large enough to reach the earth are called meteorites. Most minerals
found in meteorites are the same as those we have on earth. But,
there are some rare minerals known only in meteorites. Two of them
are cohenite and schreibersite.




MAIN KINDS OF ROCKS



Rocks are the building blocks of the earth's crust. They may be
massive, as in granite ledges, or tiny. Soil, gravel, sand and clay
are rocks. THERE ARE THREE MAIN TYPES OF ROCKS.

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,