52 OLDHAM: THE CACHAIt EARTHQUAKE OF 10TEI JANUARY 18G9. 



other, and so long as the motion was sufficient to produce a separatiou 

 of the mass by cracks at all, these cracks or fissures must have followed 

 the line of least resistance and must therefore necessarily have been 

 nearly parallel to the face of the bank. Though from this cause the 

 direction of Assuring varied from north and south to nearly east and 

 west, it was here evident that the main lines of motion must have been 

 in some line between east and north, or even, to note it more parti- 

 cularly, between north-east and north. 



* * -x- * # * 



One conclusion pointed out in the foregoing pages cannot be too 

 strongly insisted on, and even at the risk of wearisome repetition, I 

 will say that nothing is so conclusively shown, or so certain, as that 

 these great earth -fissures and cracks were in every case purely secondary 

 results of the earthquake shock ; there being no reason to suppose that an 

 earthquake shock can, but on the contrary every reason to suppose 

 that it cannot in any case, by its direct action, open fissures in solid 

 rock or clay. 1 



In order to make the mechanism of the formation of these fissures 

 clear, I will first take the case in which the beds overlying the 

 soft yielding stratum, already mentioned, may be supposed to be 

 perfectly homogeneous, and the direction of the wave-path in azimuth, 

 normal to that of the river bank. As the wave passes on its way, the 

 particles of clay are forced, first forwards and upwards, and then return 

 backwards and dowuwards to their original position ; but as the vertical 

 movement takes but little part in producing these earth-fissures, we 

 may for the moment neglect it and only consider the effects of the 

 horizontal motion impressed on the particles of clay. We may take 

 it then that the belt in which the particles of clay are in motion at any 

 one moment, under the influence of the earth-wave, is divided into two 

 equal or nearly equal zones, in the first of which the particles are in 

 motion forwards while in the other they are moving back ; and as the 



1 For a discussion of this same point, Refer Quart. Journal Geological Society, 

 London, Vol. XXVIII, p. 255, 1872. 



( 52 ) 



