AGE OF THE EAETH RAYLEIGI1. 253 



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 composi- 

 tion. 



These various complications introduce a certain amount of diffi- 

 culty and even ambiguity into the interpretation. A full discussion 

 can not be given on an occasion like the present, but the complica- 

 tions can not, I think, be considered to modify the broad result. 



A determination of the amount of helium in minerals gives an 

 alternative method of estimating geological age; but helium, unlike 

 lead, is liable to leak away, hence the estimate gives a minimum only. 

 I have found in this way ages which, speaking 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 years. Thus the helium in an Eocene 

 iron ore indicated 30,000,000 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 7X10 -6 of the whole, and the lead 

 as 22X10 -6 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. We 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 20G) and how much thorium-lead 

 (atomic weight 208) exists in common lead (atomic weight 207) be- 

 comes 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 11X10 9 

 years. This is certainly too great. Allowing for the production of 

 some of the lead from thorium, Russell finds a period of 8X10 9 years 

 as the upper limit. This is about six times the age indicated by 

 the oldest individual radio-active minerals that have been examined. 



