356 SCIENCE PROGRESS 



be assigned without very great uncertainty. In this way an 

 age of about i ,000 milHon years is derived from pre-Cambrian 

 rocks. 



HeUum is another product of disintegration of uranium, 

 and the heUum content of radio-active minerals can also be 

 utilised. In this way only a lower limit of the age of the 

 mineral can be assigned, as some leakage of the gas is probable. 

 Lord Rayleigh finds that, in general, ages of about one-third 

 those given by estimates of lead are thus obtained. 



Prof. SoUas pointed out the necessity of making sure that 

 the radio-active clock was not as much too fast as Lord Kelvin's 

 was too slow. Lord Rayleigh had assumed that, since the rate 

 of disintegration of uranium is not affected by any change of 

 temperature or pressure which we can apply, uranium has 

 always disintegrated at the same rate. Prof. Sollas drew 

 attention to the phenomenon of pleochroic haloes occurring in 

 uranium-bearing black mica. These haloes, which have been 

 studied by Prof. Joly, are formed by the X-rays expelled by 

 uranium in the course of its disintegration. In general, the 

 ranges of the rays are consistent with those found at the present 

 time. In the case of the two inner rings, however, the ranges 

 are greater than normal, and Prof. Joly has concluded from this 

 that in Caledonian times there existed a metope of the uranium 

 we now know, with different properties. If this conclusion is 

 correct, the radium clock has not been keeping uniform time, 

 and estimates of the age of the earth which are based upon it 

 are liable to an error of an uncertain amount. 



Prof. Gregory dealt with the geological estimate of the age 

 of the earth, based upon the salinity of the sea. Estimates 

 obtained in this way varied from 70 to 150 million years. He 

 pointed out that the argument suffered from three fundamental 

 objections. It was assumed that the sea was originally fresh, 

 although the oldest fauna, the Cambrian, had marine charac- 

 teristics, and the contrast between the fresh water and marine 

 fauna in Palaeozoic times was as sharp as it is to-day. There 

 was also no allowance for the large supplies of sodium chloride 

 raised from beneath the earth's surface by magmatic waters. 

 Further, a uniform rate of denudation was postulated, whereas 

 there have been alternating periods of quick and slow crustal 

 movements : the earth is now under the influence of a time of 

 quick movement with, consequently, denudation faster than 

 the average. Taking these three causes separately, Prof. 

 Gregory estimated that, to allow for them, the age of the earth 

 deduced from the salinity argument should be multiplied two- 

 fold, three- or four-fold and five-fold respectively. He con- 

 cluded by stating that the best-known geological estimates of 

 the age of the earth require to be multiplied ten- or twenty-fold 



