Dr. G. J. Stoney on Denudation and Deposition. 559 



which we call heat, there the material in which these move- 

 ments can take place is compressible; in the sense that if 

 already compressed it will yield to any extra pressure great 

 or small tending to further reduce its bulk. We also seem 

 to know with certainty (as I think, and notwithstanding 

 what Dr. Chree has urged) that it is elastic also, that is 

 that if relieved of a part of the pressure it will recover its 

 pristine volume, provided that the temperature is kept from 

 changing ; and when Dr. Chree speaks of great pressure as 

 "killing the elasticity" he has perhaps not taken into 

 account the necessary dynamical consequence, which is that 

 the density would in that case increase without limit as time 

 goes on. 



5. In paragraph (3) we had occasion to speak of the so- 

 called solid part of the earth. This is because the earth is far 

 from being solid in the sense attributed to that term in 

 physical laboratories or by the mathematicians to whose 

 investigations respecting elasticity Dr. Chree refers. If we 

 desire to represent the earth on a small scale and within the 

 brief duration of a laboratory experiment, it will in the first 

 place be necessary to substitute for a solid material something 

 very much more plastic; nor will it be easy to find a material 

 which will represent, within the brief duration of human 

 experiments, the kind of plasticity by means of which nature 

 slowly carries on her work during the lapse of geological 

 ages — a plasticity which causes the several parts of the 

 earth to yield to pressures however feeble when time enough 

 is allowed, but where they yield so imperceptibly, even under 

 great pressures, that a sustained* force is at any one time 

 transmitted across any part of the earth with almost the 

 definiteness as regards its direction with which it would be 

 transmitted by a theoretically rigid body. Accordingly 

 when the earth is described as being plastic, what is to be 

 understood is that the materials of which it is made up yield 

 to any stresses tending to change their form, if these stresses 

 continue in action for sufficiently long periods of time. The 

 yielding seems mainly to consist in slips, which sometimes 

 take place abruptly and on a molar scale by the small starts 

 which occur when there are earthquakes or during volcanic 

 eruptions ; but usually they are molecular readjustments, 

 acting without breach of continuity, and very slowly, both on 

 a large and on a small scale. Some of the results on a small 

 scale are familiar to us as foldings in strata and such like 



* By a sustained force is meant one so gradually applied and so long- 

 maintained that it does not produce appreciable wave? within the 

 earth. 



