when the earth's
surface must have been hotter, according to La Place's idea, he
decided that our globe has been cool enough for the existence of life
upon it for a period of somewhere in the neighborhood of one hundred
million years. Those who try to study the rate at which mud is being
deposited in our bays and at the mouth of our rivers, and who hence
try to deduce how long it has taken to produce the thickness of all
the stratified rock we know, arrive at a figure larger, rather than
smaller, than that mentioned above. The same is true of those who try
to count the age of the earth by the rate at 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
technical name the geologist gives to the era. This technical 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 advancement 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 any geologist.
Most of them will serve to keep very clearly before the mind of the
ungeological the period which he is studying. Like all such tables,
this must be read from the bottom up. This arrangement is used because
the oldest rocks in the series are naturally at the bottom and the
newest rocks are on the top, though occasionally a region is
sufficiently upset partly to reverse the order.

TABLE OF GEOLOGICAL