It requires but a cursory glance to see at once that running water was the main agent. The limestone is so traversed by joints and lines of shrinkage, that the water rapidly sinks down into its mass, and collects in small streams, which owe their direction to the dip of the strata and the position of the fissures. These channels are being continually deepened and widened by the mere mechanical action of the passage of stones and silt. But this is not the only way in which the rock is gradually eroded. The limestone is composed in great part of pure carbonate of lime, which is insoluble in water. It is, however, readily dissolved in any liquid containing carbonic acid, which is an essential part of our atmosphere, is invariably present in the rain-water, and is given off by all organic bodies. By this invisible agent the hard crystalline rock is always being attacked in some form or another. The very snails that take refuge in its crannies leave an enduring mark of their presence in a surface fretted with their acid exhalations, which sometimes pass current among geologists for the borings of pholades, and are the innocent cause of much speculation as to the depression of the mountain-tops beneath the sea in comparatively modern times. The carbonic acid taken up by the rain is derived, in the main, from the decomposing vegetable matter which generally forms the surface soil on the limestone. [Illustration: FIG. 7.--Diagram of an acid-worn joint, Doveholes, Derbyshire.] The view from the ancient camp on the top of Ingleborough offers a striking example of the effect of rain-water in eroding the surface of the limestone. As you look down over the dark crags of millstone grit, great, grey, pavement-like masses of limestone strike the eye, standing above the heather, perfectly bare, and in the distance resembling clearings, and in rainy weather sheets of snow. On approaching them the surface of erosion becomes more and more apparent, and the shapes due to the mere accident of varying hardness in the rock, or the varying quantity of water passing over it, present a most astonishing variety. There are, however, general principles underlying the confusion. The lines of joints in the strata being lines of weakness, searched out by the acid-laden water, have been widened into chasms, sometimes of considerable depth; and as they cross at right angles, the whole surface is formed of rectangular masses, each insulated from its fellow, and some of them detached from the strata beneath so as to form rocking-stones. The mode in which the acid has attacked one of these joints in the limestone of Doveholes in Derbyshire is represented in Figure 7, the surface being honeycombed and worn into sharp points, solely by chemical action. The minute fossil-shells also, and fragments of crinoid standing out in bold relief, lead to the same conclusion--that the denuding agent is chemical and not mechanical. Each of the upper surfaces of the blocks is traversed by