Effects of Rain.
Mere rain is a powerful agent of disintegration; and its frequent attacks leave at length, in sandstones and limestones, otherwise very durable, channels of considerable dimensions, which have sometimes been ascribed to other causes. The Devil's Arrows at Boroughbridge, in Yorkshire, are fluted from this cause from top to bottom (except on the under hanging sides, where they cease not far below the summit)—the work of two or three thousand years: and when we turn from these monuments of man to the native crags whence they were cut, "Brinham rocks," and regard the awful waste and ruin there, well marked by the pinnacles and rocking stones which remain in picturesque desolation, it is difficult to avoid indulging a long train of reflection on the processes of decay and renovation which thus seem to visit even the inanimate kingdoms of nature, subjecting all its material elements to continually renewed combinations.
On the broad limestone floors which support the noble mountains of Ingleborough, Penyghent, and Whernside, the rain channels are so abundant as to have attracted the attention of artists and tourists; and on Hutton roof crags, as well as among the limestones of the Alps, they change their direction with the slope of the ground, collect into larger furrows like valleys on a broad surface, and terminate in the large deep fissures, as small valleys often end in a great hollow of drainage. Another remarkable phenomenon of the moorland districts of the North of England, which are formed on the Yoredale series of mountain limestone, may perhaps admit of the same explanation. These are the "Swallow" holes, as they are termed, which range above the outcrop edge of the limestone beds, and act as drainage channels from the surface to the jointed calcareous rocks below. These round or irregular pits and holes are smoothed on the faces and joints of stone,
