Page:Popular Science Monthly Volume 47.djvu/324

312 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 upward. 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 practically simultaneous first arrival (A, B), the smaller will be the area of vertical shock, and the more rapidly will the intensity decrease 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 computed that the Charleston earthquake started from a depth of no less than one hundred kilometres, say sixty miles. Unfortunately, there has been much difficulty in getting reliable facts enough for these estimates, and Dutton, who investigated the same earthquake 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 anything 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, contain fragments of chrysolite, etc., whose rounded and corroded outlines and often blackened edges show plainly that they have been in process of dissolving in the lava. They therefore may represent fragments of deep-seated rocks which have liquefied when pressure has been relieved by cracks and the eruption of lava following 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