a point of the surface directly over the source (E). 



From such considerations the depths of the sources of various 

 earthquakes have been computed. For example, Schmidt com- 

 puted that the Charleston earthquake started from a depth of no 

 less than one hundred kilometres, say sixty miles. Unfortunate- 

 ly, there has been much difficulty in getting reliable facts enough 

 for these estimates, and Dutton, who investigated the same earth- 

 quake for the United States, made it but twelve or eighteen miles 

 deep. But whether it be one depth or the other does not affect 

 what we wish to show namely, that the earth is capable of 

 cracking to a depth such that if the earth's heat increases at any- 

 thing like the ratio that it does near the surface, it must there be 

 more than white hot, and would be molten and freely fluid, except 

 for the counteracting effect of pressure. If, then, the earth is 

 solid at this depth, pressure has more effect than heat and keeps 

 the earth solid. Barus has shown by experiment that for the 

 basic rocks pressure tends to solidify. Moreover, the most basic 

 rocks we know, those apparently from the greatest depths, con- 

 tain fragments of chrysolite, etc., whose rounded and corroded out- 

 lines and often blackened edges show plainly that they have been 

 in process of dissolving in the lava. They therefore may repre- 

 sent fragments of deep-seated rocks which have liquefied when 

 pressure has been relieved by cracks and the eruption of lava fol- 

 lowing thereon. 



The fact that we find the rocks in some places crumpled in 

 folds and recrystallized has been by some taken to indicate that 

 such rocks had been buried so deep beneath the surface as to be 

 remelted. But recent investigations, by cutting thin sections of 

 such rocks and studying them under the microscope, have shown 

 that a rock may be thoroughly changed into different minerals, 

 differently interwoven, and may be folded and contorted in most 

 complex fashion, without for a moment being molten or ceasing 

 to be crystalline. Recent experiments have also shown that we 

 may account for the folds and crumplings without supposing a 

 thin, flexible crust lying over a fluid interior ; while, on the other 

 hand, there are very numerous faults or cracks, where one part 

 has slidden down on the other, that can hardly be accounted for 



