MODULUS OF ELASTICITY OF ROCKS. 11 



traces one and the same curve when a specimen is bent and 

 unbent many times cyclically, the direction of the elongation of 

 the curve does not remain fixed when the amplitude of the cycle 

 — i.e. the greatest amount of stress applied to the specimen during 

 the cycle — is varied. As a general rule, the hysteresis curve 

 becomes more and more vertical when the amplitude of the 

 cycle is further and further increased. One instance is given in 

 Fig. 9. 



The amount of hysteresis, which is to be measured by the 

 area enclosed by the curve or by some function of it, is least for 

 Archaean rocks and increases rapidly for new rocks. 



Although the modulus of elasticity is never constant during 

 a cyclical strain, its variation obeys one and the same law for 

 both the on- and the ofP-curve, in so far as the centre of the 

 cycle coinsides with the neutral state of the specimen. It may 

 be necessary to make a remark on the meaning of the term 

 " Modulus of Elasticity." As there is a great amount of hysteresis 

 in the relation of stress to strain, the ordinary conception of the mo- 

 dulus of elasticity is ultimately vague and uncertain. The actual 

 resistance to the deformation in any state whatever, be it already 

 bent or twisted, elastic or plastic in that state, will be taken as 

 the measure of elasticity in that state, so that in the present 

 experiment, the modulus is measured, step by step, by the in- 

 crease of bending per 200 grams increase of the effective mass. 



The curve expressing the relation between the modulus of 

 elasticity and the amount of strain is symmetrical \vith respect to 

 the axis of ordinate. One instance for Limestone No. 29, is 

 given in Fig. 10. Each kind of rock seems to have its own 

 special character. If Hooke's law were to hold good, four branches 

 of curve, of which the right and left branches correspond to the 



