We have seen then in the preceding discussion how caves are formed
largely in rocks saturated with water, and how by land movements and
changes of sea level the caves and pores formed in this rock are moved
above permanent water levels and exposed to air. It then becomes possible
to deposit rock in the pores and caves rather than to increase their size
by solution. As you go through the caves you will notice that the walls
are wet and that water is oozing out of the pores of the rock.
This water has just passed through limestone and has dissolved parts of
the rock. The reader undoubtedly knows that water will dissolve a
substance in large quantities and more rapidly if it is hot, and that it
can hold more gases to make stronger acids if it is under pressure. So,
having been released from a relatively warm rock in which it was under
some pressure and where there was little air circulating, into a large
cave where rapidly circulating air cools the cave and evaporates the
water, this water can no longer hold all the limestone it has dissolved
and it releases part of it.
Small drops of water emerging from the lime rock on cave walls are
evaporated and calcite and other rock minerals are deposited along these
walls. Where these drops cascade along the walls a continuous elongated
ridge is deposited. If the water oozes out in an extremely fine coating
of water, the entire ceiling, walls and floor may be paved with calcite.
[Illustration: These deposits combine a rather even and general flow of
water and possible intergrowth of the stalactites to make the thickened
deposit. The ground water issued more rapidly and was concentrated at one
point to make the long tubular deposits, the tube being made by rapid
evaporation along the outer margins of the drops of water as they hang on
the stalactite before the large part of the water falls to the floor.]
Where individual drops collect on the cave ceilings, a thin deposit may
be formed on the ceiling after which the remaining water may drop to the
cave floor where more calcite is deposited. Continuous dripping results
in paired deposits extending down from the ceiling and up from the floor.
The deposit on the floor is commonly thicker and more columnar, whereas
that on the ceiling is thin and tapering much like an icicle. Those
hanging from the ceiling are called _stalactites_ and those on the floor
are _stalagmites_. Where these two deposits are joined they are known as
a _column_.
These cave formations are all composed of the mineral calcite, which
forms all lime rock. If you will notice in the cave this mineral is
crystalline, and it is remarkable that as calcite crystallizes from the
many individual drops of water it is arranged always in a particular
pattern. These crystal faces reflect light and form the many unusual and
beautiful arrangements which you will see in a visit to the caves.