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WORLD

as having passed since the appearance of the first organisms. T. Mellard Reade approached the ques- tion from an entirely different standpoint. He cal- culated that on an average about 3000 year.s are necessary to denude the upper surface of the earth one foot, and, taking the processes of denudation and deposition as equal, he arrived at an interval of ninety- five million years in round numbers. A. Geikie, who likewise bases his calculations on the deposition of stratified rocks, found as the limits 73 and 680 miUion years, while McGee, on the same basis of sedi- mentary formations, estimates that 7000 million years elapsed since the Cambrian period and double that length of time since the formation of the terrestrial crust. Another method adopted by geologists depends on the shrinkage of the earth in consequence of the formation of mountains. Nathorst and Neu- mayer suppose that the radius of the earth has become about 5 km. shorter since the Silurian period. On this hypothesis and theoretical figures concerning the annual cooling and contraction of the earth M. P. Rudzki bases his investigations, and endeavours to arrive by exact mathematical methods at the time hitherto elapsed, arriving at an interval of 200 million years; by assuming a total shrinkage of 50 km. and employing the coohng theory of Thomson, he places the age of the world at 500 miUion years.

On the development of mount ain chains is based the calculation of P. Kreichgauer also. He starts from the hj-pothesis that 1400 years are on an average neces- sary "to carry away from exposed, and not too flat, sections of mountains as much matter as is contained in an evenly spread layer one metre in depth. The most prominent of the recently formed mountain chains, whose completion is to be referred to the end of the Tertiary period, are found, on the one hand in Central Asia, with crests about 6000 metres high, and, on the other hand, in the Andes of South America \\dth crests about 5000 metres high — a mean height of 5500 metres. Of the nexi- older mountain chains, dating from the Carboniferous period, the most undisturbed, however, now possess a mean height of only 1750 metres, so that, supposing the original height to be the same in both instances, an interval of five and a quarter million years must be supposed to have elapsed (between the two formations). Biit we know that three of such intervals, which is equivalent to 16 million years, elapsed since the end of the pre-Cambrian period (that is, since the appearance of the first organisms); consequently, about sixteen and a half million years separates the pre-Cambrian period from our time. If, furthermore, we take the close of the pre-Cambrian era as the middle of the whole period since the first formation of the terrestrial crust, thirty-three milhon years have elapsed since that time. Another method might be designated as the chemical method. It was first proposed by J. Joly in 1899. Joly calculated the quantity of sodium in sea water and also in the water carried annually by the rivers to the sea, and thus estimated the interval during which erosion has been proceeding and the time of the deposition of the first sediments. In this manner he arrived at the conclusion that, to con- vey to the ocean the quantity of sodium which is contained therein, ninety million years are necessary. Basing his enquiries on the comparative absence of lime in the oceans and rivers, FAigene Dubois, in 1900, endeavoured in like manner to contribute to the solution of this question, placing the age of the world at forty-five million years. Finally, E. von Romcr approached the question from the considera- tion of the quantity of salt in the sea-water and of the amoimt carried by rivers, and estimated that an interval of HiO millions would be necessary to account for present conditions.

The most modern method for determining the age of the world is based on radio-active processes.

E. Rutherford has held that from the amount of helium or lead contained by a mineral its age can be calculated. From the analysis of the amount of helium contained by two primary minerals he esti- mated the interval since the beginning of the Cam- brian period at about 140 million jears. This new and highly interesting method of determining the age of a mineral containing radium or thoriimi has been elaborated by R. J. Strutt. Very suitable for these investigations are the crystals of zirconium in igneous rocks, since these evidently retain within them the helium engendered. From zirconium of the various igneous rocks Strutt made the following calculations for the age of the earth : Post or Late Tertiary, less than 100,000 years; Pliocene, two million years; Miocene, six million years; Mesozoic (Triassic?), 50 million years; Pala>ozoic, 140 million years; Lower Devonian, 200 million years; Archaic, from 200 to 600 million years. Boltwood developed the method of determining the age of minerals containing a large proportion of uranium from the amount of lead they contain, inasmuch as it is highly probable that lead is the final product of the developments of uranium into radio-active substances. He obtained from minerals containing uranium which belonged to the same strata values which varied between 1000 and 11,000 millions; the cause of this great variability was that he neglected in his calculation the fact that all these minerals, even in the primary, contained more or less lead, which was not generated in the mineral by radio-active processes. This error in Boltwood's calculation was first pointed out by G. F. Becker. Finally, Soddy has endeavoured to find a maximum for the age of the earth by pointing out that the age of uranium is hmited, so that minerals, even though they originally consisted entirely of metallic uraniimi, have an age of less than 1000 million years.

According to the above-mentioned theories, it can only be said: that since the beginning of the Algon- quian period, if we base our calculations on the coohng process of the earth, more than 30 million of years have elapsed and if we base our computations on the theory of radio-activity, less than 600 milUon years, so that a period of from 100 to 200 miUion years may perhaps be regarded as the most Ukely hy- pothesis.

With the question of the age of the world is very frequently connected the question of the age of man. This can be deduced only from fossilized human re- mains and from finds of human implements. Many- regard the eoliths (stone fragments resembUng primi- ti^-e tools) as vestiges of man or of some man-like being, although their artificial origin is not yet proved. No bones of Tertiary man have as yet been discovered, nor any traces of lower precursors of man. The pithecanthropus erectus Dvh. is now almost universally regarded as a large animal of the species hylobatidoe; furthermore, it is not Tertiary, as Dubois supposed, but Diluvial (probably old Diluvial), as has been shown by J. Elbert, W. Volez and the Selenka expedi- tion on the basis of geological and patoontological investigations. It is also very doubtful if the human bones, reported by Santiago Roth, Doring, and Anie- ghino to have been found in the Pampas formation in Argentina, belong to the Tertiary period; as for the neck-bone (.\tlas), found in the Tertiary strata of Monte Hermoso (.Argentina), and described by Leh- mann-Nitsche under the name Homo neogants, the attempts hitherto made to prove it of human origin are entirely unconvincing. However, although there is at present no evidence to prove the existence of Tertiary man, it is not impossible that in the near future such evidence may be forthcoming. Espe- cially inadequate have been the investigations in Africa and in tlu- East, where, jiresunuibly, we must seek the earliest abode of mankind. Indeed, even in Italy and Greece systematic investigations have only