48 GEOLOGICAL BIOLOGY. 



strata of each geological system was taken as a means of de- 

 ducing the relative duration of their formation, as was first 

 done by S. Houghton. 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 estimated as equivalent to five feet of 

 other sedimentary deposits in making up the time-ratios. 

 Dana* estimated the time-ratio for the several geological 

 periods to be as follows : 



Quaternary 1)/- 



^ ^. W Cenozoic i. 



1 ertiary f j 



Cretaceous i \ 



Jurassic i^ I Mesozoic 3^. 



Triassic . i j 



Carboniferous 2 "^ 



Devonian 2 I 



Silurian (Upper) ^\ )- Paleozoic \2\. 



Ordovician (Lower Silurian)... 6 

 Potsdam . . i 



Ward's Estimate. — Lester Ward, in the fifth annual report 

 of the United States Geological Survey, has proposed to ad- 

 just these proportions as follows : 



( Quaternary-Recent i 



3- -\ Miocene-Pliocene i 



y Eocene i 



{Cretaceous I 



Jura-Trias i 



f Permo-Carboniferous I 



Devonian i 



Silurian I 



l^ Cambrian I 



thus forming nine divisions of equal length. 



* Dana's '' Manual of Geology," 3d edition, 1874. In the latest edition, 1895, 

 these estimates are revised and the following remark is made : "There is great 

 doubt over conclusions based on this criterion [i.e., maximum thickness], because 

 thickness is dependant so generally on a progressing subsidence — no subsidence 

 giving little thickness, however many the millions of years that may pass. But 

 as it is the only available method, it is still used," p. 716; also see beyond on 

 P» 49- 



