URANIUM AND GEOLOGY — JOLY. 369 



in accordance with the requirements of geology is, I believe, not 

 realizable on any probable estimate of the allowable radium, or on 

 any concentration of it which my own experiments on igneous rocks 

 would justify. 



But we maj take refuge in a less definite statement, and assume a 

 distribution by means of which the existing thermal state of the crust 

 may be maintained. A specially rich surface layer we must recog- 

 nize, but this need be no more than a very few miles deep; after 

 which the balance of the radium may be supposed distributed to any 

 depth with which we are thermally connected. Below that our 

 knowledge is indefinite. The heat outflow at the surface is in part 

 from the surface radium, in part due to the cooling arising from the 

 diminishing amount of uranium, in part from the deep-seated radium. 

 In this manner the isogeotherms are kept in their places, and a state 

 is maintained which is in equilibrium with the thermal factors in- 

 volved, but which can not be considered steady, using the word in a 

 strictly accurate sense, in view of the decay of the uranium. 



While the existing thermal state may, I think, thus be maintained 

 by radio-active heating and radio-active decay, we find ourselves in 

 considerable difficulties if we extend this view into the past and 

 assume that the same could be said of any previous stage of the 

 earth's history. If the heat emitted by the earth, when the surface 

 was at melting temperature, was in a state of equilibrium with the 

 radio-active supplies, then, at that date, there must have been many 

 thousands of times the present amount of uranium on the earth, and 

 the period of the consistentior status must be put back by thousands 

 of millions of years. Apart from hopeless contradiction with every 

 geological indication as to the age of the earth, difficulties in solar 

 j)hysics arise. For the sun must be supposed of equal duration, and 

 we are required to assume impossible amounts of uranium to main- 

 tain its heat all that great lapse of time; and again this uranium 

 would perish at just the same rate as that upon the earth, so that at 

 the present time the solar mass must be, for by far the greater part, 

 composed of inert materials of high atomic weight — the products of 

 the transformations of the uranium family. The difficulty is best 

 appreciated when we consider that even to maintain its present rate 

 of heat loss by radium supplies, some GO per cent of its mass must be 

 composed of uranium. But there are other troubles to face if we 

 adopt this view. The earth, or rather those parts of it which are 

 sufficiently near the surface to lose heat at the requisite rate, would 

 have cooled but 1 per cent in 10^ years. Shrinkage of the outer parts 

 and crustal thickness will be proportionately small, and we must put 

 back our epochs of mountain building to suit so slow a rate of cooling 

 and shrinkage and refer the earlier events of the kind into a past of 

 inconceivable remoteness. Otherwise we must abandon the only ten- 



