FLEXURE OF ROCK. 32I 



tending to compress and the other to bend the layer. If 

 this bending component be great enough, the result will 

 be the same as when the ultimate strength of any rigid 

 substance is overcome, viz., deformation with rupturing; 

 if it be below the ultimate strength but above the elastic 

 limit and be applied gradually, flow, or in other words 

 deformation without rupturing, will take place. 



But what is the elastic limit? Experiments, as well as 

 common observation, show that the limit of elasticity de- 

 pends upon the time. Thus, if a rod or bar of glass, ice 

 or stone, supported at its ends be struck in the middle, it 

 will either break or regain, approximately, its original 

 position, showing that the limit of elasticity is practically 

 as high as the ultimate strength. On the other hand, if, 

 by means of small weights, pressure be applied gradually 

 and slowly, the same rod may be bent, and in time will 

 exhibit a permanent deformation or set. ^^ This shows 

 that even the time of a laboratory experiment, necessarily 

 limited, is suflicient to make the elastic limit, and cor- 

 respondingly the force necessary to produce flow, much 

 lower than in the case first supposed. 



All this suggests the following conclusion : Assuming 

 that the resultant of all the forces acting upon the particles 

 at any point in a layer of rock, if below the ultimate 

 strength and above the elastic limit of the layer, will pro- 

 duce flow in its own direction, we can say, that if the 

 proper ratio between this resultant and the time through 

 which it acts be maintained, flow will ensue, even though 

 the resultant be indefinitely diminished. There seems to 

 apply here the mathematical law that a side which has 

 any per cent, in its favor, no matter how small, wall, if 

 given time enough, ultimately win. 



10 Miall in Pop. Sci. Eeview, Jan., 1872. 



