4 o SCIENCE PROGRESS 



measuring their thickness and selecting as part of the data 

 required those beds which we believe to most completely 

 represent each formation. The total of these measurements 

 would tell us the age of the earth if their tale was indeed 

 complete, and if we knew the average rate at which they 

 have been deposited. We soon, however, find difficulties in 

 arriving at the quantities we require. Thus it is not easy 

 to measure the real thickness of a deposit. It may be folded 

 back upon itself, and so we may measure it twice over. 

 We may exaggerate its thickness by measuring it not quite 

 straight across the bedding or by unwittingly including volcanic 

 materials. On the other hand, there may be deposits which 

 are inaccessible to us ; or, again, an entire absence of de- 

 posits ; either because not laid down in the areas we examine, 

 or, if laid down, again washed into the sea. These sources 

 of error in part neutralise one another. Some make our 

 resulting age too long, others make it out too short. But 

 we do not know if a balance of error does not still remain. 

 Here, however, is a table of deposits which summarises a 

 great deal of our knowledge of the thickness of the strati- 

 graphical accumulations. It is due to Prof. Sollas. 1 



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 „ 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 . > Algonkian .... 14,000 

 Huronian .J 18,000 



82,000 



Archaean ? 



Total 335)°°° f eet - 



1 Address to the Geol. Soc. of London, 1909. 



