198 JR. A. Daly — Abyssal Igneous Injection. 



The shell above the zero-strain level is under tangential com- 

 pression. The shells beneath that level, for a considerable dis- 

 tance downwards, are under tension. On account of the weight 

 of all overlying shells any shell below the zero-strain level tends 

 to be stretched or (using Keade's term) to suffer " compressive 

 extension." This tendency increases with depth to a maxi- 

 mum in a level computed by Davison for a solid earth to lie 

 72 miles below the surface. The corresponding level for an 

 earth with a fluid substratum has been calculated by Fisher to 

 lie at depths of from 30 to 55 miles, depending among other 

 conditions on the temperature of solidification.* 



The hypothesis of a crust and fluid substratum. — There are 

 many reasons why the doctrine of the earth's complete solidity 

 is not acceptable to the working geologist. Some of the 

 cogent arguments against it have been summarized by De 

 Lapparentf and other able writers on the theory of the globe. 

 The strength of these arguments is great and by so much 

 favors the opposed doctrine of a fluid substratum supporting a 

 solid crust. The astronomic evidence for the hypothesis of 

 complete solidity rested at first on the calculations of Hopkins 

 in his well known paper on the combined effect of tidal pull 

 and internal fluidity upon the precession of the equinoxes. 

 Later analyses by Kelvin and Darwin proved that Hopkins' 

 conclusions could afford no " decisive argument against the 

 earth's interior liquidity." "Here we have a remarkable 

 instance of the flnal abandonment of an argument, which, from 

 the portentous difficulties of comprehending it, had proved too 

 hard for geologists to assail." J 



The stronger argument, based by Kelvin and Darwin on the 

 observed and calculated magnitude of oceanic tides, has like- 

 wise suffered destructive criticism by Fisher. The various dis- 

 cussions on this most complicated subject show clearly thai 

 not enough is known either of the constitution of matter or of 

 the oceanic tides themselves to permit of certain mathematical 

 determination of the earth's true rigidity. Fisher's luminous 

 work seems to prove that the geologist may still accept as the 

 best working hypothesis the view that the earth's "crust" is a 

 true crust and rests on a shell of fluid magma. Fisher has 

 demonstrated that, if the substratum is saturated with water- 

 gas, the bodily tide of the earth may entirely disappear, its 

 place being taken by a density tide in the substratum. This 

 would be true on account of the compressibility of the substra- 

 tum. § Since glass and presumably igneous magma are not only 



*Op. cit., p. 106. 



fSee the chapters on vulcanism in his " Traite de Geologic" 



%0. Fisher, op. cit., p. 38. 



§0. Fisher, op. cit., p. 61. 



