An abrupt cliff walls in the hollow on the Limestone side, only a few paces from which are naked patches of Yoredale rocks, clearly defining the boundary of the two series. We now came to one of the most interesting openings that we have met with. It lies about 200 yards north of Bull Pit. As often happens, immediately above the swallet, in the Limestone, is a deep chasm almost perforating the escarpment. At the base of the escarpment is a rounded archway with a turbulent stream running in. After securing a photograph we enter, and make our way down stream easily for a little distance; then the cave twists and narrows, and at a distance of 40 feet or so we are disappointed to find the channel too confined for us to force our way farther. Outside we had observed that the basin-shaped area had been flooded not long ago, and inside the vegetable débris that was plastered over the walls and roof showed that the swallet must have been completely choked during the recent wet weather. But the peculiarity of this swallet was that the solid mass of rock through which the stream had carved its way was not ordinary Limestone, but beautifully veined and crystalline like marble, and its surface smooth and polished. It had very much the same appearance as the marmorised Limestone found in the neighbourhood of intrusive lavas, such as those near Tideswell. By the action of the water it had been sculptured into fantastic shapes; in one place a corner had been cut through and a small pillar left, joined to the rock at top and bottom. We scrambled with some difficulty into the chasm behind the swallet. At the bottom, on the same side as the existing swallet, was the broad and lofty arch of a cave, which went only a few yards in, otherwise it would have broken through the escarpment. Right above the keystone of the arch was a weathered group of stalactites hanging from a ledge, and under them the broken stalagmite floor of a tiny grotto. It is a rare thing to find such deposits in the open air, and doubtless it indicates that the chasm was formed by the destruction of a larger cave. A thick deposit of earthy mud covered the floor, and at one side a big hole penetrated this to a depth of six feet, the work of a stream that had perhaps not run for ages. This deposit, though dry, was so soft that I nearly sank through into the hole. We found four birds' nests in this cave mouth, with eggs and young in them, and were disappointed not to come across the egg of a cuckoo that flew out the moment before we entered. In the wiry grass not far away from the top of the cavity we discovered a lark's nest with two eggs in it.