Lets Collect Rocks and Shells | Page 6

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.
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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,