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Various attempts have been made to establish the age of the world. Two groups of scientists have especially busied themselves with this question: physicists and geologists. The most notable attempt is that of the physicist Thomson (Lord Kelvin), who based his calculations on Laplace's theory that the earth originated in a fiery fluid magma. While in this magmatic state, the earth as a whole must have reacted to the attraction of the moon as the oceans now do, with ebb and flow. These constant and strong tides must in long intervals have retarded the rotation of the earth to such an extent that 7200 million years ago the earth must have rotated with double its present rapidity. Again, the polar flattening of the earth was likewise caused by this rotation, and Thomson calculated that this flattening could not have been effected to such a degree if the terrestrial crust had been solid, and the rotation of the earth the same as today. Consequently, from the extent of the flattening a conclusion may be drawn as to the rapidity of the rotation at the time of the superficial solidification of the globe. Thomson reckoned that, so long as the earth revolved double as quickly, the flattening at the poles must have been much greater than now, and thus estimated that the solidification of the terrestrial crust occurred less than 1000 million years ago. Thomson later approached the same problem in another way, by using Fourier's laws of thermal conductivity to arrive at the time elapsed since the upper crust became solid. His hypothesis was that at the moment of solidification the whole earth (its covering of stone and its kernel of iron) must have been of the same temperature (about 3000 degrees Celsius), and that the geothermic level on the upper surface must have been twenty-eight metres; consequently, the time elapsed was in round numbers 100 million years. Some of these suppositions are, however, uncertain. Thus, the initial temperature at the moment of the solidification of the terrestrial crust was set at too high a figure, and the geothermic level was rated too low. Besides, heat-producing processes (e.g., melting heat, heat of chemical composition, radio-active heat, etc.) were not taken into consideration, although these greatly retarded the cooling of the earth. On hypotheses similar to those employed by Lord Kelvin are based the calculations of 0. Fischer, who places the age of the world at 33 million years, and, those of Dawison, Mellard Reade, and H.G. Darwin, who place it at 100 million years.

Cl. King pointed out that the cooling could be reckoned only from the time when the terrestrial crust was stable — that is, so thick that it was no longer disturbed by the movement of the tides of fluid magma. In view of the atmospheric pressure then prevailing, the initial temperature for this terrestrial crust must be taken as 1200 degrees Celsius, and the age of the world set at 10 million years. Thomson subsequently expressed his adhesion to this view. G.F. Becker, on the other hand, pointed out that a crust of only eighty miles in thickness would satisfy the above conditions, that under this crust the red-hot magma would still remain, and that in consequence of the increase of weight a stratification of matter and a change of temperature according to the depth must necessarily be supposed. On the basis of his calculations he set the age of the world at 60 million years. According to the present condition of physical knowledge it may be said that the initial temperature of the terrestrial crust may really have been a little over 1200 degrees Celsius, since otherwise all the particles of stone would not have been knitted together. Accordingly the minimum figure for the time elapsed since the Algonquian period (when, probably, life was first possible) might be placed at 30 million years. This figure, however, appears to be too small, since during the process of cooling quantities of heat (melting heat, etc.) were released. The geologists as a body are of opinion that the interval allowed by the physicists is too short. In reply to Thomson, Sir Archibald Geikie pointed out that enormous periods must unconditionally be supposed to explain the processes on our globe. We know, for example, that the present mountains are very recent developments, that they were preceded by numerous older mountain systems, of which only scanty remains now exist or which have entirely disappeared. For the raising and levelling of each of these mountains an incalculably long period must be granted, since no important diminution can have taken place in the historical era. In the same period of several thousand years, moreover, the relation between mainland and sea has not altered, except in the case of very limited areas. Yet the study of existing continents shows that deep-bedded oceans formerly occupied their place, and that between these, in many cases, towered a mainland which was sometimes covered with primeval tropical forests, sometimes groaned (like Greenland) under a coating of ice, and again heard the sand storms roar above it.

Facts like these suggest an idea of the great duration of geological eras, but they afford us no data for an exact estimate of this duration. Only details can be calculated in this manner. Thus, for example, we know that Niagara Falls has receded about 12 kilometres since the Diluvial glacial period. On the basis of its annual recession, Lyell has ascribed to this process a period of 36,000 years; the later observations of Gilbert and Woodward have, however, reduced this figure to 7000 years. It was long hoped that the comparison of the denudation of the drainage basins of the individual rivers would afford a measure for geological eras. It has, however, been shown that the Nile lowers the level of its basin about one metre in 17,000 years, while the Po requires only 2400 years; the Indian rivers effect the same result in 5200 years, while the slow streams of Central Europe require 164,000. Equally impossible is it to arrive at any generally legitimate conclusions from the growth of sediments; every observation, however accurately carried out, has only a local value, and consequently no conclusions can be drawn from the extent of the sedimentary rocks of earlier formations. One other method has been tried. The alterations which the fossil remains reveal in successive eras have been employed to divide geological formations into smaller sections or zones. The Jura formation alone has already exhibited more than thirty of such zones; the whole Diluvial period and modern times together, on the other hand, show not the least changes in the organisms, so that the latest section of the world's history, which has already occupied many thousands of years, would be equivalent to a single one of these "zones". It is thus easily understood why evolutionists, who would see the manifold diversity of existing animal and plant forms derived from the same original living organism, make the most excessive demands of all for the most extended geological eras.

In 1900 Gilbert pointed out that only rhythmic processes are a suitable means for calculating geological eras, the rhythms of precession and eccentricity being especially of value. Precession refers to the displacement of the earth's axis, which occurs within a period of 26,000 years. But the alterations in the form of the earth's orbit involve the far more extensive rhythm of eccentricity, the orbit now approximating to the form of a circle, now to that of a comparatively narrow ellipse. Precession and eccentricity influence the climate of our globe, since the summer half-year is longer now for one, now for the other hemisphere, and thus the difference in the length of summer and winter varies. There are, consequently, for each hemisphere maximum and minimum temperatures which return periodically. These conditions form the principle on which James Croll attempted to calculate the glacial period, which lies between the Tertiary and Diluvial epochs. He calculated that a corresponding period of higher eccentricity began about 240,000 years ago and lasted until 80,000 years ago, which time he accepts as the glacial period. Other glaciations of the earth probably occurred 750,000; 850,000; 2,500,000; and 2,600,000 years ago, and may be expected in 500,000; 600,000, and 900,000 years — alternately in the northern and southern hemispheres. In point of fact the traces of a large number of these glacial periods have already been recognized — for example, the Permian carboniferous glacial period on the borders of the Indian Ocean — but to admit a rhythm of a few hundreds of thousands of years, we must suppose hundreds of glacial periods to have occurred during the enormous length of geological eras. Besides, the connection between eccentricity and glacial periods has not yet been established.

Of other attempts to calculate the age of the world a few may be mentioned. Newcomb takes as his starting-point the cooling of the sun, and finds that the longest period that can have elapsed since the formation of water on the earth is ten million years. W. Upham, on the other hand, believes that ten times that interval, or 100 million years, must be accepted as having passed since the appearance of the first organisms. T. Mellard Reade approached the question from an entirely different standpoint. He calculated that on an average about 3000 years 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 million years, while McGee, on the same basis of sedimentary 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 Neumayer 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 cooling theory of Thomson, he places the age of the world at 500 million years.

On the development of mountain chains is based the calculation of P. Kreichgauer also. He starts from the hypothesis that 1400 years are on an average necessary 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 with crests about 5000 metres high — a mean height of 5500 metres. Of the next 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). But 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 million 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 convey 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, Eugene 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 Romer approached the question from the consideration of the quantity of salt in the sea-water and of the amount carried by rivers, and estimated that an interval of 160 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 be estimated the interval since the beginning of the Cambrian period at about 140 million years. This new and highly interesting method of determining the age of a mineral containing radium or thorium 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; Palaeozoic, 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 limited, so that minerals, even though they originally consisted entirely of metallic uranium, 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 Algonquian period, if we base our calculations on the cooling 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 million years, so that a period of from 100 to 200 million years may perhaps be regarded as the most likely bypotbesis.

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 remains and from finds of human implements. Many regard the coliths (stone fragments resembling primitive 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 Dub. 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. Velez and the Selenka expedition on the basis of geological and paleontological investigations. It is also very doubtful if the human bones reported by Santiago Rotb, Döring, and Ameghino to have been found in the Pampas formation in Argentina, belong to the Tertiary period; as for the neck-bone (Atlas), found in the Tertiary strata of Monte Hermoso (Argentina), and described by Lehmann-Nitsche under the name Homo neogoeus, 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. Especially inadequate have been the investigations in Africa and in the East, where, presumably, we must seek the earliest abode of mankind. Indeed, even in Italy and Greece systematic investigations have only begun. So far France exhibits the greatest number of the abodes and hunting-places of pre-historic man. These and all others whose place in the stratification can be unequivocally determined indicate that the first appearance of man in Europe must be referred to the middle of the Quaternary glacial period. This fact has been established by the investigations of Penck, but especially by those of Boule and Obermaier, who refer the event to the third intermediary glacial period.

The age of the human race is thus largely bound up with the question of the time of the Quaternary glaciation of Europe. We have already given the calculations of James Croll, based on astronomical principles, which place the conclusion of this period about 80,000 years ago and its beginning about 240,000 years ago, so that the first appearance of man would, according to this estimate, have occurred some 160,000 years ago. But, apart from the fact that Croll's hypothesis is based on erroneous assumptions, it has been recognized that all the earlier figures for the age of mankind (Lyell, 100,000 to 200,000 years, Lapparent, 230-240,000 years) must be greatly reduced. For example, it has already been mentioned that the time which Niagara required to recede 12 km., estimated by Lyell at 36,000 years, is now given by Gilbert and Woodward as not more than 7000 years. Similar conditions (the recession of a waterfall since the glacial period) may be studied on the Mississippi in Minnesota, and Winchell came to the astonishing conclusion that this river did not require more than 8000 years to excavate its course. A study of some Scandinavian rivers leads to the same conclusion, and the waterfall on the Tosa, a tributary of Lake Maggiore which has existed since the glacial period, indicates a much shorter interval.

Another method for estimating the age of the cultural remains of Diluvial man is based on the thickness of the layers of clay which is pressed down as dust in the interior of protected caves. As an example may be taken the cave known as Teufelsloch at Stramberg, near Neutitschein in Moravia. This contains traces of man from the lower layer of the Palaeolithic age up to the present. Not far from the entrance, the thickness of the uppermost layer, which extends back to the late pre-historic period, measures 30-70 cm. Below this is found cave clay 30-50 cm. in depth with post-glacial prairie animals and cattle, and still lower 30-40 cm. of earth with glacial prairie animals. The last layer contains most of the traces of man, especially the lower stage of the Early Stone age. One may thus estimate the interval since man's first appearance at from 8000 to 10,000 years. Other calculations based on the deposits made by rivers etc. are much more uncertain, inasmuch as some catastrophe (e.g. an avalanche) might bring more matter in one day than would otherwise be conveyed in 100 years. However, the latter calculations have also their sponsors. Thus, Heim has estimated the postglacial period at 16,000 years on the basis of his observations made on a moraine in the Lake of Lucerne; Bruckner suggests 14,000 to 15,000 years, based on observations of the alluvial deposits of the Aar. Both these figures may, however, be too high. According to Morlot the Finiere required only a period of 10,000 years to form the cone-shaped bank at its mouth on the Lake of Geneva. In this bank Roman bricks were found at a depth of 1-2 m.; two metres deeper, earthen vessels and a pair of bronze tongs; and about 3 metres still deeper, rude pottery and the bones of some domestic animals. The remains dating from the Roman period form the none too reliable basis for the calculation. On the ground of these and similar calculations Schaafhausen gives the age of mankind as 10,000 to 15,000 years, which, however, is purely an estimate. One thing at least is certain: instead of the 100,000 and more years formerly given, the age of mankind may with much greater probability be placed at about 10,000 years as the mean approximation. We are thus approaching ever nearer to the chronology of the Bible, according to which the Jews reckon that 5673 years have now (1912) elapsed since the creation of the world, or rather of Adam.