Suppose we picked up an old pair of boots, and found the soles worn away in the centre. It would be easy to find out how much had been worn away over the holes by simply measuring the thickness of leather at the sides, where we will suppose that they were protected by strong nails. Geologists apply a very similar kind of method in order to find out how much rock has been removed from a certain region of the earth. One of the simplest cases of this kind is that of the area known as the Weald of Kent, Surrey, and Sussex (see illustration, Fig. 1). A great deal of denudation has taken place here, because there is ample evidence to prove that the great "formation" known as the Chalk (now seen in the North and South Downs) once stretched right across; and below this came the lower greensand and Weald clay. They spread over this area in a low arch of which we now only see the ruins. [Illustration: Fig. 1. SECTION ACROSS THE WEALD OF KENT AND SURREY.] [Illustration: Fig. 2. THE HIGHLANDS OF SCOTLAND ON A TRUE SCALE (after GEIKIE).] The dotted lines in the figure show us their former extent; but the vertical height is exaggerated, for otherwise the hills would scarcely be seen. These lines simply follow out the curves taken by the strata at each end of the denuded arch, and therefore rightly indicate its former height. By making such a drawing on a true scale, geologists can easily measure the former height of the surface of this old arch, or "anticline," of chalk, greensand, and other strata, just as an architect might restore the outlines of an old traceried window from a few portions left at the sides. This very useful and instructive method is much employed in drawing sections through mountain-chains, in order to gain some idea of the amount of denudation which they have suffered. Let us see how much has been removed from the present surface of the Weald. First there is the chalk, which we may put down at six hundred feet at least; then there is the lower greensand, say, eight hundred feet; and below that, and forming the lowest ground in the Weald, is the Weald clay, which is one thousand feet thick, and being softer, was more rapidly borne away. Along the centre runs a ridge of Hastings sand, forming higher ground on account of its greater hardness, but this formation is not much denuded. However, adding together the thicknesses of the others, we arrive at the conclusion that about twenty-four hundred feet of chalk and other strata has been removed from the present surface of the Weald. And all this denudation has probably been effected by rain and rivers, for it is very doubtful whether the sea had any share in this work.