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PRINCIPLES OF ANIMAL BIOLOGY 

 Geological Time Scale 



1 Together constituting the Carboniferous (coal age). 



Usually only a small part of this scale is represented in exposed 

 strata at one place. One of the more extensive exposures of the crust 

 is in the walls of the Grand Canyon in northern Arizona. The Colorado 

 River at its bottom is cutting its way through granite at the rate of 

 perhaps an inch in a century. Just above it in the slopes are Archeozoic 

 rocks; the rim, a mile above, is Permian. Between these are mostly 

 undisturbed strata in the order of the scale. 



How old the strata are in years may be computed from the trauvs- 

 formation of radioactive substances. The element uranium is being 

 constantly transmuted into other simpler elements, the chain ending in 

 lead and helium. The rate of change is constant, and independent of 

 surrounding conditions. Where uranium is present, the amount of it 

 and the amount of lead are ascertained as accurately as possible. If it be 



