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WORLD

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 soHdifica- tion the whole earth (its covering of stone and its kernel of iron) must have been of the same tempera- ture (about 3000° C), and that the gcothermic 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 supposi- tions are, however, uncertain. Thus, the initial temperature at the moment of the soUdifieation 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 compo.sition, radio-active heat, etc.) were not taken into consideration, although these greatly retarded the cooling of the earth. On hj-potheses similar to those employed by Lord Kelvin are based the calculations of O. Fischer, who places the age of the world at 33 miUion years, and, those of Dawison, MeUard Keade, and H. G. Darwin, who place it at 100 miUion years.

CI. lung 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° C, 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 vmder this crust the red- hot magma would still remain, and that in conse- quence of the increase of weight a stratification of matter and a change of temperature according to the depth must necessarilj' be supiwsed. On the basis of his calculations he set the age of the world at 60 miUion 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 1200oC., since otherwise all the parti- cles of stone would not have been knitted together. Accordingly the minimum figure for the time elapsed since the Algonquian period (when, probably, Ufe was first possible) might be placed at 30 million years. This figure, however, apj^ears to be too small, since during the process of cooling quantities of heat (melt- ing heat, etc.) were relea.sed. The geologists as a body are of opinion that the inter\-al allowed by the physicists is too .short. In reply to Thom.son, 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 motmtains an ine.alcu- lably long period must be granted, since no im])ortant diminution can have taken place in the historical era. In the same period of several thousand years, more- over, 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 ca.ses, towered a mainland which was sometimes covered with primeval tropical forests, sometimes groaned (hke Greenland) tmder a coating of ice, and again heard the sand storms roar above it.

Facts like these suggest an idea of the great dura- tion of geological eras, but they afford us no data for an exact estimate of this duration. Only details can be calc\ilated in this manner. Thus, for example, we know that Niagara Falls hae receded about 12 kilo- XV.— 45

metres since the Diluvial glacial period. On the basis of its annual recession, Lyell has ascribed to this process a iieriod of 36,000 years; the later observa- tions 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 hae 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 tliousands of years, would be equivalent to a single one of these "zones". It is thus easily understood why evolu- tionists, who would see the manifold diversity of existing animal and plant forms derived from the same original living organism, make the most exces- sive demands of all for the most extended geological eras.

In 1900 Gilbert pointed out that only rhythmic processes are a suitable means for calculating geo- logical eras, the rhyt hms of precession and eccent ricity 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 exten- sive rhythm of eccentricity, the orbit now approxi- mating to the form of a circle, now to that of a com- paratively narrow ellipse. Precession and eccen- tricity influence the chmate 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 simimer and winter varies. There are, con- sequently, for each hemisphere maximtmi and mini- mum 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 cal- culated that a corresponding jjcriod of higher eccen- tricity 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 7.50,000; 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 carbon- iferous 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 connexion between eccentricity and glacial periods has not yet been estabhshed.

Of other attempts to calculate the age of the world a few may be mentioned. Newcomb takes as his starting-point the coohng 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 mterval, or 100 million years, must be accepted