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 ................. T)/~ 



^ , . \ \ Cenozoic I. 



Tertiary ..................... f j 



Cretaceous .................. I } 



Jurassic ..................... i V Mesozoic 3^. 



Triassic .................... . I ) 



Carboniferous ...... ........... 2 ^| 



Devonian .................... 2 



Silurian (Upper) ............. i^ ^ Paleozoic 12^. 



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 

 3 -< Miocene-Pliocene 

 ( Eocene 

 Cretaceous 



Jura-Trias 



f Permo-Carboniferous ........... 



Devonian .................... 



Silurian ...................... 



1^ Cambrian ..................... 



thus forming nine divisions of equal length. 



I 



* Dana's "Manual of Geology," 30! 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- 



