I907.1 AND MOUNTAIN FORMATION. 395 



These combinations of circumstances are not the dynamical cause 

 of the earthquakes, but only the occasion for outbreaks when insta- 

 bility is already developed. It might thus be that earthquakes would 

 break out when the polar motion is changing most rapidly, but the 

 earthquakes do not cause the change; rather they are occasioned by 

 that which makes the change. Milne's view above quoted does not 

 seem to differ much from this, for he says that " both effects may 

 arise from the same redistribution of surface material by ocean 

 currents and meteorological causes generally," though he overlooks 

 the leakage of the oceans as the cumulative dynamical cause of in- 

 stability. 



On these grounds probably very few will agree with Professor 

 Milne, if he is the reviewer quoted in Nature, that the present ex- 

 planation fails. The facts which he assumes are not proved, and 

 even if they were, the interpretation should be exactly the reverse 

 of that which he has given in Nature. Consequently we may dis- 

 miss this whole criticism as not well taken. Nothing is more certain 

 than that movements of large masses within the earth adequate to 

 displace the polar motion do not take place. A supposed effect of this 

 kind is contradicted by all that we know of the rigidity and solidity 

 of the earth under pressure of its own mass, which would prevent 

 deep movements, and by the well-established fact that all earthquakes 

 are shallow and the shock limited to one locality. Movements in 

 the sea and in the atmosphere, however, might conspire to displace 

 the pole and vary its rate of movement about the mean position, and 

 the stresses thereby exerted upon the globe might occasion seismic 

 outbreaks where the steam pressure had already approached the 

 limits of stability. If the facts assumed by Professor Milne, are 

 confirmed by time and experience, it is in this way that they must 

 be explained ; the outbreak of earthquakes being occasioned, but not 

 caused, by the surface movements of water and air which displace 

 the pole from its mean position and vary its rate of movement in 

 different parts of the path, and thereby also the stresses exerted 

 upon the different parts of the already unstable revolving globe. 



§ i6. On an Explanation of the Squeezing up of the Continents 

 by the Settlement of the Ocean Basins given in Some Works on 



