DISCOVERY 



109 



long and technical discussion. It may, however, be 

 confidently stated that present-day rates are abnormally 

 high ; that denudation and sedimentation move in 

 rhythmic c\xles, sometimes rapidlj', as now, but at 

 other times much more slowly ; that the rate of sedi- 

 mentation is controlled not only by the supply of 

 material, but also by the rate of relative subsidence of 

 the sea floor ; and, finally, that there are innumerable 

 time-intervals unrepresented by layers of sediment 

 in almost every part of the geological record. We 

 may therefore be prepared for figures far in excess 

 of even those crudely arrived at in the above 

 estimates. 



The early geologists were apt to be somewhat casual 

 in their estimates of geological time, particularly as 

 the latter were necessarily based more on sentiment 

 than on quantitative data. More than half a century 

 ago Sir William Thomson, afterwards Lord Kehin, 

 invaded the mists of speculation in which the subject 

 was then enveloped, hoping to disi>er5e them with the 

 wand of thermodynamics. It is well known that, as 

 the earth's crust is penetrated by bore-holes and mine- 

 shafts, a steady increase of temperature is encountered. 

 On the assumption that the temperature gradient of 

 the earth was the result of simple cooling from a molten 

 state, and knowing the average thermal properties of 

 rocks, Kelvin was able to calculate within Umits the 

 time that must have elapsed since the consohdatibn of 

 the crust took place. The first hmits were 20 and 400 

 miUion years, with 100 million j-ears as the most prob- 

 able figure. Later, however, he reduced his hmits to 

 20 and 40 million years, with a tendency to favour 

 the shorter period. The long controversy which raged 

 around these results is one of the famihar details of 

 Victorian scientific history. Although a few geologists, 

 impressed by Kelvin's authority and apparently un- 

 impeachable reasoning, were willing to adapt them- 

 selves to the very severe limitations imposed, others, 

 like Geikie and Goodchild, demanded far less restricted 

 periods for the varied changes of hfe and scene revealed 

 by the long succession of records preserved in the rocks. 

 There was a hope, slight indeed at first, though after- 

 wards fulfilled beyond all expectation, that some under- 

 hing flaw would be found in the thermodynamic argu- 

 ments. It might have been pointed out that the latter 

 ignored the great convection currents represented by 

 the rise of granites and other products of igneous 

 activity ; but even had the calculations been modified 

 to include such transfer of heat, the resulting estimates 

 would have been little higher, if at all, than Kelvin's 

 final maximum. In justice to Kel\-in, however, it is 

 only fair to state that he clearly reaUsed that his esti- 

 mates would fall to the ground if it could be showTi that 

 the earth contained within herself suppUes of potential 

 energy which, Uberated as heat, would counterbalance 



the external loss by radiation. With the discover}' of 

 radio-activity the real flaw was at last detected, and it 

 was found, contrary to Kelvin's assumption, that the 

 earth is not a simply coohng body. 



The examination of pitchblende by Mme. Curie led 

 to the discovery of radium, and later it was found that 

 radium is constant!}' being formed as one of a long series 

 of elements which arise from the spontaneous atomic 

 decay of uranium. During this disintegration radia- 

 tions of three types are emitted by the imstable ele- 

 ments : a-rays, consisting of positively charged atoms 

 of hehum ; /3-rays, identified with streams of (negatively 

 charged) electrons ; and y-rays, of the same nature as 

 X- or Rontgen-raj's. After three atoms of hehum 

 have been discharged from the parent element, uranium, 

 and its immediate descendants, radium is reached. 

 Radium in turn decays, and as the rate of disintegra- 

 tion of each member of the family is proportional to 

 the quantity of it which is present, an equilibrium is 

 ultimately reached in which a constant ratio is estab- 

 hshed between the amount of uranium and that of 

 each of the unstable daughter elements. The ratio 

 of uranium to radium, for example, is 3,000,000 to i. 

 From the disintegration of radium, hehum and a gas 

 known as niton, or rachum emanation, are evolved, and 

 it is due to the easy detection and measurement of the 

 emanation that small quantities of radium or uranium 

 can be estimated. The emanation continues to break 

 down, and ultimately a stable end-product which is 

 chemically identical with lead is reached. When the 

 whole famil}' has fallen into equilibrium, the decay of 

 one atom of uranium imphes that of an atom of each of 

 the unstable daughter elements, and the generation of 

 eight atoms of hehum and one of lead. The hehum 

 and lead, being stable, gradually accumulate in pro- 

 portion as their ultimate parent, uranium, is destroyed. 

 An atomic equation representing in two stages the 

 total change is as follows : 



238 



238 



238 



It will be noticed that the atomic weight given for 

 " Pb " is 206, whereas that of ordinary lead is 207'2. 

 That this difierence of atomic weight is real has been 

 estabUshed beyond question by actual determinations 

 on lead separated from uranium-bearing minerals in 

 which the stable products of decay have accumulated 

 through long geological periods. To this aspect of the 

 subject we shall return for a more detailed discussion. 

 Thorium is another parent radioactive-element, and 

 its disintegration leads to the generation of hehum at 

 six stages with lead again as the probable end-product. 

 The " lead " of the thorium family has apparently an 



