218 Dr. Harold Jeffreys — Causes of Mountain-Building. 



Oa the other hand, the recovery after stress may be slow, though 

 after a long enough time it may become complete. This phenomenon 

 is the elastic after -iv or king. It does not lead to permanent set or to 

 indefinite flow when a constant stress is applied, and must therefore 

 be distinguished from plasticity, even though the same substance 

 may possess both properties. There are then two distinct kinds of 

 imperfection of elasticity, and the question is further complicated by 

 the fact that both are functions of the stress and of the previous 

 history of the body, some substances behaving as if perfectly elastic 

 for small stresses and very imperfectly so for large ones. For 

 instance, suppose a weight laid on a flat surface of wet clay, which 

 may be regarded as plastic. It will proceed to sink in, the clay 

 acquiring permanent set, but after a time the stresses will become 

 too small to cause set, and the weight will cease to sink, though 

 appreciable shears still exist. 



The distinctions between these various properties are of funda- 

 mental importance in all questions dealing with the behaviour of 

 rocks under stress. The idea of plasticity, in particular, must 

 always be carefully distinguished from smallness of rigidity. If two 

 similar pieces of quartz and copper, for instance, are exposed to the 

 same stress, the quartz will yield more ; but when the stress is 

 released the quartz springs back all the way, while the copper does 

 so only partially. Thus copper is more plastic than quartz ; on the 

 other hand, the change of strain caused instantaneously by the same 

 change of stress is less in the copper, which is therefore more rigid. 

 In geophysical investigations the fact that a very rigid substance 

 may also be a plastic one is continually coming into notice. 



In geological upheavals and readjustments elastic after-working 

 is probably of small importance, as the times involved are much 

 longer than those needed for the relaxation of the strain. The 

 importance of plasticity on the other hand is very great, for solid 

 siibstances may easily flow to a great extent when a lapse of time 

 of the order of a geological period is available, without the flow 

 producing any noticeable effect when earthquake waves or tides are 

 considered, so that for these movements the earth may behave as if 

 perfectly elastic. Elastic after-working acts in the opposite direction, 

 for if a stress is varying rapidly there will never be time for the 

 strain appropriate to it to be produced, and consequently short 

 period transverse waves cannot be transmitted for any considerable 

 distance. It would thus produce a greater effect on earthquake 

 waves than on vibrations of longer period, and we may therefore 

 infer from their transmission that it is not appreciable in the crust 

 of the earth. The remarkable effects of high pressure and terape*'a- 

 ture on the elastic properties of solids indicate, however, that it 

 would be dangerous to deny on this ground its possible importance 

 in the centre of the earth. It is certain that no great part of the 

 earth is fluid, for it has been shown by Love that the yielding of the 



increased beyond all limit, as Mr. Deeley supposes, it would obviously be 

 impossible to model it by hand. Non-ductile substances break when the set 

 becomes great enough, and after this occurs the series will diverge rapidly 

 instead of converging. 



