292 THE JOURNAL OF GEOLOGY. 



Ward, in the fifth annual report of the United States Geo- 

 logical survey, has proposed to adjust these proportions as fol- 

 lows : 



Quaternary-Recent - 



Miocene-Pliocene - 



Eocene - - - - 



Cretaceous - - - 



Jura-Trias _ _ _ 



Permo-Carboniferous 



Devonian _ _ _ 



Silurian _ _ _ 



Cambrian _ _ _ 



thus forming nine divisions of equal length. 



Since Dana's estimate additions have been made to the known 

 thickness of the Cambrian rocks of North America, which should 

 lengthen the Cambrian ratio to 5 in the above table, and dupli- 

 cations 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 somewhat enlarged. The Tertiary esti- 

 mate in Dana's ratios assumes the thickness to be of less ( ^ ) 

 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; but it must 

 be granted that the thickness of the sediments is the prime fac- 

 tor in determining these time -values of the geological scale. 

 However, the conditions of deposition, the fineness or coarse- 

 ness 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. 



After all these uncertainties are weighed the time -ratios formed 

 on this general basis are of great importance in studying the his- 

 tory of organisms, and the value of accuracy in the time -scale is a 

 sufficient reason for calling attention to the points in which greater 



