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. KINDS OF "METAMORPHIC" ROCK Rock of any kind may be changed to crystallized rock. Where the conditions are not favorable for crystallization the rock is made more solid, and material soaked out of the rocks above filters down into it. The lower layers of sandstone may become almost as solid as glass, and are then called "quartzite." Clay rocks are hardened into slate. Rocks changed in any of these ways are called "metamorphic" rock, from two Greek words meaning "to form over." But by "metamorphic" is usually meant rock that has been crystallized. NICE HATCHING TEMPERATURE FOR ROCKS I compared the hatching of new rocks to the hatching of new chickens, because it is done by the rocks sitting on one another. But chicken hatching and rock "hatching" are alike in still another way. The rocks need heat, but not too much heat. Too much heat melts them. It is only when they have cooled down a good deal that they begin to crystallize; and that, you see, wastes time. A nice hatching temperature for rocks is between 500 and 1000 degrees Fahrenheit.