496 Rev. 0. Fisher — Compression of the Earth's Crust. 



yielding of the substratum is also testified by the phenomena of 

 isostacy. I have therefore endeavoured to estimate the amount 

 of corrugations which would be produced by a cooling globe also 

 on this hypothesis. But although they would be slightly greater 

 than in the case of a solid globe, they still fall far short of those 

 actually existing, I therefore argue that the corrugations of the 

 crust are not due to the shrinking of the interior away from the 

 cooled ci'ust, whether we regard the interior as solid or liquid. 



If it be asked what my views are upon this vexed question, I may 

 be allowed to say that I have published them fully in my " Physics 

 of the Earth's Crust." In it I have given reasons for believing that 

 the substratum is affected by convection currents, and that these 

 ascend beneath the oceans, and flowing horizontally towards and 

 beneath the continents, and descending beneath mountain chains, 

 are the cause of the compression of the crust, and other disturbances, 

 of which we are in search. 



Before giving my reasons for believing that upward convection 

 currents exist beneath oceans, it is in the first place necessary to 

 combat the dictum of leading physicists that the interior of the 

 earth is solid. It has been asserted that, unless the earth is extremely 

 rigid, bodily tides would be produced, and that there would be no 

 rise and fall of the water relatively to the land. If the earth was 

 a smooth spheroid covered with a uniformly deep ocean this would 

 no doubt be true. But as matters stand, the tides of short period 

 are affected by local irregularities known as the establishment of 

 the port. If the substratum of the crust is liquid, isostacy requires 

 large protuberances of its underside, which would cause irregularities 

 in the tides in the magma analogous to those in the ocean, and, 

 unless these agree in time, in height, and in place, with the water 

 tides, the latter will not be obscured by them, and may even be 

 augmented. 



Of tides of long period the fortnightly is the most important ; 

 but I think I have shown in the Appendix to my " Physics of the 

 Earth's Crust " that it had not been proved by fifteen years 

 of observation that any such tide existed,^ which would be an 

 argument in favour of the liquidity of the interior. 



The peculiarities of the transmission of earthquake waves to gi'eat 

 distances through the body of the earth have been appealed to, as 

 approving to all, " except some geologists,"^ that the earth is solid. 

 The disturbance first arrives as a series of minute tremors. These 

 have been considered to be waves of compression. They are soon 

 followed by somewhat larger disturbances, which have been con- 

 sidered to be waves of distortion. Since waves of distortion could 

 not be propagated in a liquid, it is maintained that the earth is 

 hereby proved to be solid. In reply to this argument I have shown 

 that, if a liquid magma holds gas in solution, two types of waves 

 will be propagated through it with different velocities. Tremors 

 will first arrive due to the compressibility of the magma, and 

 subsequently waves caused by the extrusion of gaseous vesicles due 



1 p. 34. 2 Darwiu's " Tides," p. 236. 



