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 district find here and there evidences of this progressive
destruction. Not only do the caves wear out from above, but their
roofs are constantly falling to their floors, a process which is
greatly aided by the growth of stalactites. Forming in the crevices or
joints between the stones, these rock growths sometimes prize off
great blocks. In other cases the weight of the pendent stalactite
drags the ill-supported masses of the roof to the floor. In this way a