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 
