Melina Bryant, age 10, of Highland Home, Alabama, for her question:

Will you please tell me about the rocks?

Melina is mighty interested in the earth's rocks, such as what they are and where they are and especially about their wondrous colors. Andy can't tell everything about them in one day. So, as usual, he will do the best he can. He thinks his best plan is to tell about some of his favorite rocks    and where they belong in the world.

It's nice to know that we shall never run out of rocks. There are enough of them for all of us to have just as many as we want    with plenty left over. As you know, our world is a great big round ball, about 25,000 miles around its fat middle. And the entire ball is covered with a crust of rocks, rocks and more rocks. In some places the rocky crust is about five miles thick. In other places it is more than 40 miles thick.

We stand on rocks when we stand on the ground. They even use rocks to make the streets and cement sidewalks. They also use rocks to build buildings of stone and bricks. We walk on rocks in the woods and fields, over the hills and down the slopes. The earth has about 1,500 chemical ingredients and it took many long ages to make them into about 100 different types of rock, though, actually, no two chunks of rock are exactly mike. For example, garden dirt does not look like rock. But it is made from powdered rocks, mixed with moisture, fallen leaves and whatever else is left to rot on the ground. The sand on the beach is made from tiny grains of hard rock.

Each type of rock called for a different recipe. Crumbly white limestone was made under the sea, from tiny shells and tiny seaweeds. Sometimes a trace of mud was added to make it brown or grey. The sea also made sandstones and tinted them rosy red or grey, ginger or dark friendly brown. Limestones and sandstones are called sedimentary rocks because they were made from dregs or sediments that sank to the bottom. Later the water went away and left them high and dry.

Other rocks were made by fiery volcanos, when they erupted rivers of red hot lava. The runny lava was a mixture of many mineral ingredients. It cooled in the air and set into solid rocks. We called them igneous rocks, which means made by fire. Granites are igneous rocks, all speckled with colored grains of this and that.

Sometimes a layer of limestone got mashed by a growing mountain    and turned into waxy white marble. We call it a metamorphic rock, which means it was completely remodeled.

Andy's favorites are silicate rocks, made from chemicals called silicon and oxygen. The earth used different recipes to make dozens of gorgeous silicate rocks. Some turned out to be smooth white pebbles, some are glassy crystals, like tinted glass. Other silicates are gleaming tiger eyes and some are agates, with bright stripes like colored ribbons. Opal is a silicate rock and so is glassy carnelian, that looks like frozen honey.