Fossils are chiefly found in rocks which have been formed of sediments laid down in water, such as sandstone, shale and most limestones. These rocks, broadly speaking, have been deposited in a horizontal position, though really slightly inclined from shore to deep-water. One layer has been formed above another, so that the oldest layer is at the bottom, and the newest at the top, of the series (Fig. 11). Let us, for instance, examine a cliff showing three layers: the lower, a sandstone, we will Call A; the intermediate, a shale or clay bed, B; and the uppermost, a limestone or marl, C (Fig. 12). In forming a conclusion about the relative ages of the beds, we shall find that A is always older than B, and B than C, provided no disturbance of the strata has taken place. For instance, the beds once horizontally deposited may have been curved and folded over, or even broken and thrust out of place, within limited areas; but occurrences like these are extremely rare. Moreover, an examination of the surrounding country, or of deep cuttings in the neighbourhood, will tell us if there is any probability of this inversion of strata having taken place. [Illustration: =Fig. 11.--Horizontal Layers of Fossiliferous Clays and Sands.= In Sea Cliff, Torquay Coast, Victoria, looking towards Bird Rock. (_Original_). ] [Illustration: =Fig. 12.--Cliff-Section to Show Superposition of Strata.= A = Sandstone. B = Shale. C = Limestone.] This law of superposition holds good throughout the mass of sedimentary rocks forming the crust of the earth. (1). Thus, the position of the strata shows the relative ages of the beds. =Differences in Fossil Faunas.--= Turning once again to our ideal cliff section, if we examine the fossils obtained from bed A, we shall find them differing in the number of kinds or species common to the other beds above and below. Thus, there will be more species alike in beds A and B or in B and C. In other words the faunas of A and B are more nearly related than those of A and C. This is explained by the fact that there is a gradual change in specific forms as we pass through the time series of strata from below upwards; so that the nearer one collecting platform is to another, as a rule, the stronger is the community of species. =Guide Fossils.--= Certain kinds of fossils are typical of particular formations. They are known as guide fossils, and by their occurrence help us to gain some idea of the approximate age of rocks widely separated by ocean and continent. Thus we find fossils typical of the Middle Devonian rocks in Europe, which also occur in parts of Australia, and we therefore conclude that the Australian rocks containing those particular fossils belong to the same formation, and are nearly of the same age. (2). The included fossils, therefore, give evidence of the age of the beds. =Value of Lithological Evidence.--=