Brian smith, age l0, of Milwaukee, Wisc., for his question:

What is as oil trap?

This is a term Used by the geologists who study the wonders of the earth's crust. The oil is Petroleum, and no one knows how much of this precious product still lies undiscovered. But a likely Place to look for ft is in an oil trap deep in the rocky ground.

Petroleum or rock oil is a natural mineral often trapped in certain porous layers of the earth's crust. A geologist may point out one of these likely oil traps, but he cannot say for surf that it holds a deposit of precious petroleum. Maybe no oil seeped into the trap, and maybe it did, but later geological events helped it to escape.

A deposit of petroleum, is a highly complex mixture of hydrocarbon chemicals. Most experts suspect that they are the organic remains of ancient marine life. Many an oil deposit began where a shallow sea once swamped the land. Masses of organic material from microscopic plants and animals were left to sink into the ground.

Sometimes a deposit was left in a thick layer of sand. The oily mixture is lighter than water so it could not sink through the ground water in the earth's crust. It would float on top of the Water that saturated the rocks below it and perhaps seeP sideways in all directions through the grains of sand. If the oily mixture were near the surface, its light gases and liquids would evaporate. The petroleum would change to puddles of tar.

Sometimes an oily deposit was trapped in porous layers of sandstone or limestone. It, too, would sink down and float on the top of ground water. If it could, it too would seep sideways through the spongy pores, thinning out in all directions. If it came near the surface, its lighter chemicals also would evaporate leaving a deposit of gummy asphalt.

A trap to catch and hold the oily deposit must have a roof of rock such as shale to seal it from the air. It must have walls of rock such as granite to prevent the oil from seeping sideways. The earth's crust is made of assorted layers of rocks, some dense and solid, some soft and porous. The rocky sandwiches are bent and cracked in many underground formations. An oil trap occurs where a porous layer saturated in    petroleum is walled in with dense rocks and sealed under a dense, rocky roof. In such a trap the oily deposit may be held captive for ages, along with the natural gases sad other buried materials.

Sometimes the underground layers of dense and porous rocks are bent like a hill. At the top a few surface layers may be worn away, leaving a hump of dense shale at ground level. An oil trap may form below the shale in a hump of porous sandstone. sometimes the rocky sandwiches cracked and shifted and a layer of oily limestone was sealed inside a trap of dense granite. sometimes an oil trap formed in a sloping sandwich of dense and porous rocks roofed with a flat, dense surface layer.