A rock which consists mostly of sand is called sandstone. The Eday freestone, which is much used for putting round the doors and windows of shops and large buildings, is a sandstone. The common blue flagstone contains clay mixed with more or less sand. The sandy beds are coarse, gritty, and hard; the fine-grained flags contain more clay, and are darker in colour, softer, and smoother on the surface. Nearly all the fine flags contain lime; often it is seen in white shining crystals on the joint-faces of the stones used in building. The presence of lime in a soil improves it considerably. In different parts of Orkney the rocks differ much in appearance. In one place we find yellow and red sandstone, in another blue and gray flags, in another pudding-stone and granite. What is the meaning of this? It shows that while the whole area of our islands was covered with water, gravel was being laid down at one place, sand or muddy silt at another, and so on. We can even make out the order in which the different layers or strata were laid down. It is done in this way:—Usually the beds of rock are not now flat but tilted, and show their edges turned up in a more or less sharp slope. If we walk along any bare rocky shore we shall find that bed succeeds bed, each resting on the top of all those which underlie it. No place could be found to show this better than the shore of Hoy Sound from Stromness to Breckness. We go on and on, crossing over bed after bed of rock, till we have passed over the edges of a pile of flagstone which must be several thousand feet thick. The same thing can be seen to the east of Kirkwall, or, in fact, almost anywhere in the islands. Sometimes the beds dip or slant in different directions at different parts of the shore. Then again they may be broken by cracks or faults which bring different kinds of rock up against one another. If one could visit the whole of Orkney and examine all the rocks, making out in what order they follow one another, how often they are interrupted or repeated by faults, and what is their inclination or dip, one could tell exactly the order in which the rocks of each district were laid down on the bottom of the old lake where they were formed. This is one of the tasks which the geologist undertakes; and though it looks very difficult, yet in Orkney it is quite possible to do so with pretty fair accuracy.