Part 4

1. IGNEOUS rocks are those formed at very high temperatures or from molten materials. They come from magmas--molten mixtures of minerals, often containing gases. They come from deep below the surface of the earth. If they cool off while below the surface, they form intrusive rocks, which may later be revealed by erosion. When magmas reach the surface red hot, they form extrusive rocks, such as volcanic rocks. Thus, granite is an igneous, intrusive rock; lava is an igneous, extrusive rock. (Notice how the type of rock tells its past history--if you know what to look for.)

2. SEDIMENTARY rocks are formed by the action of wind, water, or organisms. They cover about three quarters of the Earth's surface. Most are laid down--as sediments--on the bottom of rivers, lakes and seas. Many have been moved by water, wind, waves, currents, ice or gravity. The most common sedimentary rocks are sandstones, limestones, conglomerates and shales. Oil is found in sedimentary formations.

3. METAMORPHIC rocks are those that have been changed from what they were at first into something else--by heat, pressure, or chemical action. All kinds of rocks can be changed. The result is a new crystalline structure, the formation of new minerals, or a change in the rock's texture. Slate was once shale. Marble came from limestone. Gneiss (pronounced "nice") is perhaps reworked granite.

Igneous rocks are formed at high temperatures or from molten materials. They come from deep beneath the earth. They can be intrusive or extrusive--depending on where they cooled off.

Sedimentary rocks are formed by the action of wind, water, or organisms. They usually are laid down on the bottom of rivers, lakes and seas. Most of the earth's surface is covered by these rocks. Oil is found in sedimentary formations.

Metamorphic rocks have been changed from their original state into something else. Heat, pressure, chemical action change the crystalline structure, the texture, even form new minerals. All kinds of rock can be changed.

A Word On Fossils

Perhaps you'll find rocks containing fossils--or even fossils by themselves. They should form a separate part of your collection.

Fossils are the remains--or the outlines--of former plant or animal life buried in rock. The older the rock, the simpler the plant and animal life it contains. Thus fossils can give a clue to the age of the rock strata.

Fossils can teach history. They tell us about plants and animals that are now extinct--the dinosaur, for example. They can also tell of ancient climates. Coral found in rocks in Greenland suggests it must have once been warm. Remains of fir and spruce trees have been found in the tropics.

How are fossils formed? Teeth, bone and wood don't last long in their original state. However, buried materials decompose, leaving a film of carbon as a fossil. This results in a leaf tracery, or the outlines of some simple animal. On a gigantic scale, this process of forming carbon has resulted in our great coal deposits.

Sometimes the buried material is gradually replaced by silica or other substances, making petrified objects. Wood can be replaced--cell by cell--by agate or opal from silica-bearing water. The result is petrified wood, the finest examples of which can be found in our Petrified Forest National Park in Arizona. This can happen to shells, too.

How about molds and casts of footprints of ancient animals? A brontosaurus might have stomped along in soft, warm mud eons ago. The mud hardened and later another layer of soft earth covered the print, preserving it.

COLLECTING

If you want to collect rocks and minerals just for the sake of having them, you can buy specimens. Many can be purchased for 25 cents to $1 each, while a rare specimen can cost hundreds of dollars.

The true pleasure is in finding your own samples. Later, when you have a good-sized collection, you can fill gaps by buying specimens or swapping extras with other collectors. You'll be amazed at the number of amateur collectors. Perhaps no branch of science owes more to the work of amateurs than mineralogy. Our great collection of minerals in the U.S. National Museum in Washington, D.C., was gathered almost entirely by two amateurs who devoted many years and much money to their hobby.

Where To Look

Look for pebbles by the roadside, in beds of streams and riverbanks. Go out into the country for ledges on hillsides. Every road cut, cliff, bank, excavation, or quarry shows rocks and minerals. Railroad cuts, rock pits, dump piles around mines, building sites--they'll all yield specimens. Some of the best mineral specimens collected in New York City came from skyscraper and subway excavations. Help a New England farmer clear his field and you'll have more rocks than you know what to do with.

As for reference books, many states publish guides to mineral deposits. Mineralogical magazines list mineral localities.

Tips For The Field

Don't try to collect too much at once. Work early in the day or late in the afternoon. A hot sun on bare rock can make you sizzle--especially if you're loaded with equipment and samples.

Here's the equipment to take: newspapers for wrapping samples, notebook and pencil, geologist's pick, cold chisel, magnifying glass, compass, heavy gloves, a knife, and a knapsack. Later on, you may want a Geiger counter for spotting radioactive rocks.

Be selective. Hand-sized specimens are best. If your sample is too large, trim it to size, showing its most striking feature to best advantage. When you wrap the sample in newspaper, include a note telling when and where you found it. This information will be transcribed to a filing card when you add the specimens to your display, so make it as complete and accurate as you can.

When you get home, clean specimens with soapy, warm water, applied with a soft brush. Soluble minerals like halite can't be washed, but should be rinsed with alcohol. A coat of clear lacquer will protect some samples against dirt.

Arranging Your Collection

Put a spot of enamel on the specimen. Write on the spot--in India ink--a catalog number and have this number refer to a card in a file drawer. The card should list date, place found, identification of specimen, etc.

Group your samples: metallic minerals, semiprecious stones, nonmetallic minerals. Display them on a shelf, or buy or build a mineral cabinet with partitioned drawers. For smaller samples, use a Riker mount with a glass top.

A common rock