3^2 NATURE OF EARTHQUAKES. 



Darwin's Views. According to the comprehensive idea of Dar- 

 win, earthquakes and volcanic eruptions originate in some local frac- 

 ture and displacement of the bed of the neighbouring ocean ; the 

 volcanic effect spreads as far as the subterranean sea of molten rock 

 extends, but is excited to violence at one or more points, the most 

 favourably circumstanced at the time ; the convulsive movement is 

 propagated through the solid and liquid contents of the crust of the 

 earth, as far as the nature of these materials and the force of the blow 

 permit. It is important to remark that a blow is struck the earth- 

 quake is the evidence and this can only mean some rending of the 

 solid crust of the earth. Thus a convulsion seems the necessary pre- 

 cursor of the earthquake ; and to allow of the movement traversing 

 whole continents, we must suppose the blow to be given at a consi- 

 derable depth, expressed in miles, below the surface. 1 



Michell 2 entertained the idea that the sheet of rocks constitut- 

 ing what we know of the crust of the earth was flexible enough to 

 bend to the agitation of an interior liquid ocean of melted rock. 

 The earthquake in this view is a wave in the rocks representing a 

 tide in the subjacent fluid. 



Rogers 3 supposed waves of this kind, moving parallel to certain 

 axes, to be effective in permanently raising the ground ; and attributes 

 to such an agency in early geological periods the formation of the 

 anticlinal and synclinal hollows parallel to the Appalachian chain. 



Mallet's Views. Mallet, 4 however, both by reasoning and ex- 

 periment, has shown that the earthquake movement is a wave of elas- 

 tic compression, whose rate of movement varies according to the elas- 

 ticity of the medium and the continuity of rock masses. There is 

 one rate of movement for the sea, another for the land ; one rate for 

 solid granite, another for broken granite, another for loose sand. 

 This movement near the surface is far less rapid than the elasticity 

 of the media might lead us to expect ; a circumstance dependent on 

 the many fissures of the rock, at each of which discontinuity and loss 

 of motion are occasioned. 



Velocity of Earthquake Waves. The actual velocities of some 

 earthquake movements have been approximately ascertained as under 

 in a line directly across the wave : 



Conception, earthquake, 1835 . . 30 . . Rogers, Rep. to Brit. Assoc. 1843. 

 Guadaloupe, .... 1843 . . 27 . . Do. do. 



The Conception earthquake had its crest directed ET.N.E. from the 

 western border of Alabama to Cincinnati, a distance of 500 miles. 

 On this line it was felt simultaneously. The motion was to the 

 E.S.E., and was felt at successive times simultaneously on lines 

 directed to the JST.N.E. If a measure of velocity were taken on the 



1 Stukeley assigned the improbable depth of 200 miles for the forces of an 

 earthquake in Asia Minor, A.D. 17, which embraced a circle of 300 miles in 

 diameter. 



2 Phil. Trans., 1760. 3 Brit. Assoc. Reports, 1843. 

 4 Mem. Royal Irish Acad., 1846 ; Brit. Assoc. Reports, 1850, et seq. ann. 





