LIFE IN THE PAST 157 



which the present rivers are carrying away their river 

 basins, and hence who calculate how long it has taken 

 the rivers of the globe to wash away all the rocks 

 which it is quite clear have been carried out. Still 

 others have attempted to solve the problem by seeing 

 how much salt the rivers are carrying into the sea, 

 and consequently how long it must have taken the sea 

 to become as salt as it is. A very late attempt has 

 been based on the alteration in the minerals that show 

 radio-activity. Conservative estimates, based on all 

 of these, would give us a figure on which we must 

 not count with any exactness, but which will serve at 

 least to mark the present trend of opinion. We may 

 put this figure at one hundred millions of years. 



The following table gives us the names of the 

 periods into which the geologist has divided the past 

 history of the earth. The first column gives a simple 

 name, which, in each case, is a translation of the tech- 

 nical name the geologist gives to the era. This tech- 

 nical name is also given in parenthesis. The second 

 column shows the number of years ago at which this 

 period may be placed, while the third column gives a 

 series of names most of which are in use in geology 

 and which are intended to indicate the stage of ad- 

 vancement of the higher animals in that particular 

 period. Some of these names are perhaps giving way 

 to later terms, but all of them will be understood by 



