364 Prof. J. Joly on the 



II. The Age from the Sedimentary Column. 



As the result of the observations of 2'eologists in many 

 parts of the world the maximum thickness of the strata 

 deposited in the various geological periods may be estimated 

 as follows : — 



Recent and Pleistocene 4,000 



Pliocene 18,000 



Miocene 14,000 



Oligocene 12,000 



Eocene 20,000 



63,000 



Upper Cretaceous 24,000 



Lower „ 20,000 



Jurassic. . 8,000 



Trias 17,000 



— 69,000 



Permian 12,000 



Carboniferous 29,000 



Devonian 22,000 



Silurian... 15,000 



Ordovician 17,000 



Cambrian 26,000 



63,000 



58,000 



Keweenawan 50,000 



Animikian 14,000 



Huronian 18,000 



82,000 



Archaean ? 



Total 335,000 feet 



This compilation is due to Professor Sollas (Presidential 

 Address, Geol. Soc. London, 1909). 



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 accumulated 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 fortunately no rocks have been more carefully 

 studied within recent years. The appearance of the mono- 

 graph of Van Hise and Leith (Bulletin 360. IT. S. Geol. 

 Survey, 1909) places the known facts at our disposal along 

 with explanatory remarks of the most helpful character. 



