It is easier to talk about the passing of millions of years than it is to
imagine how vast these long spans of time really are. In order to see how the
various ages of the Earth compare with each other, scientists who study
rocks and fossils, called geologists, sometimes use what is called a geological
clock.
This is how a geological clock is drawn up:
The Earth is thought to have existed for about forty five hundred million
years. If you imagine that huge length of time to be just one hour on a clock,
then all forms of life have existed for only fifteen to twenty minutes of that
hour; sea creatures for about eight minutes; land animals for less than five
minutes; mammals have spent a mere two and a half minutes on the Earth
and man and his ancestors, only about one second. To bring history, as
written by man, into this diagram would be very difficult. By this clock,
William the Conqueror invaded England less than one thousandth of a
second ago!
In order to make the dating of fossils easier, the earliest geologists divided
up the time since the Earth was formed. They first divided the time since the
Earth formed into five eras. The Azoic era was without life; the Proterozoic
era contained the very earliest forms of life; the Palaeozoic was the era in
which many sea creatures, fish and amphibians appeared; tlke reptiles ruled
the Mesozoic era and the Cenozoic era belonged to mamma~and to man. We
are, at present, in the Cenozoic era. Each era was divided into periods, as
shown in the diagram. There are further subdivisions of the periods, but not
everybody uses the same system of divisions.
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