1882.] 77 [Crosby. 



vertical as average joints is usually so great that the vibrations 

 must lose their power to break the rocks before traversing it. This 

 objection may, I think, be sufficiently answered by calling atten- 

 tion to the fact insisted upon by Mallet, that the earthquake focus 

 is not a point, nor, normally, a space or cavity of equal extent in 

 all directions, but it is a fissure. So far as I know, every deter- 

 mination of the form of the earthquake focus that has been made 

 supports the conclusion that earthquakes usually originate in (i. e 

 the original impulse is due to) the formation, suddenly, of a more 

 or less extensive fissure. The enormous strains in the earth's 

 crust, arising primarily from the contraction of the interior, in- 

 crease until the limit of the resistance offered by the rocks is 

 reached, when the latter are crushed or ruptured along a plane 

 normal to the direction of the strain. The concussion or jar 

 attending the yielding of the rocks soreads in all directions and 

 constitutes the earthquake. 



These great crust-strains, whether of the nature of tension or 

 compression, are usually conceived as horizontal ; and hence we 

 must think of the resulting fissures as normally vertical, or, at 

 least, as making a large angle with the horizon. Now it is rea- 

 sonable to suppose, and the supposition agrees exactly with 

 Mallet's conclusions based upon the Neapolitan earthquake of 

 1857, that the amplitude of the vibrations is greatest, and hence 

 that they spread with destructive force farthest, in directions at 

 right angles to the plane of rupture, the isoseismal curves being 

 elongated ellipses having the fissure in which the shock originates 

 for the minor axis. In other words, the shock or blow being 

 delivered upon vertical surfaces, the resulting vibrations ot 

 greatest amplitude and breaking power run horizontally, and pro- 

 duce vertical fractures, from the beginning, the vertical fractures 

 or joints, according to this view, commencing at the seismic verti- 

 cal, and being parallel with the primordial fissure. 



Where the chief part of the energy represented by an earth- 

 quake is delivered and propagated horizontally and at a consider- 

 able depth, the effect upon the surface, as regards the destruction 

 of human life and property as well as the developement of joint- 

 structure, must be comparatively slight; and we have, therefore, 

 cause to be thankful that earthquakes do originate mainly in ver- 



