GEOLOGICAX TIME 15 



nearly flat that but little sediment was being carried into the seas 

 and lakes and being deposited. 



It is obvjous, however that this method while very inexact is, 

 when carefully done, much better than no estimates, and when 

 the results are expressed in ratios rather than in actual durations 

 of so many years and we say that one geological period was twice 

 as long as another, or that the time that has elapsed since the last 

 glacial period is but one twenty-fifth of the duration of the whole 

 Pleistocene, we are naming results which while gross from the view- 

 point of a mathematician, physicist or chemist, are very satisfac- 

 tory for the coarse scale of geological work. 



Geologists have grouped the rocks of the earth's crust into the 

 following five major divisions, of which the two earlier are omitted 

 in the table given on a preceding page. These five divisions are 

 in the order of their age: Archeozoic, Proterozoic, Paleozoic, 

 Mesozoic, and Cenozoic. The stratified rocks that contain any 

 abundant display of the life of their time are embraced in the 

 Paleozoic (rocks with ancient life), Mesozoic (rocks with middle 

 life), and Cenozoic (rocks with modern life). If 5 be taken to 

 represent the duration of the Cenozoic, then the Mesozoic will be 

 represented by 12 and the Paleozoic by 27. Unity is commonly 

 considered as a million years but it may be more — it certainly is not 

 much less. 



