First,--as being greatest in extent,--the "avenues," or tunnels, which present conclusive evidence of having once been the channels of a subterranean stream, whose waters, having some peculiar solvent property, wore their bed lower and lower in the rock, until they cut through into some lower opening, through which they were drawn off, leaving the old channels dry. Imagine one of the narrow, crooked streets in the old part of Boston, spanned by a continuous stone archway from the summits of the buildings on either hand; then close with solid masonry every window and loop-hole by which a ray of light could struggle in, and you have for proportions and sinuosity not a bad semblance of these tunnels, which constitute four fifths of the extent of the Mammoth Cave. The second and next largest department 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.