This is not a mere supposition, but can be proved quite clearly. In many Orkney hills there are beds of rock which can be identified by the geologist by certain marks. They may contain peculiar fossils, or they may be of a special colour or structure. In Firth and Orphir, for example, there is a band of flagstone which yields roofing slabs. You can follow this band from hill to hill for several miles, often by the quarries in which it was extensively worked in former years. It occurs at much the same level in all the different hills, though sinking somewhat to the north according to the dip or slope of the rock bed. It is found on both sides of the valleys, as, for example, at Finstown, at much the same height. [Illustration: _The Hills of Orkney. Photographed from a Relief Model based on the Ordnance Survey._] The Orkney hills, then, consist of a great pile of beds of flagstone which once spread unbroken over the whole country. Out of this great mass of flagstones and sandstones the running water of the burns has carved the valley systems. The hills are the remnants which the streams have not yet removed. As time goes on the valleys deepen and broaden, and the hills get less and less. “The hills are shadows, and they flow From form to form, and nothing stands.” It has taken vast ages to do this work, and the work is still going on. It is very slow. The oldest man hardly notices any visible change in the configuration of the country. But wind, rain, frost, and running water are ever at work. Every day sees some loss, some material swept away never to return. What becomes of it? It reaches the sea, and there forms mud and sand. Time will change these into solid rock again, and may ultimately use them in building new continents. The hills crumble into dust, but it is “the dust of continents to be.” Cliffs and Beaches. On looking at a map of Orkney or Shetland we are struck with the irregularity in the shape of the islands and the winding nature of the coast line. There must be some reason for this, and a little reflection will bring it to light. If you look at the larger valleys you will notice that most of them end in salt-water bays, while the hills or ridges between the valleys run out into points or “nesses.” This is especially the case in Shetland; but in Orkney, too, there are many instances of it. The shape of the land extends beneath the water—the deep bay continues the land valley, the point and the skerry mark the position of the watersheds.