Dr. Gr. J. Stoney on Denudation and Deposition. 561 



about 8000 miles across, to one of 12,000 miles in diameter, if 

 the mutual gravitation of its parts were to cease, and if (which 

 is a further necessary condition) that great amount of energy 

 were imparted to it which would enable its several portions 

 while expanding to retain the same temperatures which they 

 have in their present compressed condition. Now, it is certain 

 that under the circumstances supposed the earth would expand; 

 and I see nothing antecedently improbable in the expansion 

 having the amount which Dr. Chree thinks incredible, or one 

 larger than it, or smaller. It is to be observed, however, that 

 no authority attaches to the amount of expansion arrived at by 

 Dr. Chree* s mode of applying my provisional hypothesis, since 

 he has taken no account of the fact that coefficients of expansion 

 are functions of both t and jJ, as is shown above in paragraph 2, 

 and that accordingly, if the temperatures are kept constant, the 

 average value of these coefficients within the earth will vary 

 with the pressures, and therefore change as the expansion 

 goes on. 



8. Dr. Chree's comments have left me under the impression 

 that he conceives that 1 infer what has occurred geologically 

 from a value of the average elasticity assumed beforehand ; 

 whereas the process I have attempted to follow is the converse 

 of this. I infer the average coefficient of compressibility from 

 the observed facts of geology ; and I endeavour to show in my 

 paper that the value so inferred, or something near it, must 

 be the true value of this constant, if the compressions and 

 expansions undergone by portions of the earth when loaded or 

 relieved from load have been a machinery by the instrumen- 

 tality of which nature has for ages been engaged in slowly 

 augmenting the inequalities of the earth's surface. 



9. It is from a consideration of the facts of geology, and 

 especially of those under the ocean, that we can deduce the 

 most definite estimate of what the average coefficient of com- 

 pressibility of the earth must be to produce the observed 

 effects. A careful study I gave to this subject many years 

 ago led me to form the opinion that all the geological data 

 I could then collect — terrestrial and oceanic — point to the 

 conclusion that if a layer of material of twice the density 

 of water were uniformly spread over any large portion of the 

 land-surface of the earth, the new surface would (after the 

 lapse of a sufficient time, and unless counteracting agencies 

 are in operation) be nearly at the same level as the old. And, 

 further, that the condensations and other movements within the 

 earth, to which the slow imposition of this deposit would give 

 rise, are of a sufficiently reversible character to enable the 

 surface to resume nearly its original position if the deposit 



