The two actions by which caves are hewn out of the calcareous rock are seen here in operation side by side. Below the level of the stream the rock is seen to be smoothed and polished by the mechanical action of the materials swept down by the current. Above the water-level the sides of the cave are honeycombed and eaten into the most fantastic and complex shapes, the resultant surface (see Fig. 7) bearing small points and keen knife-edges of stone, that stand out in relief and mark the less soluble portions of the rock. This is due to the chemical effect of the carbonic acid in the water percolating through the strata. [Illustration: FIG. 3.--Diagram of Helln Pot.] The Helln Pot, into which the stream flowing through the Long Churn Cave falls, is a fissure (Figs. 2, 3, 4) a hundred feet long by thirty feet wide, that engulfs the waters of a little stream on the surface, which are dissipated in spray long before they reach the bottom. From the top you look down on a series of ledges, green with ferns and mosses, and, about a hundred feet from the surface, an enormous fragment of rock forms a natural bridge across the chasm from one ledge to another. A little above this is the debouchement of the stream flowing through the Long Churn Cave (Fig. 3, _a_), through which Mr. Birkbeck and Mr. Metcalfe made the first perilous descent in 1847. The party, consisting of ten persons, ventured into this awful chasm with no other apparatus than ropes, planks, a turn-tree, and a fire-escape belt. On emerging from the Long Churn Cave they stood on a ledge of rock about twelve feet wide, and which gave them free access to the “bridge” (Fig. 2, _b_). This was a rock ten feet long, which rested obliquely on the ledges. Having crossed over this, they crept behind the waterfall which descended from the top, and fixed their pulley, five being let down while the rest of the party remained behind to hoist them up again. In this way they reached the bottom of the pot, which before had never been trod by the foot of man. Thence they followed the stream downwards as far as the first great waterfall, down which Mr. Metcalfe was venturesome enough to let himself with a rope, and to push onwards until daylight failed. He was within a very little of arriving at the end of the cave into which the stream flows, but was obliged to turn back to the daylight without having accomplished his purpose. The whole party eventually, after considerable danger and trouble, returned safely from this most bold adventure.