d ice
are the three great agencies that wear away the land, bring rock
fragments long distances, and deposit them where aqueous rocks are being
formed. Volcanic eruptions bring material from the earth's interior.
This material ranges all the way from huge boulders to the finest
impalpable dust, called volcanic ashes. Rivers of ice called glaciers
crowd against their banks, loosening rock masses and carrying away
fragments of all sizes, in their progress down the valley. Brooks and
rivers carry the pebbles and the larger rock masses they are able to
loosen from their walls and beds, and grind them smooth as they move
along toward lower levels.

The air itself causes rocks to crumble; percolating water robs them of
their soluble salts, reducing even solid granite to a loose mass of
quartz grains and clay. Plants and animals absorb as food the mineral
substances of rocks, when they are dissolved in water. They transform
these food elements into their own body substance, and finally give back
their dead bodies, the mineral substances of which are freed by decay to
return to the earth, and become elements of rock again.

The decay of rock is well shown by the materials that accumulate at the
base of a cliff. Angular fragments of all sizes, but all more or less
flattened, come from strata of shaly rock, that can be seen jutting out
far above. A great deal of this sort of material is found mingled with
the soil of the Northeastern States. Round pebbles in pudding-stone have
been formed in brook beds and deposited on beaches where they have
become caked in mud and finally consolidated into rock. If the beach
chanced to be sandy instead of muddy, a matrix of sandy paste holds the
larger pebbles in place. Limestone paste cements together the pebbles of
limestone conglomerates.

In St. Augustine many of the houses are built of coquina rock, a mass of
broken shells which have become cemented together by lime mud, derived
from their own decay. On the slopes of volcanoes, rock fragments of all
kinds are cemented together by the flowing lava. So we see that there
are pudding-stones of many kinds to be found. If some solvent acid is
present in the water that percolates through these rocks it may soften
the cement and thus free the pebbles, reducing the conglomerate again to
a mere heap of shell fragments, or gravel, or rounded pebbles.

The story of rock formation tells how fire and water, and the two
combined, have made, and made