water is charged with various salts of lime; when it evaporates it
leaves the material behind on the roof. Where the outflow is so slight
that the fluid does not gather into drops, it forms an incrustation of
limy matter, which often gathers in beautiful flowerlike forms, or
perhaps in the shape of a sheet of alabaster. Where drops are formed,
a small, pendent cone grows downward from the ceiling, over which the
water flows, and on which it evaporates. This cone grows slowly
downward until it may attain the floor of the chamber, which has a
height of thirty feet or more. If all the water does not evaporate,
that which trickles off the apex of the cone, striking on the floor,
is splashed out into a thin sheet, so that it evaporates in a speedy
manner, lays down its limestone, and thus builds another and ruder
cone, which grows upward toward that which is pendent above it.
Finally, they grow together, enlarged by the process which constructed
them, until a mighty column may be formed, sculptured as if by the
hands of a fantastic architect.

[Illustration: Fig. 13.--Stalactites and stalagmites on roof and floor
of a cavern. The arrows show the direction of the moving water.]

All the while that subterranean streams are cutting the caverns
downward the open-air rivers into which they discharge are deepening
their beds, and thereby preparing for the construction of yet lower
stories of caves. These open-air streams commonly flow in steep-sided,
narrow valleys, which themselves were caves until the galleries became
so wide that they could no longer support the roof. Thus we often find
that for a certain distance the roof over a large stream has fallen
in, so that the water flows in the open air. Then it will plunge
under an arch and course, it may be, for some miles, before it again
arrives at a place where the roof has disappeared, or perhaps attains