But there is one of these strange openings in the earth which has a special interest, for it is the handiwork, not of nature, but of man. I had never heard of Blue John when I came to these parts. It is the name given to a peculiar mineral of a beautiful purple shade, which is only found at one or two places in the world. It is so rare that an ordinary vase of Blue John would be valued at a great price. The Romans, with that extraordinary instinct of theirs, discovered that it was to be found in this valley, and sank a horizontal shaft deep into the mountain side. The opening of their mine has been called Blue John Gap, a clean-cut arch in the rock, the mouth all overgrown with bushes. It is a goodly passage which the Roman miners have cut, and it intersects some of the great water-worn caves, so that if you enter Blue John Gap you would do well to mark your steps and to have a good store of candles, or you may never make your way back to the daylight again. I have not yet gone deeply into it, but this very day I stood at the mouth of the arched tunnel, and peering down into the black recesses beyond, I vowed that when my health returned I would devote some holiday to exploring those mysterious depths and finding out for myself how far the Roman had penetrated into the Derbyshire hills. Strange how superstitious these countrymen are! I should have thought better of young Armitage, for he is a man of some education and character, and a very fine fellow for his station in life. I was standing at the Blue John Gap when he came across the field to me. "Well, doctor," said he, "you're not afraid, anyhow." "Afraid!" I answered. "Afraid of what?" "Of it," said he, with a jerk of his thumb towards the black vault, "of the Terror that lives in the Blue John Cave."