Wegener's Displacement Theory. 339 



beneath the oceans. Some sixty years ago Archdeacon Pratt ^ 

 concluded that the " density of the crust beneath the mountains 

 must be less than that below the plains, and still less than that below 

 the ocean-bed ", and subsequent work has tended to confirm his 

 conclusion. Lord Kelvin suggested that the differences in density 

 may be due to differences in the composition of the original crust 

 formed when the molten globe consolidated. But Airy's explanation "^ 

 comes nearer to Wegener's conception. He assumes a lighter crust 

 resting upon a heavier substratum. The latter is supposed to have 

 some fluidity, but tbe fluidity may be very imperfect — he thinks 

 " it may even be little more than that degree of yielding which (as 

 is well-known to miners) shows itself by changes in tbe floors of 

 subterraneous chambers at a great depth where their width exceeds 

 20 or 30 feet". He proves that the crust, even if it be a hundred 

 miles in thickness, cannot of itself support a plateau a hundred 

 miles broad and two miles high. But since such plateaux exist 

 there must be a corresponding downward protuberance of the lighter 

 material into the heavy stratum beneath. They are, in fact, 

 supported by flotation. 



Airy's explanation was not universally accepted. The observed 

 phenomena of tides and of precession require an earth that is as 

 rigid as a ball of steel or glass of the same size, and it was thought 

 that such rigidity excludes even the small degree of plasticity 

 assumed by Airy. But many experiments, now well-known, have 

 shown that under sufficient pressure substances as rigid as steel may 

 flow, without becoming liquid in the ordinary sense. Rigidity and 

 elasticity are not qualities belonging solely to the substances con- 

 cerned. They depend also upon the forces acting and the length of 

 time for which they act. Perhaps the oldest illustration of the point 

 is embodied in the proverb, " A bow long bent at last waxeth weak." 

 A small, but long-continued stress may produce a permanent 

 change of shape, vv^hile a greater stress acting for a shorter period 

 may leave no lasting efiect. The rigidity and elasticity required 

 by the phenomena of tides, earthquake-waves, etc., may be com- 

 patible with the plasticity demanded by modern theories of isostasy, 

 or even by the theory of Wegener. The forces concerned are 

 different. 



Up to this point the views of Wegener were foreshadowed long 

 ago. That the floor of the oceans may consist of rock more basic 

 than the land-masses is believed by some, and that the lighter 

 land-masses dip into a heavier substratum is very generally 

 conceded. But Wegener imagines that these lighter masses have 

 moved lateraUy, and are still moving, and this is a very different 

 matter. There is the force of gravity to press them downwards 

 into the Sima, but there is no known force comparable in magnitude 

 with this to move them sideways. It is possible, however, that a 



1 Figure of the Earth, 3rd ed., 1865, p. 134. 



2 Phil Trans., vol. cxlv, 1855, p. 101. 



