fire, and that later on other towns had grown up over the buried remains
of the earlier settlements. The lowest layers were, of course, the
oldest, and the position of each layer in the pile gives its date, not
in years, but with regard to the layers above and below it.

Now, from time immemorial nature has been at work building up monuments
and providing tombs which tell us what were the events going on,
and what kind of inhabitants the earth had long before man made his
appearance on its surface. The monuments are the rocks which compose the
ground under our feet, and these, like many ancient monuments of human
construction, are the tombs of the creatures that lived while they were
being built.

Many facts testify that the earth's crust did not come into existence
exactly as we find it now, but that its rocks have been built up by the
slow action of natural agencies. These rocks constantly inclose the
remains of plants and animals, and as it is evident that neither plant
nor animal could have lived in the heart of a solid rock, this fact
shows that the rock must in some way have gathered round the remains
that are now found in it. Again, many of these remains, or fossils,
belonged to animals that lived in water, the larger part, indeed, to
marine creatures. This indicates that the rock was formed beneath the
sea, and when we examine the way in which the constituents of the rock
are arranged, we frequently find it to correspond exactly with the
manner in which the sand and mud that rivers sweep down into the sea or
lakes are spread out over the bottom of the water. In a pile of rocks
formed in this way it is clear that the lowest is the oldest of all, and
that any one stratum lying above is younger than the one beneath it.
Further, the occurrence of rocks inland containing marine fossils far
above the sea level shows that the sea and land have changed places.