THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 49 



Corrections and Elements of Uncertainty in these Estimates. 

 Since Dana's estimate was published additions have been 

 made to the known thickness of the Cambrian rocks of North 

 America, which may lengthen the Cambrian ratio to 5 in the 

 above table, and duplications of thickness due to confusion 

 in regard to the Quebec group may reduce the Ordovician 

 (Lower Silurian) to 5, and the Cretaceous ratio may be some- 

 what enlarged. The Tertiary estimate in Dana's ratios 

 assumes the thickness to be of less (-J) time-value because of 

 the increased rate of deposition due to transportation of 

 rivers. This and many other factors enter in to complicate 

 the time- value of thickness of strata ; and it must be granted 

 that the thickness of the sediments is the prime factor in 

 determining these time-values of the geological scale. 



In the last edition, 1895, of the " Manual," Dana ex- 

 presses the following opinion: "The evidence at present ob- 

 tained appears to favor the conclusion that the relative duration 

 of the Cambrian and Silurian, the Devonian and the Carbon- 

 iferous eras, corresponds to the ratio 4$- : I : I, or perhaps 4 : 

 I : I, the ratio hitherto adopted; and for the Paleozoic, 

 Mesozoic, and Cenozoic, 12 13 : i." However, the condi- 

 tions of deposition, the fineness or coarseness of the clastic 

 fragments, the abundance or rarity of supply of materials, and 

 other variable conditions must be taken into consideration in 

 an accurate reduction of thickness of strata into length of time. 

 Errors, also, whose value is almost impossible of estimation, 

 arise from the intervals between strata, particularly those 

 where unconformity exists. However, after all these uncer- 

 tainties are weighed the time-ratios formed on this general 

 basis are of great importance in studying the history of organ- 

 isms, and the value of accuracy in the time-scale is a sufficient 

 reason for calling attention to the points in which greater 

 accuracy may be attained by further investigation. 



Estimates of Actual Length of Time Highly Hypothetical. 

 It is doubtful if it is possible with our present knowledge to 

 reach an estimate, in years or centuries, of the actual length 

 of geological time which is within 100 or perhaps 200 per 

 cent of the truth. We may accept Dana's estimate of a.t 



