PRESIDENTIAL ADDRESS SECTION A. 39 



ness of the deposit. When such deposits therefore occur on a 

 larg-e scale as in great earth troughs or geosynchnes, the con- 

 siderable rise of temperature that will ensue is likely to cause a 

 line of weakness to develop along the axis of the trough, and 

 the line is less likely than neighbouring areas to resist the hori- 

 zontal thrusts that will be caused by that general cooHng of sub- 

 crusted layers which we have pointed out as probable. It is to this 

 deposition of radio-active sediments in great geosynclines that 

 Joly attributes, with some degree of probability, the origin of 

 the great mountain chains of the Alps and Himalayas and analo- 

 gous systems. 



It has- been stated above that in spite of the high tempera- 

 ture of the interior revealed by mines and by volcanic action, and 

 of the supplies of heat generated by the original potential energy 

 and by the radio-active contents of its materials, it is impossible 

 to regard any continuous shell of it as being liquid ; and, in fact, 

 that the earth as a whole behaves towards quick-acting forces as if 

 it were a rigid body somewhat stiffer than first-class steel. 



Several lines o-f evidence concur in leading to this conclusion. 

 I shall refer to two. 



The first is the evidence of the tides. Kelvin and Love have 

 proved that if the earth had a liquid kernel or even if there were 

 a liquid shell below the crust, the crust would rise and fall under 

 the tidal forces of moon and sun to nearly the same extent as 

 the ocean would, and that the actual tides which depend on the 

 relative motion of crust and ocean would be much smaller than 

 they really are. The most accurate observations on the matter 

 are those of Michelson, who laid twO' horizontal iron pipes, each 

 500 feet long, underground, closed them and half-filled each 

 with water. He then observed that the water at one end of each 

 pipe went up and down, though a very minute distance, under the 

 tidal action, while the water surface at the other end went down 

 and up. He observed that the amount of rise and fall was less 

 than if the earth had been quite unyielding, but much greater 

 than was compatible with the presence of any liquid shell. The 

 axnount of yielding was such as would be expected from an earth 

 rather stififer than first-class steel. It is highly desirable that the 

 experiment should be repeated in various parts of the world, and 

 a beginning has been made therewith in South Africa. 



The general high rigidity of the earth revealed by such 

 experimeuts and by certain astronomical effects, has been fully 

 confirmed by the evidence oif earthquake waves, as shown on the 

 delicate earthquake recorders or seismometers which are now 

 to be found in many parts of the world, and of which we have 

 at least three in South Africa, namely, at Capetown, Johannes- 

 burg and Kimberley. Just as X-rays, by their passage through 

 the human body, reveal its inner structure, so do earthquake 

 waves, by their march through the innermost parts of the earth, 

 make a definite and undeniable revelation of their nature, and 

 I do not doubt that in the course of the next 20 vears we shall 



