44 PRESIDENTIAL ADDRESS SECTION A. 



deflection from the perpendicular which should have been pro- 

 duced by these surface features was 30.37 seconds of arc, while 

 the mean deflection actually observed was only 2.91 seconds. 

 Hayford and Bowie conclude that the surface features are to 

 the extent of 90 per cent., compensated by deficiencies of density 

 below the land elevations and by excess of density under the 

 ocean bottom. Observations of the intensity of gravity on conti- 

 nents, sea-coasts and oceanic islands largely confirm these results. 

 The magnitude of the compensation will be better understood 

 if one notes that 2.91 seconds of arc is the deflection that would 

 be produced by a rectangular table-land 12 miles long, 8 miles 

 broad, and 662 feet high, consisting of rock of specific gravity 

 2.77, acting on a plumb-bob at a point one mile from the middle 

 of its longer side. Hayford and Bowie's calculations go to show 

 that the most probable depth of the layer of equal pressure 

 is about 76 miles. 



Now as the surface rocks of all continents are constantly 

 being worn away, and as this process has been going on for 

 enormously long geological periods, entailing a constant increase 

 of load on oceanic borders, it is clear that if no yielding had oc- 

 curred below the crust, the pendulum deflections would have had 

 much greater values than those which actually occur. We are 

 therefore forced to postulate a weak sub-crustal layer which has 

 adjusted itself to the varying load and by a slow flux backward 

 from below the ocean to below the continents has restored the 

 vertical loads to the approximate equality which we observe. To 

 small quick-acting forces this layer is highly rigid, just as a glacier 

 is when we walk on it ; to great slow-acting inequalities of load 

 with the changes of temperature and stress they involve, and with 

 the slow fall of temperature caused by conduction towards the 

 surface, this layer is plastic, just as a glacier slowly flows down 

 hill under its own weight. 



The lines of evidence I have tried so imperfectly to trace for 

 you point then to an earth which, on the whole, is highly rigid and 

 fairly stable, especially for gentle, quickly-changing forces, but 

 which possesses within itself certain slowly-acting elements of 

 instability, namely, a greater density of rocks under the oceans 

 causing a separation of land and water, with all the consequences 

 of denudation and deposition flowing therefrom, a high tem- 

 perature at no great depth beneath the surface crust with proba- 

 ble cooling, shrinkage, and a tendency to plasticity in a layer 

 beginning at a depth of from 20 to 100 miles, and finally an 

 irregular distribution of radio-active materials carried hither and 

 thither by water and wind, and ready wherever they accumulate 

 to evolve their heat and generate fresh centres of instability and 

 weakness. Such, I believe, to be, in dim outline, the structure of 

 the interior of mother earth as seen by the feeble but steady 

 gaze of the great geologists, mathematicians, physicists and biolo- 

 gists who have devoted their lives to its study. For clearer out- 

 lines we must trust to perseverance in the great paths marked 

 out for us by our forerunners. 



