Shores and Islands vie 
world began to look more like the earth as we know it 
today. 
Attempts have been made to measure the age of the ocean 
by computing how much salt is borne into it by rivers each 
year, and dividing that number into the total amount of salt 
in the ocean today. This method gives the ocean an age of 
ninety to six hundred million years, but is.unreliable because 
it assumes (1) that the rate of salt additions to the sea water 
has always been the same as it is today, (2) that the original 
sea water was fresh, (3) that all the salt deposited in the 
ocean remains there forever. Each of these three assumptions 
is highly doubtful. 
At a point in the earth’s history some one billion years ago 
the science of geology takes over and brings us to the pres- 
ent. It derives most of its knowledge from a minute study of 
layers of rock all over the world. Though not an exact science, 
it is much more precise and reliable than the fancies out- 
lined above. 
THE GEOLOGICAL TIME SCALE 
The following timetable shows the progression of geologi- 
cal time from the earliest known rocks down to the present 
(starting about one and one half billion years ago). 
Archeozoic Era 
This is the very earliest era. During it the oldest known 
rocks were formed. It was during this remote time that life 
probably first appeared in the form of tiny single-celled ani- 
mals and plants. No fossils are known from this era. The 
rocks are highly metamorphosed, made up of all kinds of 
igneous and sedimentary types, but mostly the former. 
