262 



PKOCEEDINGS OP THE GEOLOGICAL SOCIETY. [April 10, 



rise to the fissure shall have been previously in a state not very far 

 removed from one of static instability. 



Thus, for example, if fig. 2 represent a transverse section of a 

 mountain-line on both flanks of which incoherent formations (i. e. 



Fig. 2. — Ideal Section of Mountain. 



clays, gravels, sands, &c.) repose, and a shock transverse to the. anti- 

 clinal axis be transmitted through the whole in the general direction 

 a to b, the effect of the wave of shock in its transit through the 

 entire mass will be, at the arrival side S, to cause the incoherent beds 

 2^ lying on the inclined flank to slip downward during the first 

 semipliase of motion of the wave-particle. 



Actual slippage must occur to a greater or less extent whenever 

 the energy of the impressed movement upon the mass oip is equal to 

 or greater than the resistance due to friction of the mass against its 

 own inclined bed, or of its own materials against each other in the 

 plane of the angle of their repose, usually denominated ^ by writers 

 on mechanics. During the same first semiphase the incoherent 

 masses r reposing on the flank N are pressed more firmly against 

 their inclined beds ; but the instant after the wave-particle has 

 attained its maximum velocity, and so the second semiphase (or 

 reverse movement of the oscillation) commences, the ordinary pressure 

 of these masses perpendicular to their beds (due to gravity) is more 

 or less reduced, the masses themselves, to which forward motion in 

 the direction a to 6 had been previously given, tending now to be 

 left behind. And as their equilibrium in situ depends upon their co- 

 efiicients of friction, either internal or upon their beds, and the effect of 

 these is greater as the pressure on the bed is greater, so the tendency 

 to slip is now induced on all the masses at the side JST through the 

 momentary reduction of their pressure upon their beds. 



Thus the conditions for slip are first produced on that side of the 

 chain from which the wave of shock travels, and afterwards on that 

 side towards which its transit is directed. In the second semiphase 

 the tendency to slip in the beds p is diminished, as the movement 

 of the wave-particle (now from h to a) tends to force them up their 

 inclined beds. 



The fissures, if produced, may be only inceptive, and the slip be 

 limited to the direction transverse to the wave-motion nearly ; or the 

 slip, once begun, may assume all the magnitude of great landslips. 



In either case, and at both sides of the chain, the general direction 

 of the fissures, if produced, will tend to one transverse to the wave- 



