April i8, 1895] 



NATURE 



58: 



which still have attached to them delicate antennas. In 

 sandstones we find most delicate ripple marks and the 

 marks of rain-drops. But over and above all this, 

 denudation along coast-lines can hardly be regarded as 

 of much importance compared with subaerial denudation 

 (Sir A. Geikie, Trans. Q^a\. Soc. of Glasgow, iS68). 

 Was there more rain .' and did it fall more suddenly .' 

 Did the wind blow more strongly .' Were atmospheric 

 actions more vigorous in the past ? There is no great 

 reason for believing that they were. As Prof G. Darwin 

 observes, fossil trees do not seem to have been built 

 more strongly than modern trees, and this gives some 

 evidence as to the relative violence of aerial forces. 



All the geological evidence points to rates of denuda- 

 tion and deposition in the past which may, on the average, 

 have been greater than the average rate at present, but 

 which were not on the average greater than the greatest 

 rates at present. 



The pakcontologist now comes in. A study of fossils 

 shows that there has been a gradual development, some- 

 times more quickly perhaps, and sometimes more slowly, 

 but on the whole a continuous development of animal 

 life in the past. We believe from all our study of nature 

 that the development has been continuous. As more 

 and more strata are studied, many of the apparent dis- 

 continuities are being converted into continuities. Now 

 even in the lower parts of the Cambrian, Brachiopoda are 

 found. Biologists tell us that in all probability these 

 were gradually developed from creatures like worms ; 

 their structures are sufficiently comple.x for us to know 

 that the time taken to develop the Brachiopod from the 

 worm may have been as great as the age of known 

 fossiliferous rocks. There are many rocks, evidently 

 sedimentary, enormously older than the Cambrian, and 

 when laid down there was certainly water on the earth, 

 and hence it was neither too hot nor too cold for animal 

 life. In these lower formations there are conglomerates 

 containing pieces of still older rocks. Although in pre- 

 Cambrian strata traces of animal remains are said to 

 occur, we may say that the pateontological record is 

 almost lost below the Cambrian, most of the earlier rocks 

 having been subjected to great metamorphic action. 

 If we keep to our principle of continuity in nature's 

 actions, we see that the first beginning of life must have 

 taken place at a date many times earlier than the very 

 earliest geological record. 



But the most experienced geologists and palasonto- 

 logists state that they are satisfied with a i^vi hundred 

 million years as the possible age of life or the existence 

 of water on the earth. 



2. The considerations drawn from tidal retardation 

 are as follows : — ■ 



(a) The shape of the earth now is the same as its shape 

 when it solidified, [fi) The shape of a liquid earth tells 

 us its rate of revolution on its axis, therefore we know 

 the rate of revolution of the earth on its axis when it 

 solidified. (y) Assuming that we know, with a fair 

 amount of accuracy, the rate at which the length of the 

 day is altering, we know the date of the earth's solidi- 

 fication, and certainly this is later than looo million years 

 ago. 



When I referred to the fallacy in this argument, 

 I did not know that it had already been pointed out by 

 the Rev. M. H. Close and Mr. Clarence King and Prof. 

 George Darwin. It lies in the fact that (n) is certainly 

 wrong. A solid body like the earth will, under the action 

 of great forces, alter its shape in time. Such alteration 

 is continually going on. Again (y) is very doubtful. 



(3) 1 now come to the considerations from the cooling 

 of the earth. Lord Kelvin proved that, if the earth was 

 once at a uniform temperature of 7000' F. or 3S70" C, of 

 material the heat properties of which are the same as the 

 average of three rocks experimented upon at Edinburgh 

 — these remaining constant throughout — and if the rate 



NO. 1329, VOL. 51] 



of increase of temperature downwards in the crust is now 

 I Centigrade degree for every 90 feet, 100 million years 

 have elapsed since cooling began ; but there is a possible 

 maximum of 400 millions. 



In the article on this subject, published in Nature, 

 January 3, 1895, I showed that, if we assume greater 

 conductivity in the interior than at the surface, we in- 

 crease this limit of age. I took a number of examples, 

 which could be worked mathematically. I did not pre- 

 tend that any one of these represented the actual state 

 of the earth. They merely proved that there were 

 i)ossible internal conditions which might give enormously 

 greater ages than physicists had been inclined to allow. 

 Of my various results, I did not give one as more correct 

 than another, although some may have seemed more 

 probable than others. It was not my object to obtain a 

 correct estimate. Indeed I tried to show that it was im- 

 possible for a physicist to obtain such an estimate, as 

 there were all kinds of possible assumptions which led to 

 many different answers. 



The validity of my reasoning in no degree rests upon 

 the accuracy of R. Weber's results as quoted by me. In- 

 deed, I only discovered these results when writing to Prof 

 Tait. In N.-^TURE, February 7, p. 341, I have shown the 

 extent to which the possible limit of the earth's age is 

 increased if k and c increase with temperature and 

 k c remains constant. But I published this as an in- 

 teresting mathematical result, and was careful to add — • 

 " It must be understood that my conclusions are really 

 independent of whether R. Weber's results are correct or 

 not.'' It is comparatively unimportant, but R. Weber 

 has published another set of results which confirm those 

 which I quoted. The results, published on March 7 

 for the first time, differ so utterly from the two previous 

 sets, that I venture to think there may be mistakes in 

 transcribing. However that may be, I am not concerned 

 either to support or refute them. 



I mentioned the possible great quasi-conductivity due 

 to the interior of the earth being a honey-combed mass 

 containing liquid, and to the possible greater con- 

 duction due to the presence of iron and other metals. 

 Almost anything is possible as to the present internal 

 state of the earth. Dr. Ramsay seems to think that 

 there must be great quantities of sulphides inside, and 

 these would probably be much better conductors than 

 the surface rocks. 



Prof. Schuster, in discussing the diurnal variation of 

 terrestrial magnetism {Phil. Trans. 1889, p. 467), comes 

 to the conclusion that the electric conductivity of the 

 earth must be considerably greater inside than at the 

 surface. 



In all probability there are no great masses of liquid 

 inside the earth at the present time, but it is quite 

 possible that until recent times convection in such masses 

 may have been conveying heat from the very inner earth 

 towards its surface, and the latent heat given out by such 

 masses of liquid as they solidified would be another 

 potent factor. Some distinguished geologists say that the 

 e.xcessive folding which has occurred on the earth's sur- 

 face cannot be accounted for by the current assumption of 

 physicists, which involves the result that, practically, no 

 cooling has yet taken place below the depth of 120 

 miles : my assumption is that cooling has taken place to 

 much greater depths. 



All these things, like the numbers published by R. 

 Weber, support the argument if they are correct, but 

 they do not in any way destroy it if they are wrong. I 

 was not looking for ^. probable age of the earth from the 

 point of view of mere physics. 1 wished to show that the 

 physics' higher limit was greater than a few hundred of 

 millions of years. 



Mr. Clarence King's paper appears somewhat incon- 

 clusive. He assumes, possibly rightly, that the earth's 

 crust may have the properties of Diabase ; experiment has 



