Rocks around the block.

The stone buildings around you are bursting with life--or prehistoric life, that is. Shells from ancient sea creatures, leaves from primeval plants, and skeletons of animals long extinct "live" on in the stones as fossils.

But you don't have to rush to your library to learn about fossils. Instead, you can read the writing in the stone walls. In fact, you can learn a lot about fossils and rocks--the natural material from which building stones are cut--just by walking down a city street.

The first thing you'll notice is the variety of the stones--from swirling green marbles and speckled pink granite to chalky white limestone. Geologists, scientists who study rocks and fossils, can read these colors and textures to figure out which naturally occurring, solid chemical compounds (minerals) make up the rock. In addition, the texture can tell them where and how a rock formed.

BUILDING STONES

Based on this information, geologists classify rocks into three main groups: igneous, metamorphic, and sedimentary. You should be able to spot all three on a city street.

You can usually tell an igneous rock by its speckled appearance and grainy texture without either stripes or bands. The rocky surface looks similar to ground pepper, but can be many different colors, anything from white to gray to red.

Igneous rock forms from molten magma, melted mineral material that comes from other rocks deep within Earth's mantle. The ceaseless movement of Earth's interior causes the magma to rise toward the surface. As it reaches the upper mantle, just below Earth's crust, the magma cools very slowly. The minerals crystallize, or solidify, in a regular pattern, giving the rock its evenly speckled appearance.

Look around. Maybe a bank or railroad station along your walk is made of granite. This igneous rock hardens 16 kilometers (10 miles) or more below Earth's surface. Erosion of the surface by wind and rain, along with movements of Earth's crust, may help to expose the granite. When that happens, the rock can be quarried, or cut for use. Builders often use granite on the exteriors of buildings because it is very durable.

But some builders prefer the less uniform texture of metamorphic rock. Like your favorite morphing" computer image, these rocks have formed by changing from another type.

The changes take at least 10,000 years. They occur below Earth's surface, but not so deep that temperatures melt the minerals into magma. Still, enormous amounts of pressure and heat can build up at depths of less than 16 kilometers (10 miles). If the pressure is great enough, it can crush existing rocks, and change their mineral composition chemically without melting them.

The pressure and heat cause some of the minerals to separate into bands, or layers. Look for these bands in the stones of a building of polished marble, one kind of metamorphic rock. You'll probably notice that the layers are irregular, sometimes even swirled. The swirls appear because the rock was folded under pressure.

FOSSIL BONES

If the layers in the stone building you see lie in roughly parallel lines, the rock could be sedimentary. Sedimentary rocks form from sediment--tiny pieces of rocks and minerals that break or wash off larger rocks and shells through erosion. If you fill a bottle with water from a pond, a river, or the sea and let it settle, you will see the sediment collect at the bottom.

This is exactly how sedimentary rocks start to form. Sandy particles in a pond, for example, settle in the water and collect on the bottom of the pond. Layers and layers of sediment collect one on top of the other. Sometimes animal bones and shells and parts of plants get buried with the sediment. Over thousands of years, the buried particles of sediment are cemented together. The result? Layers of sedimentary rock containing the fossilized remains of plants and animals--clues about the past.

Did you know, for example, that Indiana was once covered by a tropical sea? The evidence lies in Indiana limestone, a type of sedimentary rock. Composed almost entirely of shell fragments from tropical sea creatures, the rock formed nearly 300 million years ago. Movements of Earth's crust since then probably caused the sea to spill off and the land to move northward. But the evidence of a sunny past remains. It's written on the walls of all the buildings ever made with Indiana limestone

These remnants of once-living things are a dead giving away of the stone's identity as sedimentary rock. That's because fossils are found only in sedimentary rock, never in metamorphic or igneous rock. But if sedimentary rock changes into metamorphic and igneous rock when it is buried inside Earth, you might wonder, what happens to the fossils? The answer is that they are destroyed in the process. They simply can't take the heat and pressure.

That's an important clue when you're on the lookout for possible marble (metamorphic) fake-outs. If you're not sure you're standing on a marble floor, check for the fossil evidence. If you see sea shells and you're not by the sea shore, you might be standing on what used to be a sea shore--a polished limestone floor. Sedimentary, my dear Watson!

Sidney Horestein is a geologist at the American Museum of Natural History in New York City. Article reprinted from Science World 2/9/90.