The science of life : fully illustrated in tone and line and including many diagrams

THE EVIDENCE

obviously aquatic, for their limbs are converted into paddles.

In the same clay you will be likely to find other fossils, the spiral ammonites, often beautifully patterned. These, too, though their shells are to be found in millions embedded in various rocks, have never been discovered alive. But we know that these shells were inhabited by creatures not unlike ihe pearly nautilus of to-day, and more distantly resembling the cuttlefish and octopus.

It would not take much reflection, one might think, to realize that in any such sedimentary deposit, whether thick or thin, the lower layers must have been laid down before those above them. But such were our ancestors’ prejudices and preconceptions, based for the most part upon the belief in the sudden creation of the world at a not very remote period, that it was not until the turn of the eighteenth century that this fundamental but elementary idea was properly put forward, to become from thenceforward the basis of geology. William Smith, an English surveyor, as his work took him from one part of the country to another, noted that a number of characteristic rocks, such as chalk, oolite limestone, red sandstone, or gault clay, occurred as layers which covered large areas of country. Moreover, wherever these layers occurred, they were always in the same order. The gault clay, for instance, was always close below the chalk, the greensand always immediately below the gault, the oolite limestone many layers below the chalk, the red sandstone several layers below the oolite, and so forth. And, a third point, each layer of rock was characterized not merely by the material of which it is made, but also by the fossils which it contains. This last was of vital importance ; often two layers of clay or of sandstone may be nearly indistinguishable in their consistency and materials, but easily distinguished by their contained fossils. For instance, the London clay, over which London is built, lies above the chalk layer. It contains fossil fruits of palms and conifers, some nautilus shells, humerous characteristic sea-snails, and a few mammals. No ammonites or ichthyosaurs have ever been discovered in it. The gault clay, on the other hand, from below the chalk, has no plant fruits, but does contain ammonites, often uncoiled in a peculiar way instead of regularly spiral ; while the Oxford clay, a thick layer close above the oolite limestone, has huge numbers of ammonites, almost all built as regular spirals.

Such facts as these obviously mean that

OF THE ROCKS

we ought to be able to arrange all the sedimentary rocks of the world in a series, according to their age; and, this once accomplished, all the fossils in the earth’s crust will fall into their time-sequence, too. The task has been accomplished for the great majority of layers. As a result, we can say that one kind of fossil belonged to an animal which lived and died before another kind of animal found fossilized in another layer ; and the bewildering variety of life becomes more orderly through receiving an arrangement in time. To take merely the same examples we first mentioned, ammonites are found to be absent from all layers below the coal measures and from all above the chalk, but present in all congenial layers between these limits ; while ichthyosaurs, though they also have the chalk as their upper limit, only extend downwards through about two-thirds of the layers in which ammonites are found.

This fundamental principle, that the different layers of the earth’s crust can be arranged in a time-sequence, is the basis of that department of science known as_ paleontology. These sheets of inert matter are the pages of the book of our planet’s history. They le scattered over the globe, often torn, defaced, or crumpled. But patience and reason combined have been able to reconstruct whole chapters and sections of that great book. In it we can read not only the physical changes that the world has experienced—when the Rockies were built, or the Scottish Highlands worn down to mere stumps of their former grandeur, the date of great Ice Ages, eons before the last Ice Age, or of the appalling flow of lava which overwhelmed a quarter of a million square miles in North-Western India—but also the history of Life, printed on the pages of the book in the form of fossils, hieroglyphs which to persevering study reveal readily enough the secret of their picture-writing.

The principle is both fundamental and simple ; but there are sometimes difficulties in applying it. Part of the record may have been destroyed or defaced till the life-story it contains becomes illegible ; or a whole set of pages may have been crumpled or turned upside down into reverse order (as in the upthrust of some mountain ranges), so that their proper rearrangement 1s a matter of the greatest difficulty; or an isolated page or chapter from some out-of the-way corner of the globe may be hard to place. oo er

Happily such difficulties only concern pat ts of the record ; whole chapters of it have the

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