proper foods for aquaria, or you can prepare your
own. Fine vermicelli is good for gold fish, scraped lean beef is just
what the sunfish and Paradise fish want. Ant eggs suit many fish, and
powdered dog biscuit will fill many mouths. It is evident that an
article so brief as this is only suggestive. The libraries contain
many books of which two are recommended:

"Home Aquarium and How to Care For It." By Eugene Smith, 1902.
Published by Dutton, New York.

"Book of Aquaria." By Bateman and Bennett, 1890. Published by L.
Upcott Gill, 170 Strand, W. C., London.

ROCKS AND PEBBLES

_United States Geological Survey_

Geologists study the materials of the earth's crust, the processes
continually changing its surface, and the forms and structures thus
produced. In a day's tramp one may see much under each of these heads.

The earth's crust is made up chiefly of the hard rocks, which outcrop
in many places, but are largely covered by thin, loose, surface
materials. Rocks may be igneous, which have cooled from a melted
condition; or sedimentary, which are made of layers spread one upon
another by water currents or waves, or by winds.

Igneous rocks, while still molten, have been forced into other rocks
from below, or poured out on the surface from volcanoes. They are
chiefly made of crystals of various minerals, such as quartz, felspar,
mica, and pyrite. Granite often contains large crystals of felspar or
mica. Some igneous rocks, especially lavas, are glassy; others are so
fine grained that the crystals cannot be seen.

In places one may find veins filling cracks in the rocks, and {113}
made of material deposited from solution in water. Many valuable
minerals and ores occur in such veins, and fine specimens can
sometimes be obtained from them.

{112}

[Illustration: Fold in stratified rock]

[Illustration: Wearing the soft and hard beds by rain and wind]

[Illustration: Quartz vein in rock]

{113 continued}

Sedimentary rock are formed of material usually derived from the
breaking up and wearing away of older rocks. When first deposited, the
materials are loose, but later, when covered by other beds, they
become hardened into solid rock. If the layers were of sand, the rock
is sandstone; if of clay, it is shale. Rocks made of layers of pebbles
are called conglomerate or pudding-stone; those of limy material,
derived perhaps from shells, are limestone. Many sedimentary rocks
contain fossils, which a