Questions such as these are deeply interesting, and present to the geologist some of the most fascinating problems to be met with in the whole range of this science. And though at first sight they might seem hopelessly beyond our reach, yet even here the prospect is by no means unpromising; and it is quite possible to show that they can be answered to some extent. Here we shall find our illustration of the cathedral (see chapter v., pages [143]-[147]) holds good once more.
It is perhaps hardly necessary to explain that by looking at a Gothic cathedral one can say at what period or periods it was built. Perhaps it has a Norman nave, with great pillars and rounded arches. Then the chancel might be Early English, with pointed windows and deep mouldings, and other features that serve to mark the style of the building, and therefore its date,—because different styles prevailed at different periods. Other parts might contain work easily recognised as belonging to the "Perpendicular" period.
Now, as there have been periods in the history of architecture and art, so there have been periods in the history of our earth. What these periods were, and how we have learned to recognise them, we must first very briefly describe.[33]
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.
But it often happens that neither the strata which should come above nor those that lie below can be seen. Then our second rule comes in: We can determine the age of the rock in question by its fossils. The reason of this has perhaps already been guessed by the reader. It is that as different kinds of plants and animals have prevailed at different periods of the world's history, so there have been "styles," or fashions, in creation, as well as in art. At one geological period certain curious types of fishes flourished which are now almost extinct, only a few old-fashioned survivals being found in one or two out-of-the-way places. At another period certain types of reptiles flourished vigorously, and were the leaders in their day; but they have altogether vanished and become extinct. So one type after another has appeared on the scene, played its humble part in the great drama of life; and then—"exit!" another takes its place.
In the oldest and lowest of the series of rocks we find no certain trace of life at all. In the next series we find only lowly creatures, such as shell-fish, corals, and crab-like animals that have no backbone. In a higher group of rocks fishes appear for the first time. Later on, we come across the remains of amphibious creatures for the first time. Then follows (after a long unrecorded interval) an era when reptiles and birds existed in great numbers. After another long interval we come to strata containing many and diverse remains of mammals or quadrupeds. So we have an "Age of Fishes," an "Age of Reptiles," and an "Age of Mammals." Some tribes of these creatures died out, but others lived on to the present day. Thus we see that there has been a continuous progress in life as the world grew older, for higher types kept coming in.