Underground rivers appear to be natural to limestone caves. The reason of their existence is to be found in the fact that the mountains in which they are formed are, in geological parlance, "dykes." They must not be confounded with old river beds, such as are encountered by miners—where the surface of the earth has been raised by deposits of alluvium, or where the geological condition of things has been changed by volcanic action. These cave rivers have all been formed by water finding the lowest attainable level in its passage to the sea, and by the solid limestone rocks which have barred its direct course, and have been undermined by its subtle but persistent action. The fluid, dammed back by the mountains, has simply obeyed the laws of gravitation and accumulated force, as evidenced in the trickling silvery thread which follows the course of ant-tracks; in the laughing rill which makes its bed among the pebbles; in the babbling brook which leaps to the swelling river; and in the mighty torrent whose strength and velocity proclaim the majesty of hydraulic power. In all parts of the world where limestone dykes and caves exist, it is reasonable to expect to find subterranean rivers. The eye of the seer can follow the water drips—
"Down through caverns and gulfs profound,
To the dreary fountain-head
Of lakes and rivers underground."
He can see them again when the rain is done—
"On the bridge of colours seven,
Climbing up once more to heaven,
Past the setting sun."
But the underground rivers found in caves perform vagaries outside the sweet imaginings of the poet and the prevision of the seer. Far from the beaten track they turbulently force their way through recesses and tunnels and pockets of the earth, before they are again warmed with sunshine, and glow in the harmonious colours which form the Bow of Promise.
The Rev. Richard Taylor, in his "Te ika a maui," refers to interesting caves near Mokau (New Zealand), in some of which bones of the moa have been discovered. About a mile from Pukemapau he came to a limestone range, and entered a large cave called Tanaureure. At the bottom of a chasm he found a fine crystal spring, about a foot or so deep, but appears not to have been particularly inquisitive as to whence the water came or whither it went.
A little distance up one of the tributaries of the Rewa River, in Fiji, is a crystal streamlet which flows on towards a lofty ridge, near to which it sinks into the earth. At the mouth of a dark cavern can be heard the roaring. It is a grand expansive excavation, but
"Dark as was chaos ere the infant sun
Was roll'd together, or had tried his beams
Athwart the gloom profound."
The water rushes through narrow chasms as through a race, collects in a large pool, and flows through a distant outlet, marked by a speck of light, like a tiny star.
At the Weathercote Cave, in Yorkshire, a stream swallowed up by a rocky mouth is thus described by Walter White in his book entitled "A Month in Yorkshire":—