Caves are formed in calcareous strata by the chemical action of water laden with carbonic acid, and by the mechanical action of streams. In consequence of the original structure of the Limestone, the joints of which run at right angles to the bedding planes, these eroded hollows have two dominant forms: the vertical pot, swallet, or hole, produced by the widening of a master-joint; and the horizontal water-channel, running in the same direction as the line of stratification. But the strata being commonly tilted, these pits and abysses are often a long way out of the vertical, and the caverns that follow the strata very steep. Many of these ancient watercourses are now dry, but others are still traversed by streams, and present the explorer with most formidable obstacles. The complete exploration of any cave system would involve the tracing out of all its passages from the point where the stream or streams enter the earth to the point of exit. But I know not a single instance where such a task has been worked out in its entirety. In many cases the streams enter the ground merely as small rivulets, and begin to excavate passages practicable to man only at a considerable depth. "Siphons," or traps, as they ought to be called, complete or partial chokes, and a variety of other causes, may put insuperable obstacles in the explorer's way. Take two of the most important cave problems still awaiting solution, one in Yorkshire, the other in Somerset. A large beck is precipitated into the abyss of Gaping Ghyll, 360 feet deep, and emerges from an opening in the hillside, a mile away, close to the mouth of Ingleborough Cave, which was itself an earlier exit. Several parties have descended Gaping Ghyll, and followed the passages at the bottom to a distance of more than 1000 feet. Then impenetrable water-sinks, and muddy chambers with no outlet, have been encountered, and the communication with the lower cavern has hitherto proved undiscoverable. Both the dry galleries and the canals of Ingleborough Cave have been explored, with great toil and daring, to a considerable distance upwards, with similar results; and though many speleologists are still absorbed in this problem, there is little hope that it will be cleared up without adopting the drastic and costly measure of cutting through the obstructions. The other problem is that of Wookey Hole, the cave in Britain which has the longest history, and which is still yielding interesting discoveries. A number of streams disappear into the earth on the Mendip plateau, 2 miles away and 700 feet above, and find their issue in the source of the Axe at Wookey Hole. Two of the Mendip swallets have been explored to a great depth. Swildon's Hole, an exquisite series of terraced galleries and stalactite grottoes, has been penetrated to a depth of 300 feet. But a more determined attempt has been made to reach the bottom of the Eastwater Cavern. This was discovered in 1902 by my friend Mr. Balch, of Wells, by means of