26 A. J. Jukes- £ro7vne — Mellard Meade's Mountain Building. 



sediments must be withdrawn from the crust below, that this crust 

 losing heat will contract, and that the expanding sediment will 

 follow a retreating crust, so that no upheaval will take place at all, 

 the total volume of crust and sediment remaining unaltered. 



But these are not the conditions of Mr. Eeade's theory.^ The 

 crust is depressed by tlie weight of sediment, and it is the lower 

 part of the original crust which is the first to expand by receiving 

 heat from below. It is true that this only transfers the application 

 of Mr. Davison's argument ; the heat is conveyed from one layer 

 to another, and if the crust is receiving heat from the plastic under- 

 stratum, the latter will be cooled and lessened in volume unless its 

 loss is made good. But what is to prevent the loss from being 

 made good ? 



Mr. Fisher has a remark on this point which is quoted by Mr. 

 Davison, though he avoids the consideration of it. Mr. Fisher 

 observes that " there can be no absolute increase in the amount of 

 heat beneath the area in question except such as is supplied to it 

 laterally." ^ This means that in his opinion some heat could be 

 supplied laterally, and that heat lost by one part of the interior will 

 be made good by the conduction of heat from the surrounding parts 

 until the temperature of the whole is equalized ; this may be a 

 slow process, but it is surely a correct view, and one that Mr. 

 Davison is bound to consider. 



If the local loss of heat is thus distributed over the whole internal 

 mass of the earth, there will be an absolute increase in the amount 

 of heat below the sinking area, because the heat gained will be 

 localized in the thickened crust. This thickened crust, composed 

 partly of ancient crust material and partly of recent sediment, will 

 expand ; but how, in what manner, and to what extent, will it 

 expand? These are the questions now before us. 



Mr. Eeade assumes that as soon as the depressed tract of crust 

 begins to expand, the expansion will be localized and will form 

 a ridge parallel to the longer diameter of the depressed area. He 

 rests this belief on the results of certain experiments and observa- 

 tions made on the expansion of metal plates which did so ridge up 

 when heated. But the conditions of these experiments did not 

 resemble those of a weighted crust free to bend downwards, and 

 consequently they do not afford a sound basis for his relief. 



The ridging up of the leaden floor of a pantry-sink exj)0sed to 

 alternate expansions and contractions by the contact of hot and cold 

 water; or the heating of a sheet of lead which is screwed down to 

 a block of wood, cannot afford any clue to the manner in which 

 expansion would occur in a lenticular mass of material, heated from 

 below, slowly sinking and able to expand in almost any direction, 

 but especially in any upward direction. Without closely imitating 



1 Mr. Davison informs me that his note had no special reference to Mr. Eeade's 

 book, but was simply a criticism on the " fundamental principle " of the expansion 

 theory. 



^ Physics of the Earth's Crust, second edition, p. 123, repeated from the first 

 edition of 1881. 



