AGE OE THE EARTH — JOLY. 277 



obviously, give us no information as to the relative durations of the 

 geological epochs. The latter question can be approached in two 

 ways. (1) By means of the stratigraphical column or measured 

 maxima of detrital and chemical deposits, assuming that these were 

 laid down at an approximately uniform rate; and (2) by the radio- 

 active method. I shall first consider the former method. 



THE AGE FROM THE SEDIMENTARY COLUMN. 



As the result of the observations of geologists in many parts of the 

 world, the maximum thickness of the strata deposited in the various 

 geological periods may be estimated as follows : 



Feet. 



Recent and Pleistocene 4, 000 



Pliocene 13, 000 



Miocene 14, 000 



Oligocene 12, 000 



Eocene 20, 000 



63, 000 



Upper Cretaceous 24, 000 



Lower Cretaceous 20, 000 



Jurassic 8, 000 



Trias 17, 000 



69,000 



Permian 12, 000 



Carboniferous '. 29, 000 



Devonian 22, 000 



63,000 



Silurian 15, 000 



Ordovician 17, 000 



Cambrian 26, 000 



58,000 



Keweenawan 50, 000 



Animikian 14, 000 



Huronian 18, 000 



82,000 



Archaean ? 



Total 335, 000 



This compilation is due to Prof. Sollas. 1 



It is not probable that there will be in the future any very large 

 amendment of these figures so far as they refer to post-Algonkian 

 time. The Jurassic, as Sollas observes, seems deficient. The pre- 

 Cambrian is the most obscure among the estimates. It claims our 

 special attention, not only with reference to the thickness of accu- 

 mulated sediments, but in so far as the observations may throw 

 light on the denudative conditions of the time. 



In no part of the world are pre-Cambrian rocks better developed 

 and exposed than in and around the Archaean shield of Canada; and 



1 Presidential Address, Geological Society, London, 1909. 



