THE AGE OF THE EARTH.

New Zealand Herald, Volume XXXVII, Issue 11544, 1 December 1900, Page 5 (Supplement)

THE AGE OF THE EARTH.

AN OXFORD EXPERT'S CONSERVATIVE ESTIMATE. >

TWENTY-SIX MILLION YEARS. The latest utterance on the age of the earth, is that of Professor W. J. Sollas, who occupies the chair of geology at Oxford, and is president of the geological section of the British Association for the Advancement of Science. This address, at the annual meeting of that organisation, was devoted to reviewing the estimates of others and elucidating his own opinion on this interesting subject. It should be premised that inasmuch as matter is believed to be indestructible the scientist has difficulty in understanding how it could have he-id a beginning.' Hence, the material composing the earth is to be regarded as having existed forever, though its condition may have undergone many changes. However, even though it may once have been a nebula, " without form arid void,'' and though at one period in its history it was perhaps merged with the far greater mass comprising the other planets and the sin:, there came a time when it was detached from them all. acquired a globular shape, and was probably (though not certainly) so hot as to be entirely molten. It is customary) therefore, to specify some particular stage in its development as the starting point for calculations of the earth's age. Lord Kelvin, for instance, dated his estimate from the time when the globe became cool enough to be habitable by the lowest known forms of life. Kelvin placed the interval which has elapsed since that era at between twenty and forty million years, but he believed that it was nearer the former figure than the latter. This computation was based principally on the rate at which various kinds of rock will radiate heat, and on the assumption that the rate has remained uniform from temperatures like 2000 and 2500 down to that now observed at the earth's surface. Professor Sollas remarks, however, that another factor in Kelvin's estimate was the supposition that the temperature of the earth increases one degree for every fifty-one feet of depth, whereas other authorities make the rate less rapid. The Oxford geologist believes that one degree for 80 or 90 feet would be nearer the correct average. He specifies several instances where it is even slower than this. In the Lake Superior copper mines the rate is one degree for every 220. feet of descent. A modification such as is here suggested would bring Kelvin's figures up to about 50,000,000. Then there is Professor George Darwin's fascinating calculation, based on the belief that the moon was thrown oft" the earth in a tidal wave. At that time the latter body was supposed to be rotating on its axis in a period of from two to four hours. The subsequent retardation of its speed through the attraction of the moon would, according to Professor Darwin's mathematics, require something like fifty-six or fifty-seven million years at least, and possibly more. Professor Sollas thinks that this theory has strong claims to acceptance,, and he picks no flaws in it, though eventually offering a different estimate of his own.

Joly based another computation on the supposed rate at which salt had been carried into the ocean by rivers. But Professor Sollas offers several reasons for suspecting that Joly did not adopt a sufficiently rapid rate. He would therefore cut down Joly's eighty or ninety million a year nearly one-half. Indeed, Professor Sollas courteously betrays a considerable lack of confidence in the-ocean salt system of computation. The particular class of evidence to which the Oxford geologist attaches the largest importance is the depth of those rock formations which are clearly of sedimentary origin. Taking these at their greatest thickness, wherever this may occur, he finds an aggregate deposit of about fifty miles, or 2c4,000 feet. Great difficulty is experienced in hitting upon an approximate accurate average rate of deposition. But Professor Hollas, after presenting elaborate reasons for so doing, adopts one foot a century as the most satisfactory standard; and with this "basis he gets a little over 26,000,000 years as the time which has elapsed since the formation of sedimentary rocks began. The whole tendency of his argument is avowedly to harmonise the estimates of others as far as possible, and it will be observed that he comes much nearer to Lord Kelvin's figures than either Professor Darwin's or Professor Joly's. ' ■ Those who have followed the discussion of this subject for more than a quarter of a century will recall that Huxley deemed a period of even 100,000,000 years hardly adequate for the play of forces whose activity is self-recorded in the rocks. But while speaking as a geologist, Huxley also advocated the theory held by progressive zoologists thirty or more years ago that the evolution of life from the lowest forms to the highest could not have taken place in anything like the limits allowed by Kelvin. There again came a demand for a hundred million or more. Without referring to Huxley by name, Professor Sollas raises the question as to how far years will satisfy the biologists, and then replies, " amply."' This particular argument hinges on the susceptibility to change shown by various forms of animal life whose fossil remains have been discovered. Some are much more mutable than others; and Professor Sollas insists that it is not fair to take the slowest and most changeless families as the standard. . This course, be declares, is like judging of the velocity of a river by the condition of stagnant pools beside the stream. But he holds that biologists are not yet in a position to give independent evidence anyhow. " Nor," he adds, " can they hope to be so until they have vastly extended those promising investigations which they are now only beginning to make into the rate of the variation of species."