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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.
Fig. 11—Horizontal Layers of Fossiliferous Clays and Sands.
In Sea Cliff, Torquay Coast, Victoria, looking towards Bird Rock.
(Original).
Fig. 12—Cliff-Section to Show Superposition of Strata.
A = Sandstone. B = Shale. C = Limestone.