e all the planets, must have been once in a state
of intense heat throughout, as its mass inside is probably now.

That it must be cooling, and giving off its heat into space.

That, therefore, as it cools, its crust must contract.

That, therefore, in contracting, wrinkles (for the loftiest mountain
chains are nothing but tiny wrinkles, compared with the whole mass of
the earth), wrinkles, I say, must form on its surface from time to
time. And that the mountain chains are these wrinkles.

Be that as it may, we may safely say this. That wherever the
internal heat of the earth tends (as in the case of volcanoes)
towards a particular spot, that spot must expand, and swell up,
bulging the rocks out, and probably cracking them, and inserting
melting lava into those cracks from below. On the other hand, if the
internal heat leaves that spot again, and it cools, then it must
contract more or less, in falling inward toward the centre of the
earth; and so the beds must be crumpled, and crushed, and shifted
against each other still more, as those of our mountains have been.

But here may arise, in some of my readers' minds, a reasonable
question--If these upheaved beds were once horizontal, should we not
be likely to find them, in some places, horizontal still?

A reasonable question, and one which admits of a full answer.

They know, of course, that there has been a gradual, but steady,
change in the animals of this planet; and that the relative age of
beds can, on the strength of that known change, be determined
generally by the fossils, usually shells, peculiar to them: so that
if we find the same fashion of shells, and still more the same
species of shells, in two beds in different quarters of the world,
then we have a right to say--These beds were laid down at least about
the same time. That is a general rule among all geologists, and not
to be gainsaid.

Now I think I may say, that, granting that we can recognise a bed by
its fossils, there are few or no beds which are found in one place
upheaved, broken, and altered by heat, which are not found in some
other place still horizontal, unbroken, unaltered, and more or less
as they were at first.

From the most recent beds; from the upheaved coral-rocks of the West
Indies, and the upheaved and faulted boulder clay and chalk of the
Isle of Moen in Denmark--downwards through all the strata, down to
that very ancient one in which the be