Page:Popular Science Monthly Volume 47.djvu/322

310 fact that the mountain ranges and continents are lifted so high above the normal level. To be sure, their weight is not so very great in comparison with that of the earth, nor the distance they project above the general level. But then the breadth of the base in comparison with the height is very great, and if we compute the thrust which so broad an arch as that of the Rocky Mountain plateau, for example, must exert on its abutments, we find that the earth, if not entirely solid, must have a solid crust some hundreds of miles thick; or else possibly that the density of the mountains and the part of the crust beneath them is much lighter than the average, so that they can rise by floating on a liquid interior to their present height. There are, in fact, some indications that these plateaus, and the continents generally, really have lighter matter beneath them than the sea basins do, so that the above argument against the fluidity of the earth has not much weight. Another more important argument for the solidity of the earth may be derived from earthquakes. Sometimes these convulsions of Nature are caused merely by the jar due to a giving way or cracking in the earth's crust. Such cracks we often find in studying the rocks, where on one side of the crack the beds do not match those on the other side, but a particular bed when it comes to the crack line is not found on the other side where we should expect it to come, but some distance to the right or left. Such cracks are technically known as faults, and the displacement produced is sometimes several thousand feet. Such faults or cracks have occurred in the red sandstone area of the Connecticut River, and are well marked. Similar faults have tilted the western plateaus in great blocks. Indeed, even the very line of displacement and sudden elevation have been sometimes noticed after earthquakes, notably in New Zealand and very recently in Japan, after the earthquake described by Koto, that cost so many lives (Fig. 3).

Now, these jars known as earthquakes spread with wavelike motion and decreasing intensity from their source, like the ripples from a pebble thrown into a pond. By careful study of the time at which the jar arrives at different points and of the direction of disturbance we can form some idea of its source, just as one can tell from the ripples at what point the stone was thrown in, even though too late to see the splash. In Japan, a country much afflicted with earthquakes—although, as a friend writes me, the shocks are commonly so slight that the only attention one pays to them is to stop shaving—their study has been so far advanced that one can actually tell what path a particle describes under the influence of a given quake, and what position it occupied at any moment, and a model of such a path was exhibited at the Chicago Exposition.

Let us suppose, for example, that the shock started from the