LIFE IN SPACE AND TIME 25 



conditions, e.g. pressure, temperature, and the like, and 

 presumably depends upon the structure of the minute 

 nuclei of the atoms. Radium is one of the shorter-lived 

 products in a series which begins with uranium, the heaviest 

 atom known, and ends with lead; but the lead which is 

 so formed differs in atomic weight from common lead 

 (206 instead of 207) and can therefore be identified by a 

 careful analysis. If lead of this pecuHar type is found in a 

 uranium mineral, there is every reason to beheve that 

 it was not an original constituent, but has been formed 

 in situ by radioactive change in the crystal where it is 

 found. The rate of change is very slow, no less than 

 66,000,000 years being required for the transformation of i per 

 cent of the uranium. Hence, if we find lead of this pecuHar 

 sort in a mineral, amounting to 15 or 20 per cent of the 

 weight of the uranium which is present, it is evident that 

 it must have Iain undisturbed in the rock for a prodigious 

 time before the miner and the chemist broke upon its rest. 

 The ages of minerals in many eruptive rocks have been found 

 in this way, that is, the time since the rocks themselves 

 were molten. The geological period during which the 

 intrusion of the mass occurred can often be fixed, which 

 leads to the determination of a large number of datum points 

 along the scale of geological time. Jeffreys (1924), reviewing 

 the evidence, gives the following summary: 



Period Years 



Eocene 60,000,000 



Carboniferous 300,000,000 



Upper Pre-Cambrian 550,000,000 



Lower Pre-Cambrian 1,300,000,000 



Life was already abundant in Cambrian times, and had 

 developed a multitude of highly organized forms, so that 

 its origin must be placed much farther back, and probably 

 not less than a billion years ago. Duiing all this time, 

 the physical conditions on the Earth's surface, and, in 

 particular, the temperature, must have been very similar 



