So much would be clear.  And we might dig also and burrow a little way down, through a few different kinds of rock, where they were not too thick.  But that would be all.  There our powers would cease.

Now how different.  Through the heavings and tiltings of the earth’s crust, the lower layers are often pushed quite up to the surface, so that we are able to examine them and their fossils without the least difficulty, and very often without digging underground at all.

You must not suppose that the real order of the rocks is changed by these movements, for generally speaking it is not.  The lower kinds are rarely if ever found placed over the upper kinds; only the ends of them are seen peeping out above ground.

It is as if you had a pile of copy-books lying flat one upon another, and were to put your finger under the lowest and push it up.  All those above would be pushed up also, and perhaps they would slip a little way down, so that you would have a row of edges showing side by side, at very much the same height.  The arrangement of the copy-books would not be changed, for the lowest would still be the lowest in actual position; but a general tilting or upheaval would have taken place.

Just such a tilting or upheaval has taken place again and again with the rocks forming our earth-crust.  The edges of the lower rocks often show side by side with those of higher layers.

But geologists know them apart.  They are able to tell confidently whether such and such a rock, peeping out at the earth’s surface, belongs really to a lower or a higher kind.  For there is a certain sort of order followed in the arrangement of rock-layers all over the earth, and it is well known that some rocks are never found below some other rocks, that certain particular kinds are never placed above certain other kinds.  Thus it follows that the fossils found in one description of rock, must be the fossils of animals which lived and died before the animals whose fossil remains are found in another neighboring rock, just because this last rock-layer was built upon the ocean-floor above and therefore later than the other.

All this is part of the foreign language of geology—­part of the piecing and arranging of the torn volume.  Many mistakes are made; many blunders are possible; but the mistakes and blunders are being gradually corrected, and certain rules by which to read and understand are becoming more and more clear.

It has been already said that unstratified rocks are those which have been at some period, whether lately or very long ago, in a liquid state from intense heat, and which have since cooled, either quickly or slowly, crystallizing as they cooled.

Unstratified Rocks may be divided into two distinct classes.

[Illustration:  SECTION OF A LAVA BOMB.]

First.—­Volcanic Rocks, such as lava.  These have been quickly cooled at the surface of the earth, or not far below it.