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 best slates are found, this rule, I believe, stands true. It stands true, certainly, of the ancient Silurian rocks of Wales, Cumberland, Ireland, and Scotland.