July 19, 1900] 



NA TURE 



277 



A more general rhythm has been ascribed to the tidal retard- 

 ation of rotation and the resulting change of the earth's form. 

 If the body of the earth has a rather high rigidity, we should 

 expect that it would for a lime resist the tendency to become 

 more nearly spherical, while the water of the ocean would ac- 

 commodate itself to the changing; conditions of equilibrium by 

 seeking the higher latitudes. Eventually, however, the solid 

 earth would yield to the strain and its figure become adjusted to 

 the slower rotation, and then the mobile water would return. 

 Thus would be caused periodic transgressions by the sea, oc- 

 curring alternately in high and low latitudes. 



Another general rhythm has been recently suggested by 

 Chamberlin in connection with the hypothesis that secular varia- 

 tions of climate are chiefly due to variations of the quantity of 

 carbon dioxide in the atmosphere.^ The system of inter- 

 dependent factors he works out is too complex for presentation 

 at this time, and I must content myself with saying that his 

 explanation of the moraines of recession involves the interaction 

 of a peculiar atmospheric condition with a condition of glacia- 

 tion, each condition tending to aggravate the other, until the 

 cumulative results brought about a reaction and the climatic 

 pendulum swung in the opposite direction. With each suc- 

 cessive o.scillation the momentum was less, and an equilibrium 

 was finally reached. 



Few of these original rhythms have been used in computations 

 of geologic time, and it is not believed that they have any posi- 

 tive value for that purpose. Nevertheless, account must be 

 taken of them, because they compete with imposed rhythms for 

 the explanation of many phenomena, and the imposed rhythms, 

 wherever established, yield estimates of time. 



The tidal period, or the half of the lunar day, is the shortest 

 imposed rhythm appealed to in the explanation of the features 

 of sedimentation. It is quite conceivable that the bottom of a 

 quiet bay may receive at each tide a thin deposit of mud which 

 could be distinguished in the resulting rock as a papery layer or 

 lamina. If one could in some way identify a rock thus formed, 

 he might learn how many half-days its making required by 

 counting its lamince, just as the years of a tree's age are learned 

 by counting its rings of growth. 



The next imposed rhythm of geologic importance is the year. 

 There are rivers, like the Nile, having but one notable flood in 

 each year, and so depositing annual layers of sediment on their 

 alluvial plains and on the sea beds near their mouths. Where 

 oceanic currents are annually reversed i)y monsoons, sedimenta- 

 tion may be regularly varied, or interrupted, once a year. 

 Streams from a glacier cease to run in winter, and this 

 annual interruption may give a definite structure to resulting 

 deposits. It is therefore probable that some of the lamince or 

 strata of rocks represent years, but the circumstances are 

 rarely such that the investigator can bar out the possibility that 

 part of the markings or separations were caused by original 

 rhythms of unknown period. 



The number of rhythms existing in the solar system is very 

 large, but there are only two, in addition to the two just 

 mentioned, which seem competent to write themselves in a 

 legible way in the geologic record. These are the rhythms of 

 precession and eccentricity. 



Because the earth's orbit is not quite circular and the sun's 

 position is a little out of the centre, or is eccentric, the two 

 hemispheres into which the earth is divided by the equator do 

 not receive their heat in the same way. The northern summer, 

 or the period during which the northern hemisphere is inclined 

 toward the sun, occurs when the earth is farthest from the sun, 

 and the northern winter occurs when the earth is nearest to the 

 sun, or in that part of the orbit called perihelion. These 

 relations are exactly reversed for the southern hemisphere. The 

 general effect of this is that the southern summer is hotter than 

 the northern, and the southern winter is colder than the 

 northern. In the southern part of the planet there is more 

 contrast between summer and winter than in the northern. 

 The sun sends to each half the same total quantity of heat in 

 the course of a year, but the difference in distribution makes the 

 climates different. The physics of the atmosphere is so intricate 

 a subject that meteorologists are not fully agreed as to the 

 theoretical con.sequences of these differences of solar heating, 

 but it is generally believed that they are important, involving 

 differences in the force of the wind*, in the vel( city and course 

 of ocean currents, in vegetation, and in the extent of glaciers. 



1 An aUerapt to fr.-ime a working hypothesis of the cause of glacial 

 periods on an atmospheric basis. Joum. Cl-o!., vol. vii., 1899. 



NO. 1603, VOL. 62] 



Now, the point of interest in the present connection is that 

 the astronomical relations which occasion these peculiarities are 

 not constant, but undergo a slow periodic change. The relation 

 of the seasons to the orbit is gradually shifting, so that each 

 sea.son in turn coincides with the perihelion ; and the climatic 

 peculiarities of the two hemispheres, so far as they depend on 

 planetary motions, are periodically reversed. The time in which 

 the cycle of change is completed, or the period of the rhythm, is 

 not always the same, but averages 21,000 years. It is commonly 

 called the precessional period.^ 



Assuming that the climates of many parts of the earth are 

 subject to a secular cycle, with contrasted pha.ses every 10,500 

 years, we should expect to find records of the cycle in the 

 sediments. A moist climate would tend to leach the cal- 

 careous matter from the rock, leaving an earthy soil behind, 

 and in a succeeding drier climate the soil would be carried away ; 

 and thus the adjacent ocean would receive first calcareous and 

 then earthy sediments. The increase of glaciers in one hemi- 

 sphere would not only modify adjacent sediments directly, but, 

 by adding matter on that side, would make a small difference in 

 the position of the earth's centre of gravity. The ocean would 

 move somewhat toward the weighted hemisphere, encroaching on 

 some coasts and drawing down on others ; and even a small 

 change of that sort would modify the conditions of erosion and 

 deposition to an appreciable extent in many localities. 



BIytt ascribed to this astronomical cause the alterations of 

 bog and forest in Scandinavia, as well as other sedimentary 

 rhythms observed in Europe ; and it has seemed to me com- 

 petent to account for certain alternations of strata in the Cre- 

 taceous formations of Colorado. Croll used it to explain inter- 

 glacial epochs, and Taylor has recently applied it to the moraines 

 of recession. 



The remaining astronomical rhythm of geological import is 

 the variation of eccentricity. At the present time our greatest 

 distance from the sun exceeds our least distance by its thirtieth 

 part, but the difference is not usually so small as this. It may 

 increase to the seventh part of the whole distance, and it may 

 fall to zero. Between these limits it fluctuates in a somewhat 

 irregular way, m which the property of periodicity is not con- 

 spicuous. The effect of its fluctuation is inseparable from the 

 precessional effect, and is related to it as a modifying condition. 

 When the eccentricity is large the precessional rhythm is em- 

 phasised ; when it is small the precessional effect is weak. 



The variation of eccentricity is connected with the most 

 celebrated of all attempts to determine a limited portion of 

 geological time. In the elaboration of the theory of the Ice age 

 which bears his name, Croll correlated two important epochs 

 of glaciation with epochs of high eccentricity computed to have 

 occurred about 100,000 and 210,000 years ago. As the analysis 

 of the glacial history progresses, these correlations will eventu- 

 ally be established or disproved, and should they be established 

 it is possible that similar correlations may be made between 

 events far more remote. 



The studies of these several rhythms, while they have led 

 to the computation of various epochs and stages of geologic 

 time, have not yet furnished an estimate either of the entire 

 age of the earth or of any large part of it. Nevertheless, I 

 believe that they may profitably be followed with that end in 

 view. 



The system of rock layers, great and small, constituting the 

 record of sedimentation, may be compared to the scroll of a 

 chronograph. The geological scroll bears many separate lines, 

 one for each district where rocks are well displayed, but these 

 are not independent, for they are labelled by fossils, and by 

 means of these labels can be arranged in proper relation. In 

 each time line are little jogs — changes in kind of rock or breaks 

 in continuity — and these jogs record contemporary events. A 

 new mountain was uplifted, perhaps, on the neighbouring con- 

 tinent, or an old uplift received a new impulse. Through what 

 Davis calls stream piracy a river gained or lost the drainage of a 

 tract of country. Escaping lava threw a dam across the course 

 of a stream, or some Krakatoa strewed ashes over the land and 

 gave the rivers a new material to work on. The jc^s may be 

 faint or strong, many or few, and for long distances the lines 

 may run smooth and straight ; but so long as the jogs are 

 irregular they give no clue to time. Here and there, however, 

 the even line will betray a regularly recurring indentation or 



the period of the precession of the 

 equinoxes as referred to perihelion; but the perihelion is itself in motion. 

 As referred to a fixed star the prece.ssion of the equinoxes has an average 

 period of about 25,700 years. 



Strictly speaking, 21,000 years 

 red to 



