310 THE POPULAR SCIENCE MONTHLY. 



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 pla- 

 teau, 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 de- 

 rived 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 oc- 

 curred 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 



