20 THE HISTORY OF CREATION. 
down to the present day—ainto a hundred equal parts, and if 
then, corresponding to the thickness of the systems of 
strata, we calculate the relative duration of the time of the 
five main divisions or periods according to percentages, we 
obtain the following result :-— 
I. Archilithic, or primordial period e ° » 53.6 
II. Paleolithic, or primary period . ° ° e 32.1 
III. Mesolithic, or secondary period . : « 118 
IV. Cenolithic, or tertiary period . . . 2.3 
VY. Anthropolithic, or quaternary period : » O5 
Total ... 100.0 
According to this, the length of the archilithie period, 
during which no land-living animals or plants as yet existed, 
amounts to more than one half, more than 53 per cent.; on the 
other hand the length of the anthropolithic era, during which 
man has existed, amounts to scarcely one-half per cent. of 
the whole length of the organic history of the earth. It is, 
however, quite impossible to calculate the length of these 
periods, even approximately, by years. 
The thickness of the strata of mud at present deposited 
during a century, and which has been used as a basis for 
this calculation, is of course quite different in different parts 
of the earth under the different conditions in which these 
deposits take place. It is very slight at the bottom of the 
deep sea, in the beds of broad rivers with a short course, and 
in inland seas which receive very scanty supplies of water. 
It is comparatively great on the sea-shores exposed to strong 
breakers, at the estuaries of large rivers with long courses, 
and in inland seas with copious supplies of water. At the 
mouth of the Mississippi, which earries with it a consider- 
