August, 1913. 



KNOWLEDGE. 



297 



other appearances must, of necessity, be disregarded. 

 When it comes to measurements of the rate of 

 accumulation of the various strata, still greater 

 difficulties are encountered. Theoretically, it is 

 only necessary to measure the annual silt-loads of 

 various rivers, as well as the catchment area, and 

 the rate of deposition ensues. But, in practice, the 

 rates are found to be so divergent and variable that 

 the average counts for little. As Joly points out, 

 we are at liberty to assume anything from a foot to 

 a few inches deposition per century. Taking the 

 rate of accumulation to be four inches in the 

 century, the age of the sedimentary column, which 

 is between sixty and seventy miles thick by the best 

 estimate, then works out to be about one hundred 

 million years. Sollas gets thirty millions ; Joly gets 

 eighty million years. The latter is probably the 

 more likely of the two, so we shall not be far wrong 

 if we consider our one hundred-million year com- 

 putation as fairly near the mark. 



IV. — Radioactivity. 



Among the many wonderful possibilities which 

 radioactivity has to offer, the determination by its 

 aid of the age of the earth may be the least 

 important ; but it is undoubtedly one of the most 

 interesting, opening up as it does such a splendid 

 field for discussion and speculation. 



In the first place, it deals the death-blow to 

 Kelvin's thermodynamic method. The disintegra- 

 tion of a radioactive body is known to be accom- 

 panied by a spontaneous evolution of heat energy, 

 and, considering the widespread character of these 

 bodies throughout the earth's crust, the total heat 

 developed in this way can be no negligible quantity. 

 Indeed, so far as present estimates go, it more than 

 accounts for the earth's annual loss by radiation — 

 truly an embarrassing position for the earth, as will 

 be shown in a moment. The supposition, then, 

 that the earth is merely a cooling body, whose 

 primitive stock of heat is being slowly depleted, must 

 be modified in order to take account of the new 

 factor. Let us examine afresh our conception of 

 the life-history of the earth. 



The fact that the molten earth contains a con- 

 siderable store of long-lived radioactive elements 

 would not appreciably retard its cooling until the 

 consistentior status was reached. Then, on the 

 formation of the surface crust, the rate of cooling 

 would be reduced to a very small fraction of its 

 former value ; comparable, in fact, with the heat 

 liberated during elemental disintegration. Near the 

 surface of the earth this heat is sufficient, we have 

 seen, to make good the radiation loss ; whereas in 

 the interior, where escape is impossible, the heat 

 generated cannot but have accumulated during the 

 long geological epochs. The final result is evident. 

 Not from without, by collision with some wandering 

 star, but from within, by her own irrepressible 

 vulcanicity, is the destruction to come which is to 

 return the earth to her pristine state, to begin again 

 her life-history, perhaps for the nth time, where n 



represents an unknown quantity. In a similar way 

 the long duration of the sun's heat is accounted for, 

 and another theory of Kelvin's exploded. 



But let us get back to age determination. The 

 basis of the radioactivity method will be clearly 

 understood if it be remembered that radium is an 

 element whose parentage is known and whose 

 descendants to many generations have been fairly 

 definitely established. In other words, this singular 

 element has been proved to be one of a series which 

 begins with uranium and proceeds down a scale of 

 radio-elements of diminishing atomic weight until 

 the final stable substance, now thought to be lead, is 

 reached. The disintegrating process is accompanied 

 at almost every stage by a loss of what are called 

 a particles, which themselves have recently been 

 shown to be identical with helium atoms. 



Of course, the process of degradation is incon- 

 ceivably slow, though the time-rate for each stage 

 has actually been calculated from laboratory observa- 

 tions. The number of helium atoms discarded 

 during each of the transformations is likewise 

 known. Assuming, then, that all these discharged 

 atoms have accumulated in situ in any geological 

 formation, it is only necessary to determine in a 

 specimen mineral the ratio of the occluded helium to 

 the still-remaining radioactive element in order to 

 arrive at an estimate of the age of the strata from 

 which the mineral was taken. 



The Hon. R. J. Strutt, in particular, has examined 

 from this point of view many minerals taken from 

 strata of the different geological epochs. His results 

 are too numerous to quote, but so far they have not 

 been particularly satisfactory. Even in material 

 obviously contemporaneous, for instance, the varia- 

 tions in the geological ages turn out to be very 

 considerable. The weak point of the whole method 

 seems to lie, not in the theoretical assumptions 

 involved, but rather in the improbability of a 

 complete helium accumulation. It appears likely 

 that the varying experiences to which the helium- 

 bearing formations have been subjected throughout 

 their existence — such as changes of temperature and 

 pressure or the solvent action of percolating waters — 

 have all contributed to an appreciable alteration in 

 the amount of helium accumulated. The advantage 

 that the method otherwise possesses, in that each 

 determination is an independent estimate of the 

 geological age, is thus swamped by the disability just 

 referred to, and any results which are the outcome 

 of such a method must consequently be received 

 with caution. 



The problem has recently been approached from 

 another standpoint, and, at first sight, one more 

 reliable. If lead is really the ultimate product of 

 the disintegration of uranium — and much evidence 

 has been adduced in support of this view — then from 

 the uranium-lead ratio supplementary estimates are 

 possible. Professor Boltwood has attacked the 

 problem of the age of the earth from this assumption, 

 and is led thereby to attribute to a number of 

 uranium-bearing minerals an age which ranges from 



