562 Dr. Gr. J. Stoney on Denudation and Deposition. 



were gradually removed. It follows from this, that a heavier 

 deposit would cause the new surface to be lower than the old 

 had been, and that by the imposition of a deposit of less 

 specific gravity the surface would be raised. The reverse of 

 these effects would take place on the removal of a superficial 

 layer. The observations seemed also to show that the size of 

 the loaded area, provided it exceeds a certain limit of size, 

 makes little difference in the effect. From this we may infer 

 that the plasticity of the earth acts in such a manner that the 

 loading of a large area produces for a limited time (which, 

 however, is probably many millions of years) nearly the same 

 compression of the portion of the earth between it and the 

 centre, as an equal loading of the whole surface of the earth 

 would produce upon the whole volume of the earth. If 

 we may assume this, we find that the earth's average 

 coefficient of compressibility has the value 1 assigned to 

 it, viz. about the same as the coefficient of compressibility 

 of glass. 



10. It is, however, of no particular importance whether this 

 estimate is correct. What is of importance and at the same 

 time certain is that the compressibility of the materials of 

 which the earth consists and their plasticity operate in such- 

 wise that a uniform pressure, great or small, over a large part 

 of the surface (if applied so gradually as to avoid generating 

 earth-waves, and also so as to give time for the plasticity to 

 act) will inevitably cause the surface to which the pressure is 

 applied to descend, except in those situations at which other 

 counteracting movements are going on. And, contrariwise, 

 a relief of pressure over a large part of the earth will of 

 necessitv cause the surface from ivhich the pressure is taken off, 

 to rise, unless prevented by interfering agencies. These 

 qualitative results are certain. 



11. As to what their quantitative value is, this must be 

 ascertained by observation of the apparent elevations and 

 depressions, since we do not possess the means of fixing 

 a priori either the average coefficient of compressibility or the 

 degree and kind of the earth's complex plasticity. As already 

 stated, a comprehensive survey of the geological evidence 

 seemed to show that the compressibility and the plasticity are 

 such that the limiting specific gravity of matter which leaves 

 the level unchanged when it is deposited on or removed from 

 the land-surface of the earth, is nearly 2, probably a little less 

 than 2. In other words, either the spreading of a layer of 

 lighter material over a sufficiently wide expanse of the earth's 

 land-surface, or the removal of a layer of heavier material, 

 will bring the new surface to a higher level than the old ; 

 provided always that other agencies, such as deep-seated 



