114 



IGNEOUS AGENCIES, 



an earthquake. We conclude, therefore, that by far the most common 

 cause of earthquakes is either the formation of a great fissure or else 

 the readjustment of the walls of such a fissure. Even in the case of 

 the tremors accompanying volcanic eruptions, it is probable that their 

 true cause is the fracturing of the mountain by the eruptive forces and 

 the readjusment of the broken parts. 



Proximate Cause. — But whatever be our view of the ultimate cause 

 of earthquakes, there can be no doubt that the proximate or immediate 

 cause of the observed effects is the arrival of an earth- jar — the emer- 

 gence, on the earth- surface, of a succession of elastic earth- waves, pro- 

 duced by a violent concussion of some kind in the interior. Evidently, 

 therefore, the discussion of earthquake-phenomena is nothing more 

 than the discussion of the laws of propagation and the effects of elastic 

 earth-waves occurring under peculiar and very complex conditions. 



Application to Earthquakes.— Suppose, then, a concussion of any 

 kind to occur at a considerable depth (x, Fig. 96), say ten or twenty miles, 

 beneath the earth-surface, S S. Taking, for simplicity sake, the origin 

 as a point, a series of elastic spherical waves, similar to sound-waves, 

 will be generated, consisting of alternate compressed and rarefied shells, 

 the whole expanding with great rapidity in all directions until they 

 reach the surface at a. From this point of first emergence immediately 

 above the focus x, the still- enlarging spherical shells would outcrop in 

 rapidly-expanding circular waves, b" b", c" c," d" d" (Fig. 96), similar 



Fig. 96.— Section and Perspective View of a Portion of the Earth's Crust shaken by an Earthquake, 

 showing the origin, x, sections of the spherical waves, a, b, c, d, etc., and perspective of surface- 

 wave, b", c", d", etc. 



in form to water-waves, but very different in character. This we will 

 call the surface-iuave. The circles here drawn would equally represent a 

 series of waves, or the same wave in successive degrees of enlargement. 

 This surface-ivave would not be a normal wave propagating itself 

 like a water-wave. It would be only the outcropping or emergence of 

 the ever- widening spherical wave on the earth-surface. Both its velocity 

 of transit along the surface, and the direction of its vibration in relation 

 to the surface, will vary continually according to a simple law. The 

 direction of vibration, being along the radii x a, x b, x c, etc., will be 



