[Illustration: MAP OF THE MENDIP DISTRICT OF SOMERSET, SHOWING SWALLETS, CAVES, AND OUTLETS.] Reference to the sketch map of the district will show that the majority of the more important swallets lie along the line of the great depression referred to. These comprise by no means all the swallets of Mendip, yet they are the chief ones. It is obvious that the whole of the mass of material represented by this great depression has been removed in suspension by way of these swallets; and one is compelled to ask, How long has this work been going on? What time is represented by so vast a work? On the threshold of the inquiry we are met by such an amount of evidence bearing upon it that the subject must be dealt with separately. For, upon the upturned edges of the Carboniferous Limestone rocks, which can have been brought down to their present plane of denudation only by long-continued water action, have been deposited, and still remain _in situ_, great masses of the basement beds of the Secondary rocks, lying in such a manner as to convince us that swallet action had prepared the denuded surfaces upon which they lie. And upon this hinges the whole question of the antiquity of the caverns of Mendip. But whilst the age of our caverns is a debatable matter, no one can question the accuracy of the theory of ravine formation from the collapse of cavern roofs, as evidenced by the instances supplied by Mendip. Through crevices and cracks, here, there, and everywhere, the percolating waters find their way. Now some crevice is enlarged into a passage; now some weak point in the passage becomes a chamber; and on the water rushes, steadily joining forces and accumulating, until on the level of the lower land it finds an outlet, and rushes forth a considerable stream. In its headlong course the water again and again leaps down some great series of potholes, as down some giant stairway, forming many fine cascades, whose deafening roar goes on for ever where there is no ear to hear and where no footstep ever treads the rocky ways. Along the course of the larger streams huge chambers occur; for the ever-eddying water, bearing sand along in its course, eats out the sides of its channel, or, revolving stones in its bed, carves out the pothole by friction. Or some pendent mass of rock has its support undermined and comes crashing into the streamway, only to be broken up and carried away by the ceaseless energy of the stream, so ever enlarging the chambers upwards towards the light of day. But whilst this action is going on underground, a more potent factor is at work where the subterranean stream first sees the light. Here very soon the action of the water alone gives rise to a little cliff overhead. Now rain and frost, wind and tempest, loosen, bit by bit, the fragments of rock forming the face of the cliff, which fall away into the river, to be broken up and carried away. Little by little the face of the cliff recedes, along the line of the subterranean river,