THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 6l 



limestones is an uncertain quantity, and the rate of deposi- 

 tion of limestones is a matter of vague estimation. 



Errors Affecting the Values of Actual, not Relative Time-lengths. 

 — But allowing that the various data are quantities of only ap- 

 proximate values, in making the estimates the errors are of 

 such a nature that they do not materially affect the time-ratios. 

 These time-ratios, it must be remembered, are the reliable 

 facts that we get from the computation ; whether the total 

 time be 48,000,000 or 480,000,000, the probability is that 

 che proportions derived by this method of calculation are 

 correct to the degree of accuracy of our knowledge of the 

 facts themselves. 



By comparing the three series of values, assigned upon 

 this principle to the several divisions of the time-scale, by 

 Dana, Walcott, and the author, as tabulated in the above 

 scheme (p. 54), it will be seen, reducing them to percent- 

 ages, that there is a general agreement in the results. 



The percentages for the three grand divisions are, accord- 

 ing to the three computers, as follows : 



Dana. Walcott. Williams. Average of the 



three estimates. 



Cenozoic 6.25 10.526 15 10 + 



Mesozoic . . , , 18.75 26.315 20 2 1 -j- 



Paleozoic 75-00 63.156 65 68 — 



100.00 99-997 100 99 + 



Various Estimates of the Length of Geological Time. — Many 

 estimates, varying greatly in amount, have been made as to the 

 total length of time represented in the formation of the pres- 

 ent stratified crust of the earth. The extremes are seen in 

 McGee's estimate * that the demands of evolution and the facts 

 of geology warrant the assumption that 7,000,000,000 years 

 have passed since the earliest fossiliferous rocks were formed, 

 and twice as long, 14,000,000,000, since the earth began its 

 planetary form, and in the old conception, on the other hand, 

 which was supposed to be interpreted from the Bible record, 

 of 6000 years from the beginning of creation to the present 

 time. Both of these are probably far outside the limits of fact. 



* Am. Anthropologist, October, 1892, vol. v pp. 327-344. 



