EARTHQUAKES. 113 



twenty-five feet.* In the Sonora earthquake of 1887 there was a fissure 

 for a hundred miles and a vertical slip of eight feet.f We might 

 multiply examples if necessary. Nearly all earthquakes, if carefully 

 studied, have shown such fissures and slips. Fissures and faults, 

 formed in previous geological times, are found intersecting the earth 

 in all directions. We see them, in these cases, formed under our 

 eyes, and in connection with earthquakes. 



Ultimate Cause of Earthquakes. — The connection of earthquakes 

 with the two other forms of igneous agency suggests each a possible 

 cause. Preceding and accompanying volcanic eruptions, especially of 

 the explosive type, occur subterranean explosions, which are often heard 

 hundreds of miles. Such eruptions are also accompanied with escape of 

 immense quantities of steam and gas. These facts, together with the 

 association of earthquakes with volcanoes, have suggested the idea that 

 the sudden formation or the sudden collapse of vapor is the cause of 

 earthquakes. According to this view, an earthquake is, on a grand 

 scale, a phenomenon similar to the jar produced by the explosion of a 

 keg of gunpowder buried in the earth. 



But the association of earthquakes with bodily movements of the 

 earth's crust over large areas, suggests another and far more probable 

 cause. It is well known that there are operating, in the interior of the 

 earth, forces tending to elevate or depress or to crush together laterally 

 the earth's crust. We shall discuss the nature of these forces here- 

 after. Suffice it to say now that in this way mountain-ranges are 

 formed and continents elevated. One effect of these forces is to break 

 the earth's crust into great blocks many miles in length and breadth 

 and several miles in thickness. These blocks do not remain in their 

 original position, but are always slipped one on another, producing dis- 

 placement often thousands of feet. Such great crust-blocks separated 

 by profound fissures, and slipped one on another, are found everywhere. 

 They are, in fact, among the commonest of geological occurrences. 

 They will be described in Part II. Some of these fissures are doubt- 

 less now forming ; many of them are still slipping. Suppose, then, 

 a subterranean force, tending to elevate a portion of the earth's crust, 

 and gradually increasing but resisted by the rigidity of the crust. It 

 is evident that the time would finally come when the crust would break, 

 by a fracture extending perhaps hundreds of miles in length and 

 through several miles in depth of solid rock. Such a fracture would cer- 

 tainly cause an earth-jar great enough to produce all the dreadful 

 effects of an earthquake. But, again, the enormous crust-blocks thus 

 formed would inevitably from time to time settle, or readjust them- 

 selves to new positions. Every such readjustment would also produce 



* Gilbert, Nat., vol. xxix, p. 45, 1883. f Science, vol. x, p. 81, 1887. 



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