Nothing is known, however,
with certainty about these different periods or where one ends and another
begins, and no one knows whether the first, or oldest, layer has yet been
discovered. One geologist says, "I have found it," and presently another
penetrates a little deeper, goes a little farther back, and finds one lower
still. Nor can anyone say certainly where a fossil-fern or the mummy of
an old-world fish appeared for the first time, and though many plants and
animals which are found in a fossil state have long been extinct, yet
there are many more which appear at a very ancient date and have continued
unchanged to the present time.
There is a famous cliff in Dorsetshire upon which may be read, almost as
upon a map, the record of the changes which have passed over it during its
life-history.
On examining the strata, or layers which lie one above the other,
geologists find the first, or lowest of all, to be Portland stone, which
was formed by the accumulation of lime at the bottom of the sea.
The second layer shows that this sea-bed in time became dry land, and was
covered with soil--what had once been the seashore gradually giving place
to a forest.
But how do we know that such a wonderful change was wrought in process of
time?
We have clear proof that it was so from the vegetable soil still remaining,
and the numbers of trees the remains of which are embedded in the rock,
many of them standing upright as when growing.
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