1006 



SCIENCE. 



[N. S. Vol. XI. No. 287. 



gist is able to make an estimate of the re- 

 lative progress in evolution before and after 

 the Eocambrian epoch. The only absolute 

 blank left by the time ratios pertains to an 

 azoic age vyhich may have intervened be- 

 tween 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 estimated the age of Lake 

 Michigan, and Upham the age of the glacial 

 lake Agassiz ; and from the erosion of the 

 Niagara gorge the age of the river flowing 

 through it has been estimated. But while 

 these discussions have yielded conceptions 

 of the nature of geologic time, and have 

 served to illustrate the extreme complexity 

 of the conditions which affect its measure- 

 ment, they have accomplished little toward 

 the determination of the length of a geo- 

 logic period ; for they have pertained only 

 to a small fraction of what geologists call a 

 pei'iod, and that fraction was of a some- 

 what abnormal character. 



Wholly independent avenues of approach 

 are opened by the study of processes per- 

 taining 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 geo- 

 logic record could not begin until that con- 

 dition 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 dif- 



ferent from what actually obtains. 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 in- 

 quiry has also yielded an obscure outer . 

 limit to the antiquity of the earth as a hab- 

 itable globe. 



One of the most elaborate of all the com- 

 putations starts with the assumption that at 

 an initial epoch, when the outer part of the 

 earth was consolidated from a liquid con- 

 dition, the whole body of the planet had 

 approximately the same temperature ; and 

 that as the surface afterward cooled by out- 

 ward 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 knowledge of 

 certain properties of rock — namely, its fusi- 

 bility, conductivity 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, ils is but a minute fraction of the 

 whole amount with which it continually 

 parts, for its radiation is equal in all direc- 

 tions, and the earth is but a speck in the 

 solar sky. On the assumption that this 

 immense loss of heat is accompanied by a 

 corresponding loss of volume, the sun is 



