810 J. BARRELL MEASUREMENTS OV GEJOLOGlC TIME 



attempts to sound the depths of geologic time have in nearly all cases 

 assumed a uniformity of processes or have made but slight allowance for 

 their variation. In this part a review will be given of the relations of 

 methods to results and the assumptions on which those methods rest. The 

 trend of the modifications indicated by the recognition of the importance 

 of rhythms may then be perceived. It is not the intention to give a formal 

 review of the many previous estimates which have been made of geologic 

 time, since that has been done by others/^ but merely to present that 

 which seems essential in the present connection. 



One of the earliest estimates was based on the evidence of life trans- 

 formations in successive periods. Lyell divided the geological series into 

 twelve periods and estimated — perhaps guessed would be a better word — ■ 

 that 20,000,000 years were demanded for a complete change in the species 

 of each period, or 240,000,000 years in all. This estimate excluded the 

 Primordial of Barrande and the "antecedent Laurentian formations.^' ^^ 

 Darwin considered that CrolTs estimate of 60,000,000 years since the 

 Cambrian period and the previous 140,000,000 3^ears can hardly be suffi- 

 cient for organic evolution. 



From the physical side, the early geologists saw no indications of 

 infancy or of old age on the face of Nature. Nothing less than inter- 

 minable tim'e broken into periods by revolutions appeared to be the testi- 

 mony of the rocks. Lord Kelvin first called attention to the funda- 

 mentally erroneous nature of this conception. Of Kelvin's work A. Geikie 

 states : 



"He pointed out that from the high internal temperature of our globe, in- 

 creasing inwards as it does, and from the rate of loss of its heat, a limit may 

 be fixed to the planet's antiquity. He showed that so far from there being no 

 sign of a beginning, and no prospect of an end, to the present economy, every 

 lineament of the solar system bears witness to a gradual dissipation of energy 

 from some definite starting point. 



"But physical inquiry continued to be pushed forward with regard to the 

 early history and antiquity of the earth. Further consideration of the influ- 

 ence of tidal friction in retarding the earth's rotation, and of the sun's rate of 

 cooling, led to sweeping reductions of the time allowable for the evolution of 

 the planet. The geologist found himself in the plight of Lear when his body- 

 guard of 100 knights was cut down. 'What need you five-and-twenty, ten or 

 five?' demands the inexorable physicist, as he remorselessly strikes slice after 



3^ See especiaUy C. D. Walcott : Geologic time as indicated by the sedimentary rocks 

 of North America. Journal of Geology, vol. i, 1893, pp. 639-676; Smithsonian Report 

 for 1893, pp. 301-334, 1894. 



Arthur Holmes : The age of the earth. Harper and Bros. London and New York, 

 1913. 



sTrinciples of Geology, 10th ed., 1867, vol. 1, p. 301. Quoted from C. D. Walcott, 

 Smithsonian Report for 1893, p. 303, 1894. 



