ELEMENTS OF THE GEOLOGICAL TIME -SCALE. 291 



continuous. There were series of successive faunas or even sub- 

 faunas, in which were expressed the general features of the evo- 

 lution of life on the globe. The species preserved and known 

 present but a very imperfect representation of the species that 

 were living ; but of those preserved in one formation there are 

 generally found in the succeeding formations representatives of 

 the same or closely allied genera ; so that for-the kinds of organ- 

 isms whose remains are best preserved the record is fairly con- 

 tinuous for the grander rock -systems in terms of the generic, and 

 in some cases of the specific characters. 



While the conditions of deposition for a particular region 

 remained relatively constant and uniform, the strata were accum- 

 ulated in successive beds one upon another, and then the thick- 

 ness of the deposits of the same kind, with proportionate thickness 

 for deposits of different kinds, constitute a scale of definite time 

 value ; a foot of deposit representing a period of time, and the 

 relative time-separation for two faunas would be represented by 

 the thickness of the strata between them. It was on this princi- 

 ple that the time -ratios of Dana were estimated. The maximum 

 thickness of the known strata of each geological system was 

 taken. The limestones were assumed to represent five times the 

 time -value that is represented by the other sedimentary deposits 

 per foot ; or in other words, every foot of limestone was esti- 

 mated as equivalent to five feet of other sedimentary deposits in 

 making up the time -ratios. On this principle Dana estimated 

 the time -ratio for the several geological periods to be as fol- 

 lows : 



Quaternary - - - Va. \ r- 



■^ .• 0/ r (-enozoic i. 



iertiary _ - - Va S 



Cretaceous - - - i ) 



Jurassic - - - ^}i r Mesozoic 3 -f-- 



Triassic - - - i j 



Carboniferous - - 2 



Devonian - - - 2 



Silurian (Upper) - i ; 



Ordovician (Lower Silurian ) 6 

 Potsdam - . - - i 



>- Palaeozoic 12 -|-. 



