Denise vitale., age 8, of bradenville. Pa., for her question:

How did coal get in the ground?

Our world, they tell us, has had at least four billion birthdays. That long, long time is very hard for our minds to grasp. We change, and every year different things happen to us. In the earth's long life story, many things have happened and many things have changed. It took millions of years to make the beds of coal in the ground.

A lump of glossy black coal does not look much like a tree. But the beds of coal buried in the ground are a gift from the plant world. You may be able to split a big lump of coal into flat layers. Sometimes you may find a leafy pattern that looks like a fern pressed between the sooty pages of coal. And that is just what it is.

The pattern was made by a plant that grew perhaps 250 million years ago. The world was very different in those days. If we could go back to that time, we would find no flowers, no oaks or elms. There were no dogs and cats, no furry animals at all. There were no birds, and even the dinosaurs had not yet arrived.

The world was warm and wet. Vast stretches of the land were covered with marshy swamps. Strange forests grew with their roots in the boggy water. There were rushes and soggy mosses, tree sized ferns and giant horsetails. Whopping salamanders as big as crocodiles wallowed in the mud. Dragonflies with wings four feet wide zoomed through the air.

These were the forests that gave us our beds of buried coal. The strange trees lived and died. Their woody trunks and leafy fronds fell into the still water and stayed there. They did not decay, and through the ages the swamps became clogged with

Fallen trees. Then the face of the earth changed, and the swamplands became dry land. The fallen coal forest was soon covered with dirt and blowing dust. Sometimes a landslide covered the layers of old plants. In time, the remains of the forests were buried lirider deep layers of dirt and rocks. The plant life was pressed and squeezed flat. Gradually the plant material changed and became glossy black carbon. Coal is the carbon material left from these strange forests that flourished on the earth perhaps 250 million years ago.

Coal making was very slow, and it happened stage by stage. The fallen plants became peat, which is a spongy brawn material that can be dried and burned. After millions of years, the peat became lignite, which is a soft, brown coal. The lignite became soft, black bituminous coal. Some of the bituminous coal became hard, black anthracite coal.