56 OLDHAM : THE CACHAR EARTHQUAKE OP 10TH JANUARY 1869. 



homogeneity in the cla3's will cause an irregularity in the fissures as 

 great or even greater than where the wave is normal to the river 

 bunks. 



The effects above described are produced almost, or quite, entirely by 

 the mere throwing off of masses of clay, and the influence of those resist- 

 ances tending to restrain the motion of the independent masses of clay 

 have not been considered, except so far as they offer a resistance to 

 actual rupture. We may assume that the effect of the momentum im- 

 pressed on the clay is the same as that of a single force, equal to it in 

 intensity, and acting at the centre of gravity of the detached mass, in the 

 direction of the wave-path ; if then the resultant of all the resistances 

 passes through this same point, the centre of gravity, the mass will be 

 thrown off without any tendency to overturning and the surface of the 

 detached portion will retain its horizontality; but if, on the other band, 

 this resultant does not pass through the centre of gravity a dynamic 

 couple will be set up tending to overturn the mass, the amount of over- 

 throw being determined by the moment of the couple and the resistance 

 of the contiguous masses of clay against which it may be thrown. 

 To this cause, combined with the actual Assuring of the ground, the 

 wildly broken and chaotic appearance, so strikingly shown in PI. II 

 fig. 2 and V, fig. 2, is due, which on examination is seen to be entirely 

 owing to the fact that each separate portion of clay is more or less over- 

 turned on its base. 



Having thus disposed of the earth -fissures, we may now turn to 

 another very remarkable feature, likewise a mere secondary effect of 

 the earthquake, and, like the above-mentioned fissures, due to the 

 existence of a soft waterlogged stratum underlying a thick series 

 of impermeable clays, and like them never found where this soft stra- 

 tum does not occur, though not always found when it is present. 

 The feature referred to is the production of the so-called ' volcanic 

 mud craters/ about which there is nothing volcanic, being produced 

 entirely by the washing down of soil by water, previously forced up by 

 the earthquake shock, and which after the passage of the shock once 

 ( 56 ) 



