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

BOOK 3

CHAPTER 2

THE EVIDENCE OF THE ROCKS

§ 1. The Nature and Scale of the Record of the Rocks. § 3. A Sample Section in the History of Life: The Evolu§ 4. The Continuity of Evolution as Shown by Sea-urchins.

Finds in the Record. tion of Horses.

§ 2. Defects and Happy

5. © Missing Links.”

§ 1 The Nature and Scale of the Record of the Rocks

N order to make the nature of the record

of the rocks perfectly clear it is necessary to remind the reader of certain elementary geological facts. They will probably be familiar to him, but we do not want to have any “missing links” in our chain of argument. We warn him of this beforehand, so that if the note of the professional lecturer creeps into our discourse he will forgive itfor the sake of its explicitness.

It is only after decades of patient work, we must remember, that the fact of Evolution obtruded itself as a necessity in the face of the paleontologist. Right up to the end of the eighteenth century the comparatively few fossils then known were almost universally regarded as mere curiosities; many dismissed them as sports of Nature, freaks of the earth, and not really the remains of flesh-andblood, while at most they got credit for being witnesses to the universal biblical Deluge. Nothing more could be expected until geology had made her profound advance of introducing time, and time on a vast scale, into our ideas about the earth’s crust.

Let us recall how that extension of time dawned upon the human intelligence.

Everybody knows that flowing waters bring down sediment and deposit it in layers on the floor of seas and lakes, or on floodplains. Sometimes the deposit reaches the surface, as in deltas. Consider the great tongue of the Mississippi delta, built out into the sea for sixty miles and more ; the Mississippi brings down every year over four hundred million tons of sediment. At other times the deposit spreads over the bottom, as when a pond is gradually filled up, or an alluvial meadow built, layer upon layer, from the silt laid down by successive floods. Layers of material may be laid down in other ways; the great spit of Dungeness, on England’s south coast, grows out to sea at the rate of over five feet a year, from shingle brought along the coast by the waves and

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currents. In moor country, deep layers of peat are formed by the successive death of the bottom parts of the bog plants. Currents and waves deposit stretches and banks of sand in quiet bays. After a volcanic eruption vast quantities of dust and pumice and rock fragments fall in the neighbouring sea and sink to the bottom. And from the surface layers of the ocean a constant rain of billions of skeletons of animals and plants, many of them microscopic, but as we have already shown, incredibly abundant, is always falling softly towards the depths.

In these and other ways new materials are to-day being accumulated in the form of sheets or layers of varying extent in innumerable regions of the globe, and they must obviously have been accumulating in the same sort of way through all the ages since liquid water has existed on our planet. These accumulations of slowly deposited layers are what we call sedimentary or Stratified rocks! (as opposed to the igneous rocks, forced in among the other layers of the crust in a molten state from below, or belched out over the surface by volcanoes), and the sheets themselves are technically called by the Latin word for layers—strata.

In these stratified rocks fossils are often found entombed—the remains and _ traces of dead animals and plants which were often strikingly different from any creatures alive to-day. We need give but a couple of examples. If you happen to spend your holiday on the Dorset coast near Lyme Regis and search the crumbling cliffs near by (or, indeed, if you explore any clay quarry across from Dorset to Peterborough), you will be pretty sure to find some bi-concave bone: discs, the vertebrae of some large animal. Persistent and lucky hunters have found whole skeletons containing such vertebre. They are of giant reptiles, christened Ichthyosaurs, or ‘‘ fish-lizards,” wholly unlike anything existing to-day,

1 By the geologist, all constituents of the earth’s crust, except the actual soil, are called racks, whether they are hard granite or limestone, friable chalk or sandstone, or soft clay or loess.