560 Dr. G. J. Stoney on Denudation and D eposition. 



phenomena ; which are most conspicuous in the older forma- 

 tions where the stresses that produced them may have been 

 in operation for enormous periods of time. I have seen a 

 slab of white marble which formed the lintel of a very old 

 chimney-piece that had so far yielded in less than a single 

 century to the small stress produced by its own weight, 

 aided by the alternate heatings and coolings to which it had 

 been subjected, that it had become bent until it was lower 

 by almost an inch in the middle than at the ends. The 

 length of the slab, so far as I can recollect, was about four 

 feet, its width about five inches, and its thickness about 

 | of an inch. There does not seem to be any solid material 

 known to us in which there is not some plasticity of this 

 kind — if bent, the molecular and intra-molecular motions 

 that are ever going on accommodate themselves in some few 

 instances to the new state of things ; and though the cases in 

 which this happens may be very few compared with the 

 whole number of molecular motions going on, every such 

 accommodation when once effected is a step towards the body's 

 assuming, in the long run, its new strained form. In a body 

 like white marble, which is a congeries of small crystals, this 

 accommodation may perhaps take place with most facility as 

 molecular slips parallel to the planes of cleavage or along the 

 junctions between crystals. In non- crystalline rocks, which 

 are a more irregular jumble of materials, the new adjustment 

 probably occurs with more facility in the positions of partial 

 discontinuity ; while in slaty rocks it seems to have operated 

 chiefly along the planes of cleavage, as well as (on a molar 

 scale, as in all stratified rocks) throughout the looser material 

 which is interposed between the strata. The slips which 

 bring about the adjustment assume their largest proportions 

 when they take the form of geological " faults," the pro- 

 duction of each of which is accompanied by an earthquake. 



6. Whether we are right in supposing that the adjustments 

 operate with most effect in the situations indicated above, what 

 is certain is that plasticity has brought about, after the lapse 

 of geological ages, changes of form in the parts of the solid 

 earth exposed to our view, which are accompanied by bendings 

 vastly more curved than those required for the movements on 

 a great scale which are everywhere going on within the earth ; 

 which force us to recognize that the earth though elastic 

 {i.e. resilient) as regards its volume is not quite elastic as 

 regards its form. That this is the case seems certain. 



7. If I understand Dr. Chree aright, he regards as incredible 

 any hypothesis which would imply that the earth, owing to its 

 elasticity, would expand from its present size, which is a globe 



