underground water slowly seeping through all subsoils and porous
rocks in other regions.

Even when gathered into definite channels, ground water does not
have the erosive power of surface streams, since it carries with
it little or no rock waste. Regions whose underground drainage is
so perfect that the development of surface streams has been
retarded or prevented escape to a large extent the leveling action
of surface running waters, and may therefore stand higher than the
surrounding country. The hill honeycombed by Luray Cavern,
Virginia, has been attributed to this cause.

CAVERN DEPOSITS. Even in the zone of solution water may under
certain circumstances deposit as well as erode. As it trickles
from the roof of caverns, the lime carbonate which it has taken
into solution from the layers of limestone above is deposited by
evaporation in the air in icicle-like pendants called STALACTITES.
As the drops splash on the floor there are built up in the same
way thicker masses called STALAGMITES, which may grow to join the
stalactites above, forming pillars. A stalagmitic crust often
seals with rock the earth which accumulates in caverns, together
with whatever relics of cave dwellers, either animals or men, it
may contain.

Can you explain why slender stalactites formed by the drip of
single drops are often hollow pipes?

THE ZONE OF CEMENTATION. With increasing depth subterranean water
becomes more and more sluggish in its movements and more and more
highly charged with minerals dissolved from the rocks above. At