1872.] OLDHAM AND MALLET — CACHAE EAETHQUAKE. 263 



path and to parallelism with the anticlinal axis ; but their actual 

 directions may vary to almost any extent, dependent on the con- 

 figm-ation of the rocky beds on which the masses p and r reposed, 

 and on the intestine motions of the masses themselves, dependent on 

 variable depth, nature of material, wetness or dryness, &c. &c. 



The surface of the fissure formed is in such direction as fulfils 

 the law of least action, i. e. in that of least resistance of the 

 mass fissured. This surface is almost always curved, usually of 

 double curvature, and conforms commonly more or less to the ex- 

 ternal contour of the mass before motion was given to it, and to that 

 of the unmoved bed on Avhich it was moved. 



These were the conditions of the chief part of the earth -fissures 

 examined by the writer after the Neapolitan shock of 1857, of most 

 of those of the Calabrian one of 1783, and generally of most earth- 

 quakes giving rise to fissures at all. 



But earth-fissures may also be produced in masses which do not 

 repose on inclined beds, but rest upon an almost, or quite, level 

 surface, as in the case of this Cachar earthquake. 



Here the same general dynamic relations as the preceding come 

 into play, though in a rather difi'erent manner ; and the essential 

 condition still is that the fissured mass must have had unequal 

 support at opposite sides of the plane of the fissure formed. 



Whatever may be the nature of the originating impulse generating 

 an earthquake-wave, its actual energy is employed mainly in com- 

 pressing the particles of the masses immediately surrounding the 

 centre of impulse, which, in virtue of their elasticity, transmit the 

 wave-motion (neglecting transversal vibrations) outwards or onwards, 

 through the surrounding masses, the volume in wave-motion at 

 any instant, i. e. the total volume of the particles approaching each 

 other up to the point of maximum condensation and receding from 

 each other to the point of quiescence, being constant. 



With the relation of this volume to the originating impulse, the 

 rate of translation of the wave, the maximum velocity of the wave- 

 particle, or the condition of decay or extinction of the wave, we are 

 not now concerned. 



At any point, however, in its transit the work or energy in the 

 wave is being expended in forcing new particles in advance into 

 wave-motion, as those in the rear give up their own motion and return 

 to rest ; and were the medium perfectly elastic and of infinite dimen- 

 sions, such transmitted motion would go on for ever. 



The resistance to the energy in the wave, however, is afforded at 

 any instant by the yet undisturbed particles in advance of it ; and 

 if these suddenly cease to exist, that is to say, if the elastic medium 

 suddenly terminates, the energy in the wave is unresisted in advance 

 of it, and must be otherwise expended. 



Under the actual circumstances of the imperfectly elastic solids 

 with which we are acquainted, it is expended by throwing off", or 

 tending to throw off, a layer or superficial stratum of the mass through 

 which the wave has passed from the terminal surface from which it 

 chiefly emerges : there is also at this surface generated a reflected 



