2; 6 



NA TURE 



[July 19, 1900 



the Eocambrian, does not represent the beginnings of life, but a 

 well advanced stage, characterised by development along many 

 divergent lines ; and by comparing Eocambrian life with existing 

 life the paleontologist is able to make an estimate of the relative 

 progress in evolution before and after the Eocambrian epoch. 

 The only absolute blank left by the time ratios pertains to an 

 azoic age which may have intervened between the development 

 of a habitable earth crust and the actual beginning of life. 



Erosion and deposition have been used also, in a variety of 

 ways, to compute the length of very recent geologic epochs. 

 Thus, from the accumulation of sand in beaches, Andrews esti- 

 mated the age of Lake Michigan, and Upham the age of the 

 glacial lake Agassiz ; and from the erosion of tlie Niagara 

 gorge the age of the river flowing through it has been estimated. 

 But while these discussions have yielded conceptions of the 

 nature of geological time, and have served to illustrate the extreme 

 complexity of the conditions which affect its measurement, they 

 have accomplished little toward the determination of the length 

 of a geologic period ; for they have pertained only to a small 

 fraction of what geologists call a period, and that fraction was 

 of a somewhat abnormal character. 



Wholly independent avenues of approach are opened by the 

 study of processes pertaining to the earth as a planet, and with 

 these the name of Kelvin is prominently associated. 



As the rotation of the earth causes the tides, and as the 

 tides expend energy, the tides must act as a brake, checking 

 the speed of rotation. Therefore the earth has in the past 

 spun faster than now, and its rate of spinning at any remote 

 point of time may be computed. Assuming that the whole 

 globe is solid and rigid, and that the geologic record could 

 not begin until that condition had been attained, there could 

 not have been great checking of rotation since consolidation. 

 For if there had been, it would have resulted in the gathering 

 of the oceans about the poles and the baring of the land near 

 the equator, a condition very different from what actually ob- 

 tains. This line of reasoning yields an obscure outer limit to 

 the age of the earth. 



On the assumption that the globe lacks something of perfect 

 rigidity, G. H. Darwin has traced back the history of the 

 •earth and the moon to an epoch when the two bodies were 

 united, their separation having been followed by the gradual 

 enlargement of the moon's orbit and the gradual retardation 

 of the earth's rotation ; and this line of inquiry has also yielded 

 an obscure outer limit to the antiquity of the earth as a habit- 

 able globe. 



One of the most elaborate of all the computations starts 

 with the assumption that at an initial epoch, when the outer 

 part of the earth was consolidated from a liquid condition, 

 the whole body of the planet had approximately the same 

 temperature ; and that as the surface afterward cooled by out- 

 >Yard radiation there was a flow of heat to the surface by 

 conduction from below. The rate of this flow has diminished 

 from that epoch to the present time according to a definite 

 law, and the present rate, being known from observation, 

 affords a measure of the age of the crust. The strength of 

 this computation lies in its definiteness and the simplicity of 

 its data ; its weakness in the fact that it postulates a know- 

 ledge of certain properties of rock — namely, its fusibility, con- 

 ■ductivity and viscosity — when subjected to pressures and 

 temperatures far greater than have ever been investigated 

 ■experimentally. 



A parallel line of discussion pertains to the sun. Great as is 

 the quantity of heat which that incandescent globe yields to the 

 earth, it is but a minute fraction of the whole amount with which 

 it continually parts, for its radiation is equal in all directions, and 

 the earth is but a speck in the solar sky. On the assumption 

 that this immense loss of heat is accompanied by a correspond- 

 ing loss of volume, the sun is shrinking at a definite rate, and a 

 computation based on this rate has told how many millions of 

 years ago the sun's diameter should have been equal to the present 

 •diameter of the earth's orbit. M inifestly the earth cannot have 

 been ready for habitation before the passage of that epoch, and 

 so the computation yields a superior limit to the extent of 

 geologic time. 



Before passing to the next division of the subject — the com- 

 putations based on rhythms — a few words may be given to the 

 results which have been obtained from the study of continuous 

 processes. Realising that your patience may have been strained 

 by the kaleidoscopic character of the rapid review which his 

 seemed unavoidable, I shall spare you the recitation of numerical 



details, and merely state in general terms that the geologists, or 

 those who have reasoned from the rocks and fossils, have de- 

 duced values for the earth's age very much larger than have been 

 obtained by the physicists, or those who have reasoned from 

 earth cooling, sun cooling and tidal friction. In order to express 

 their results in millions of years, the geologist must employ from 

 three to five digits, while the physicists need but one or two. 

 When these enormous discrepancies were first realised, it was 

 seen that serious errors must -exist in some of the observational 

 data, or else in some of the theories employed ; and geologists 

 undertook with zeal the revision of their computations, making 

 as earnest an effort for reconciliation as had been made a genera- 

 tion earlier to adjust the elements of the Hebrew cosmogony to 

 the facts of geology. But after rediscussing the measurements 

 and readjusting the assumptions so as to reduce the time estimates 

 in every reasonable way — and perhaps in some that were not so 

 reasonable — they were still unable to compress the chapters of 

 geologic history between the narrow covers of physical limitation ; 

 and there the matter rests for the present. 



The rocks which were formed as sediments show many traces 

 of rhythm Some are composed of layers, thin as paper, which 

 alternate in colour, so that when broken across they exhibit 

 delicate binding. In the time of their making there was a 

 periodic change in the character of the mud that settled from the 

 water. Others are banded on a larger scale ; and there are also 

 handings of texture where the colour is uniform. Many forma- 

 tions are divided into separate strata, as though the process of 

 accretion had been periodically interrupted. Series of hard strata 

 are often separated by films or thin layers of softer material. 

 Strata of two kinds are sometimes seen to alternate through 

 many repetitions. Borings in the delta of the Mississippi sho^v 

 soils and remains of trees at many levels, alternating with river 

 silts. The rock series in which coal occurs are mon )tonous 

 repetitions of shale and sandstone. Belgian geologists have 

 been so impressed by the recurrence of short sequences of strata 

 that they have based an elaborate system of rock notaiion 

 upon it. 



Passing to still greater units, the large aggregates of strata 

 sometimes called systems show in many cases a regular sequence, 

 which Newberry called a " circle of deposition." When com- 

 plete it comprises a sandstone or conglomerate, at base, then 

 shale, limestone, shale and sandstone. This sequence is explained 

 as the result of the gradual encroachment, or transgression as it 

 is called, of the sea over the land and its subsequent recession. 



In certain bogs of Scandinavia deep accumulations of peat are 

 traversed horizontally by layers including tree stumps in such 

 way as to indicate that the ground has been alternately covered 

 by forest and boggy moss. The broad glaciers of the Ice age 

 grew alternately smaller and larger — or else were repeatedly 

 dissipated and reformed — and their final waning was charac- 

 terised by a series of halts or partial readvances, recorded in 

 concentric belts of ice-brought drift. Of these belts, called 

 moraines of recession, Taylor enumerates seventeen in a single 

 system. 



In explanation of these and other repetitive series incorpor- 

 ated in the structure of the earth's crust, a variety of rhythmic 

 causes have been adduced ; and mention will be made of the 

 more important, beginning with those which have the chiracter 

 of original rhythms. 



A river flowing through its delta clogg its channel with 

 sediment, and from time to time shifts its course to a new line, 

 reaching the sea by a new mouth. Such changes interrupt and 

 vary sedimentation in neighbouring parts of the sea. Storms of 

 rain make floods, and each flood may cause a sep irate stratum 

 of sediment. Storms of wind give destructive force to the waves 

 that beat the shore, and each storm may cause the deposit of an 

 individual layer of sediment. Varying winds may drive currents 

 this way and that, causing alternations in sedimentation. 



To explain the forest beds buried in the Mississippi silts it has 

 been suggested that the soft deposits of the delta from time to 

 time settled and spread out under their own weight. . Various 

 alternations of strata, and especially those of the Coal measures, 

 have been ascribed to successive local subsidences of the earth's 

 crusf, caused by the addition of loads of deposit. It has been 

 suggested also that land undergoing erosion may rise up from 

 time to time because relieved of load, and the character of 

 sediment might be changed by such rising. Subterranean 

 forces, of whatever origin, seemingly slumber while strains are 

 accumulating, and then become suddenly manifest in disloca- 

 tions and eruptions, and such catastrophes affect sedimentation. 



NO. 1603, VOL. 62] 



