s 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
a field occupied by rocks of another character, in which caverns were
not formed. At places these old river caverns are abandoned by the
streams, which find other courses. They form natural tunnels, which
are not infrequently of considerable length. One such in southwestern
Virginia has been made useful for a railway passing from one valley to
another, thus sparing the expense of a costly excavation. Where the
remnant of the arch is small, it is commonly known as a natural
bridge, of which that in Rockbridge County, in Virginia, is a very
noble example. Arches of this sort are not uncommon in many cavern
countries; five such exist in Carter County, Kentucky, a district in
the eastern part of that State which abounds in caverns, though none
of them are of conspicuous height or beauty.[7]

[Footnote 7: It is reported that one of these natural bridges of Carter
County has recently fallen down. This is the natural end of these
features. As before remarked, they are but the remnants of much more
extensive roofs which the processes of decay have brought to ruin.]

At this stage of his studies on cavern work the student will readily
conceive that, as the surface of the country overlying the cave is
incessantly wearing down, the upper stories of the system are
continually disappearing, while new ones are forming at the present
drainage level of the country. In fact, the attentive eye can in such
a dist