As the streams that flow on the surface of the earth are constantly altering their courses, thus also the subterranean waters are ever active in excavating new channels in the bosom of the rock. Finding at length new outlets on a lower level, they abandon their ancient beds, and the explorer now wanders dry-footed where once a foaming river gushed along. The Cave of Adelsberg is a remarkable example of the changes which the subterranean waters, aided by time or by the disrupting power of earthquakes, may thus bring about; for the Poik now flows beneath its galleries in the same north-easterly direction, in a channel which is for the greatest part unknown and inexplorable, so that the dry cave of the present day must evidently have been the old river- bed. But nowhere can be found such perfect, unequivocal, and abundant proofs of the action of running water in corroding and excavating new passages in a soluble rock as in the huge Mammoth Cave. The rough-hewn block in the quarry does not bear more distinct proof of the hammer and the chisel of the workman than these interminable galleries afford of its denuding and dissolving power. At Niagara we see a vast chasm evidently cut by water for seven miles, and still in progress; but we cannot see beneath the cataract the water-worn surface, nor the rounded angles of the precipice; while the frosts and rains of countless winters have reduced the walls of the chasm itself to a talus of crumbling and moss-grown rocks. But in the Mammoth Cave we see a freshness and perfection of surface such as can be found only where the destructive agencies of meteoric causes are wholly absent. Here we have the dry beds of subterranean rivers exactly as they were left thousands of years ago by the streams which flowed through them when Niagara was young. No angle is less sharp, no groove or excavation less perfect, than it was originally left when the waters were suddenly drained off by cutting their way to some lower level. The very sand and rounded pebbles, which now pave the galleries and which anciently formed the bed of the stream, have remained in many of the more distant galleries untrodden even by the foot of man. ‘The rush of ideas was strange and overpowering,’ says Professor Silliman, ‘as I stood in one of these before unvisited avenues, in which the glow of a lamp had never before shone, and considered the complex chain of phenomena which were before me. There were the delicate silicious forms of cyathophylli and encrinites protruding from the softer limestone, which had yielded to the dissolving power of the water; these carried me back to that vast and desolate ocean in which they flourished, and were entombed as the crystalline matrix was slowly cast around them, mute chroniclers of a distant epoch. Then succeeded the long periods of the upper secondary, and, these past, the slow but resistless force of the contracting sphere elevated and drained the rocky beds of the ancient ocean. The action of