ffiESIDENTlAL ADDRESS. 693 



however, tliat the assumed factors may be supposed to vary considerably, and yet 

 the final results prove such as, I believe, caunot be ignored. Indeed those who 

 are in the way of making such calculations, and who enter into tlie question, will 

 find that my assumptions are not specially favourable, but are, in fact, made on 

 quite independent grounds. Again, a certain class of effects has been entirely left 

 out of account, effects which will go towards enhancing, and in some cases 

 greatly enhancing, the radio-thermal activity. I refer to the thickening of the 

 crust arising from tangential pressure, and, at a later stage, the piling up and 

 overthrusting of mountain building materials. In such cases the temperature of 

 the deeper parts of the thickened mass must still further rise under the influence 

 of the contained radium. Tfiese effects only take place, indeed, after yielding has 

 commenced, but they add to the element of instability wlueh the presence of the 

 accumulated radio-active deposits occasions, and doubtless increase thermal meta- 

 morphic actions in the deeper sediments, and result in the refusion of rocks in the 

 upper part of the crust.' 



The effect of accumulated sediment is thus necessarily a reduction in the 

 thickness of that part of the upper crust which is capable of resisting a com- 

 pressive stress. Over the area of sedimentation, and more especially along the 

 deepest line of synclinal depression, the crust of the globe for a period assumes 

 the properties belonging to an earlier age, yielding up some of the rigidity which 

 was the slow inheritance of secular cooling. Along this area of wcaln:c>s -from 

 its mode of formation generallj^ much elongated in form — the stressed crust for 

 many hundreds, perliaps thousands, of miles finds relief, and flexure takes place 

 in the only possible direction ; that is, on the whole upwards. In this way the 

 prolonged anticline bearing upwards on its crest the whole mass of deposits is 

 formed, and so are born the mountain ranges in all their diversity of form and 

 structure. 



We have in these effects an intervention of radium in the dynamics of the 

 earth's crust, which must have influenced the entire history of our globe, and 

 ■which, I believe, affords a key to the instability of the crust. For after the 

 events of mountain building are accomplished, stability is not attained, but in 

 presence of the forces of denudation tlie whole sequence of events has to commence 

 over again. Every fresh accession of snow to the firn, every passing cloud con- 

 tributing its small addition to the torrent, assists to spread out once more on the 

 floor of the ocean the heat-producing substance. With this rhythmic succession 

 of events appear bound up those positive or negative movements of the strand 

 which cover and uncover the continents, and have swayed the entire course of 

 evolution of terrestrial life. 



Oceanic Deposits. — The displacements of the crust which we have been con- 

 sidering are now known to be by no means confined to the oceanic margins. The 

 evidence seems conclusive that long continued movements have been in progress 

 over certain areas of the sea floor, attended with the formation of those numerous 

 volcanic cones upon which the coral island finds foundation. Here there are 

 jdainly revealed signs of instability and yielding of the crust (although, perhaps, 

 of minor intensity) such as are associated with the greater movements which 

 terminate in mountain building. I think it will be found, when the facts are 

 considered, that we have here phenomena continuous with those already dealt 

 with, and although the conditional element of a sufficient sedimentary accumula- 

 tion must remain speculative, the evidence we possess is in favour of its existence. 



One of the most interesting outstanding problems of deep-sea physiography is that 

 of the rates of accumulation of the several sorts of deposit. In the case of the more 

 rapidly collecting sediments there seems uo serious reason why the matter should 

 not be dealt with observationally, I hope it may be accomplished in our time. 



' Professor C. iSchmidt (Basel) has recently given reasons for the view that the 

 Mesozoio schists of the Simplon at the period of their folding were probably from 

 15,000 to 20,000 metres beneath the surface {Ec. Geol. Helvetica, vol. ix. No. 4, 

 p. 590). As another instance consider the compression of the Laramide range 

 (Dawson, Bull. Gcol. Soc. Am., xii. p. 87). 



