partment is that of the "chambers." These
are places in the general course of the former river where the roof fell
in before the withdrawal of the waters, opening great spaces upward; the
fallen mass forming a sort of island in the centre of the stream, and
crowding the waters on either side of it against the walls of the cave,
so that they were worn out to twice the average width, and finally
itself disappearing under the combined action of the current and the
solvent properties of the water.

The air in all these tunnels and chambers is remarkably dry and pure.
Wood seems never to decay here; as is instanced in the wooden pipes and
vats of the saltpetre-makers, upon which the lapse of a half-century has
not had any visible effect.

The general width of the tunnels or avenues is about forty or fifty
feet, and the average height about thirty feet; but this uniformity is
broken every few hundred yards by chambers, varying in width from eighty
to two hundred feet, and in height from seventy to two hundred and
fifty feet. The floor is formed in some places of sand, but generally of
indurated mud, so hard that it is impossible to make any indentation in
it with the heel of the boot, and remarkably even and smooth, so that
almost anywhere one can walk with as much ease as on city sidewalks. The
walls also are clean and smooth, as in the arched crypts of some mighty
cathedral. A cross section of almost any one of these tunnels would show
an elliptic outline, the vertical diameter being the shortest, and the
bottom being filled with indurated mud or sand to a sufficient depth to
make a level floor.

The third division or class of openings is that of the "domes" and
"pits." These were formed by the same kind of agency as the tunnels and
chambers, namely, by the action of water holding carbonic acid in
solution; but acting in a different manner, and at a period long after
the subterranean river had ceased to flow through its tunnels.

The solvent acid of the water must be acquired in percolating through
the several hundreds of feet of superincumbent earth and sandstone, as
there is but one of these domes, "Sandstone Dome," that extends upward
to the sandstone. The solvent water then, after finding its way into the
vertical crevices of the limestone, gradually rounded them out like
wells, until the pieces which occasionally fell from the top formed a
sort of floor. Through the interstices of this floor, the dissolved
substanc