October 27, 192 1] 



NATURE 



281 



Thus take the broggerite found in the pre- 

 Cambrian rocks at Moss, Norway. The lead in 

 this mineral has an atomic weight of 20606 as 

 determined by Honigschmied and Fraulein St. 

 Horovitz. The ratio of lead to uranium is 1:3. 

 Taking the lead as all produced by uranium 

 at the rate above given, we get an age of 925 

 million years. Some minerals from other archaean 

 rocks in Norway give a rather longer age. 



In other cases there is some complication, 

 owing to the fact that thorium is associated with 

 uranium in the mineral and that it, too, produces 

 helium and an isotope of lead of atomic weight 

 probably 208 exactly, about one unit higher than 

 common lead. 



In a third class of cases the uranium mineral, 



^ pitchblende, occurs in a metalliferous vein, and 



the lead isotope produced in the mineral is diluted 



with common lead which entered into its original 



composition. 



These various complications introduce a certain 

 amount of difficulty and even ambiguity into the 

 interpretation. A full discussion cannot be given 

 on an occasion like the present, but the complica- 

 tions cannot, I think, be considered to modify 

 the broad result. 

 f A determination of the amount of helium in 

 minerals gives an alternative method of estimating 

 'i geological age; but helium, unlike lead, is liable 

 to leak awav. hence the estimate gives a minimum 

 only. I have found in this way ages which, speak- 

 ing generally, are about one-third of the values 

 which estimations of lead have given, and are, 

 therefore, generally confirmatory, having regard 

 to leakage of helium. 



The helium method is applicable in some cases 

 to materials found in the younger formations, and 

 proves that the ages even of these are to be 

 reckoned in millions of vears. Thus the helium 

 in an Eocene iron ore indicated thirty million years 

 at least. 



Returning now to the estimation of lead, H. N. 

 Russell has recently applied this line of reasoning 

 to the earth's crust as a whole. He takes the 

 uranium in the earth as 7 x lo"* of the whole, 



and the lead as 22 x lO"^ of the whole. It is 

 necessary to remark that we do not know very 

 definitely whether the lead distributed in the rocks 

 in small proportion and very difficult of extraction 

 is the same mixture of isotope as the lead of 

 mineral veins. W'e call the latter "common lead," 

 but nearly all the lead in the earth's crust is of 

 the former kind. 



Even if we did know that "rock lead" were 

 the same as "vein lead," we should still not be 

 in a position to say what fraction of it was 

 uranium-lead, as we do not know whether an 

 isotope having an atomic weight 207 exists. If 

 it does, obviously the problem how much uranium- 

 lead (atomic weight 206J and how much thorium- 

 lead (atomic weight 208) exists in common lead 

 (atomic weight 207J becomes indeterminate in the 

 absence of further data. An analysis of lead by 

 positive rays will probably soon become feasible, 

 and with a determination of the atomic weight of 

 "rock lead " will do much to clear up the matter. 



If all the lead were uranium-lead, and had 

 been generated since formation of the earth's crust, 

 the time required would be 1 1 x 10^ years. This is 

 certainly too great. Allowing for the production 

 of some of the lead from thorium, Russell, finds a 

 period of 8 x 10^ years as the upper limit. This 

 is about six times the age indicated by the oldest 

 individual radio-active minerals that have been 

 examined. 



I have now traversed that part of our subject 

 of w'hich I feel competent to speak. The upshoc 

 is that radio-active methods of research indicate 

 a moderate multiple of 1000 million years as the 

 duration of the earth's crust as suitable for the 

 habitation of living beings, and that no other con"- 

 siderations from the side of pure physics or astro- 

 nomy afford any definite presumption against this 

 estimate. 



The arguments from geology and biology I 

 must leave to our colleagues from other sections. 

 May I venture to say that I for one consider the 

 topics with which they will deal as not less in- 

 teresting and important than those which it has 

 been my privilege to try to lay before you. 



By Prof. W. J. Sollas, F.R.S. 



T_T UXLEY once sagely remarked that the zoo- 

 -»- -*- logist must take his time from the geo- 

 logical clock. The geologist is thus charged with 

 a great responsibihty which he would willingly 

 share with the physicist and astronomer. One of 

 the earliest attempts to determine the age of the 

 earth by purely geological means was made by 

 the late Dr. Samuel Haughton, who based his 

 calculations on the rate of deposition of sediment 

 supposed to be evenly distributed over the whole 

 floor of the ocean. This led to the conclusion that 

 the time which must have elapsed since the first 

 appearance of the dry land is of the same order of 

 magnitude as that now presented for our con- 

 sideration by Lord Rayleigh. 



Soon, however, it was discovered, as a result 

 NO. 2713, VOL. 108] 



of exploration by the Challenger, that deposition 

 is limited to a comparatively narrow belt border- 

 ing the continents — a limitation due to several 

 causes, chief among them the fact that sediment 

 sinks much more rapidly in salt water than fresh. 

 On taking account of this factor Haughton's 

 f>eriod was reduced to about 100 million years. 

 At the same time a new method was devised by 

 Prof- Joly which depends on the rate at which 

 sodium is supplied to the sea, and this led to a 

 similar result. 



Antecedent to these attempts, another method, 

 based on the rate at which the earth is losing 

 heat, had been employed by Lord Kelvin, and 

 this gave at first an estimate concordant with the 

 preceding — i.e. 100 million years. Later, how- 



