[16] "Pudding stone" is a rock with pebbles all through it, like the plums in a Christmas pudding. Its book name is "conglomerate."
"And what they did not eat that night, the queen next morning fried!"
DOWN IN THE GREAT MELTING-POT
But how is old rock warmed over and made into new? You might easily guess that as the heart of the earth is melted rock the rock layers lying next to it would be melted, too, and so started on their way to becoming crystallized rock. Crystallization in rock takes place from the surface down, in the same way that maple syrup turns to sugar, as it does if allowed to stand undisturbed. So, as the central mass of rock is cooling from above toward the centre, we may suppose granite is still being formed away down there, miles under our feet.
But there are other ways in which rocks make their own heat—rocks far above this central molten heart of the world. One of these ways might remind you of how the mother hen gets her chickens to come out of the eggs, for rocks hatch out new rocks by sitting on one another!
THREE CHAPTERS IN THE STORY OF MARBLE
If you're ever in New York City up around 192d Street, you can read the three chapters in the life of a piece of marble right in the rocks themselves, for there you'll see this mass of rock with that granite dike pushing its way through. The rock on either side of the dike is limestone, and this limestone, owing to the heat of the lava which afterward hardened and became a "dike," is full of crystals; that is, began to turn to marble because of the heat. See how the lava crumpled the limestone as it pushed its way up into the original crack?
The pressure of the upper rocks generates heat in those beneath.
Then when these deeply buried rocks come up into the upper world as parts of mountain chains, and the covering of the softer rocks is, by the rivers and by weathering, worn away, we find the granite. The wrinkling of the rocks which makes mountains also creates immense pressure, and this is another great source of made-over rock. Such rock is found almost entirely in mountain regions. Some rocks, as shown in pebbles stretched out like a piece of gum, are heated by pressure without being crystallized. Often one of these stretched pebbles is the only thing in a crystallized rock that shows what kind of rock it was originally, all the finer material in it has been so changed. The deeper down in the earth the rocks are the more apt they are to be crystallized, because the rocks piled above them help to hold in the heat, just as thick blankets keep you warmest on a cold winter night.