There are two simple rules by which the age of an ordinary sedimentary rock may be ascertained. This is fixed (1) By its position with regard to others; (2) By the nature of its embedded animal or vegetable remains, known as fossils. These rules may easily be illustrated by a reference to the methods of the antiquary. For instance, suppose you were going to build a house, and the foundations had just been dug out; you might on examining them find several old layers of soil, showing that the site or neighbourhood had been formerly occupied. You might find in one layer stone implements, in another Roman or early British pottery, and yet again portions of brick or stonework, together with tools or articles of domestic use, belonging, say, to the time of Queen Elizabeth. Now, which of these layers would be the oldest? It is quite clear that the lowest layers must have been there the longest, because the others accumulated on the top of them. The explorations made of late years under Jerusalem have led to the interesting discovery that the modern city is built up on the remains of thirteen former cities of Jerusalem, all of which have been destroyed in one way or another. Here, again, it is quite clear that the oldest layer of débris must be that which lies at the bottom, and the newest will be the one on the top. Again, you know that the "Stone Age" in Britain came before the Roman occupation. Those old stone implements were made by a barbarous race, who knew very little of agriculture or the arts of civilisation. Then in succeeding centuries various arts were introduced, many relics of which are found buried in the soil; and hence, since different styles of art and architecture prevailed at different periods, the works of art or industry embedded in any old layers of soil serve to fix the date of those layers. These layers of soil and débris correspond to the layers or strata of the sedimentary rocks, in which the different chapters of the world's history are recorded. Geology is only another kind of history; and the same principles which guide the archæologist searching buried cities also guide the geologist in reading the stony record. As the illustrious Hutton said, "The ruins of an older world are visible in the present state of our planet." The successive layers of ruin in this case are to be seen in the great series of the stratified rocks; and we may lay it down as an axiom that the lowest strata are the oldest, unless by some subsequent disturbance the order should have been reversed, which, fortunately, is a rare occurrence, though examples are to be found in some mountain-chains with violent foldings.