That portion of the limestone dyke in which the caves are found runs six miles north and south; and the Grand Arch and the Devil's Coach House--the two principal "day caves"--are formed right through the mountain, near the centre, in an easterly and westerly direction. On the one side McEwan's Creek flows towards the Devil's Coach House, and on the other side is a natural watercourse leading to the Grand Arch, which is only a few yards distant from it. Bearing in mind how the watercourses converge towards these two central caves, and with what force, in times of heavy rain, the floods scour them, a good understanding may be obtained of the mechanical causes of the enormous excavations which excite amazement as well as admiration. If a visit be paid in winter, when the frost is sharp and the ground is "hoar with rime;" when every bough and every blade of grass is covered with congealed dew and adorned with forms of crystallisation which rival the rarest beauties of the caves; when rocks are split and crumbled by sudden alternations of heat and cold; there will be abundant illustration of the effect of water and light, and the variation of temperature in causing geological transformation. Limestone is not soluble in water without the addition of carbonic acid. An exploration of the caves, however, shows that the mountains are not composed entirely of limestone, but that other substances constitute part of their bulk. The principal causes of the formation of the "day caves," the Grand Arch and the Devil's Coach House, are the mechanical action of water and the variation of temperature. As regards the interior caves, where night reigns supreme, chemical combination has played a more important part. But the effect of water power is everywhere observable in graceful contours, caused by continuous motion, or in stony efflorescence, produced by intermittent humidity or dryness of the atmosphere. The action of the former is the more marked and striking, the latter more elaborate, and microscopically beautiful. The hygrometric condition of the caves is recorded in lovely forms, which lend enchantment to ornate bowers, sparkling grottoes, and fairy cities. [Illustration: THE GRAND ARCH ENTRANCE.] [Illustration: THE GRAND ARCH--LOOKING EAST.] THE GRAND ARCH.