Hoble Smith, age 12, of Greenbus for his question:

What makes trees petrified?

There was a time when ginkgo trees grew wild in our land. High up on the cliffs where the busy Columbia River cuts its ways through desert lava beds these trees have left their records far us in stone. There, to the sunshine, an anBient ginkgo forest grew millions and millions of years ago. Yet their dainty fern like leaves, their small round seeds and the beautiful cell structure of their woods is pre¬served for us to see in petrified fossils.

Petrified woods, logs and whole forests have been discovered in several places in our country, especially in the west, from the southern deserts to the Dakotas. If you were lucky enough to come across a petrified log in the desert you might mistake it for a stone, In a sense it is a stone    for the word petrified means turned to stone. And that is just about what has happened to the petrified fossils of woods, leaves and whole forests.

The job of turning to stone begins as soon as the forest tree falls to the ground. And it may take millions of years to complete. Trees and plants, as you know, are made of tiny cells. Under a magnifying glass or microscope, these cells look like square or oblong honeycombs. In a slice of tree truce they are arranged in circles spreading out from the center. The walls of these cell. are made of a substance called cellulose    a very tough mixture of hydrogen, oxygen and carbon.

The insides of plant cells are filled with softer substances  ¬the protoplasm, or living matter, of the plant and the liquid sap. These softer materials inside the harder cell wall are the first to decay and disappear when the tree falls to the ground.

This was also true of the ancient trees now preserved as petrified fossils. Some of these ancient trees became water logged, often

deep underground. The busy water had been lapping at rocks and stones and dissolving their chemicals. Just as the sea carries dissolved salt, so this ground water carried loads of minerals such as calcite, aragonite and quartz. As the molecules of the plants were dissolved and carried away in the water, they were replaced by the minerals in the water.

Bit by bit these minerals gelled along the walls of the plant cells. Bit by bit they replaced the softer tissues inside the cells, When the water carried the mineral quartz, the plant cells were often replaced in opal in exactly the same shapes as they grew all those million of years ago.

Your petrified log in the desert would show all these tiny cellsif it were cut open. Many of these ancient logs have been sliced and polished. Their beautiful patterns and designs were made by living plants millions and millions of years ago,