PRESIDENTIA.L ADDRESS. 695 



elapse in order that tlie final temperature-rise in the sediments must be attained. 

 The question we have to answer is: Wil] the rate of rise of temperature due tc 

 radium keep pace with the rate of deposition, or must a certain period elapse after 

 the sedimentation is completed to any particular depth, before the basal tempera- 

 ture proper to the depth is attained ? 



The answer appears to be, on an approximate method of solution, that for rates 

 of deposition such as we believe to prevail in terrigenous deposits— even so great 

 as one foot in a century, and up to depths of accumulation of 10 kilometres and even 

 more — the heating waits on the sedimentation. Or, in other words, there is 

 thermal equilibrium at every stage of growth of the deposit; and the basal tem- 

 perature due to radio-active heating may at any instant be computed by the con- 

 ductivity equation. For accumulations of still greater magnitude the final and 

 maximum temperature appears to lag somewhat behind the rate of deposition. 



From this we may infer that the great events of geological history have 

 primarily waited upon the rates of denudation and sedimentation. The sites of 

 the terrigenous deposits and the marginal oceanic precipitates have many times 

 been convulsed during geological time because the rates of accumulation thereon 

 have been rapid. The comparative tranquillity of the ocean floor far removed 

 from the land may be referred to the absence of the inciting cause of disturbance. 

 If, however, favourable conditions prevail for such a period that the local 

 accumulations attain the sufficient depth, here, too, the stability must break 

 down and the permanency be interrupted. 



Upheaval of tlie ocean floor, owing to the laws of deep-sea sedimentation, 

 should be attended with effects accelerative of deposition — a fact which may not 

 be without influence. But although ultimately sharing the instability of the 

 continental margins, the cycle of change is tuned to a slower periodicity. From 

 the operation of these causes, possibly, have come and gone those continents 

 which many believe to have once replaced the wastes of the oceans, and which 

 with all their wpalth of life and scenic beauty have disappeared so completely 

 that they scarce have left a wreck behind. But those forgotten worlds may be 

 again restored. The rolled-up crust of the earth is still rich in energy borrowed 

 from earlier times, and the slow but mighty influences of denudation and deposi- 

 tion are for ever at work. And so, perchance, in some remote age the vanished 

 Gondwana Land, the lost Atlantis, may once again arise, the seeds of resurrection 

 even now being sown upon their graves from the endless harvests of pelagic life. 



APPENDIX A. 



Convective Movement of Uranium to the Earth's Surface (p. 680). — The estimate 

 of temperature given assumes (1) that the mass of igneous material is spherical, 

 and (2) that its surface is kept at constant teoaperature, heat escaping freely. 

 The first assumption is in favour of increasing the estimate of temperature, and 

 probably would not generally be true, especially of a mass moving upwards. The 

 second assumption tends to give a lower estimate of temperature, and is certainly 

 misleading, as the surrounding materials are non-conducting, and must favour the 

 accumulation of radio-active heat. 



On assumptions (1) and (2) and on Barus' results for the thermal expansion of 

 diabase between 1100° and 1500°,' and results of my own on basalt,* which are 

 in approximate agreement, and assuming the mean excess of temperature to be 

 500° and the surrounding material to be at a fluid temperature, the force of 

 buoyancy comes out at over 60 dynes per cubic centimetre of the spherical mass. 

 This is an under-estimate. 



If we may assume that the Deccan Trap is indeed an instance of such an over- 

 heated mass escaping at the surface, and that similar radio-active masses rising up 

 from beneath at various times in the past may have afl'ected the crust, we have at 

 our disposal a local source of energv of plutonic origin which may account for much. 



» Phil Mag., xxxv. p. 173. ' Trans. Ji.J).S., vi. p. 208. 



