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

BOOK 3

has been so great that the rocks have been what we may call accordion-pleated—thrown into a whole series of deep folds, so that when seen in section on an exposed face they look like a fan. Not only that, but what the geologists call faulting may take place. Under the stresses and strains of mountain-building or of earthquake, great layers of rock may crack across, and one side slip down or be forced up and made to ride over the other side. At the time of the San Francisco earthquake a fault hundreds of miles long was produced, in which the whole country to one side of the crack fell suddenly from five to ten feet; but in mountain regions areas of rock may be faulted down hundreds of feet.

During these magnificent crumplings whole pages of the record have been made altogether illegible. Even when life has succeeded in writing its story on the rocks, the writing has too often been obliterated again. ‘The rock-layers may be subjected to colossal pressures under great depths of newer deposits or scorched by contact with huge intruding lakes of molten material from below, so that any contained fossils are squeezed, distorted, or baked out of recognition or even out of existence. The rocks themselves change their very character. Such transformed rocks are called metamorphic ; marble, for instance, is thus metamorphosed out of limestone, quartzite or gneiss out of sandstone, and so forth.

This difficulty becomes more serious as we go farther back in the record. For rain and wind and frost never stop their scouring and splitting and wearing, and the younger sediments must all be derived from the débris of the old. Whole mountain ranges, with their contained fossils, have been destroyed, worn down to level plateaus to furnish material for new layers, often hundreds or thousands of feet thick, which in their turn

will be upheaved, and in their turn eroded ' away. Entire chapters of the Book of Earth have thus been pulped to furnish material for new pages. Luckily, however, the making of the book went on simultaneously in many regions, each one often of huge extent, and it is rare that all the records of a whole Age have perished.

Then we must remember that fossilization is the fate of very few animals and plants. Only one in a million makes its mark in the Book of Life. The great majority of dead things simply decay and disappear and their material is returned to the general circulation of Nature to be built up into the bodies of new organisms. But once in a while a corpse

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THE SCIENCE OF LIFE

CHAPTER e2

is preserved more permanently ; it falls into mud or silt or some place where the bacteria of decay cannot get at it. Insects have been caught and sealed and preserved for countless years in the fossilized resin we call amber. The bodies of mammoths can be dug out of frozen mud-cliffs in Siberia with the skin and flesh still preserved. And even when the flesh decays away the bones may escape the dissolving action of rain or other waters and ultimately find their way into the palzontologist’s cabinet.

But it should be remembered that these direct preservations of the material of extinct creatures are the rarest accidents. Most fossils (for any dug-up trace of an organic being is called a fossil) are not actual surviving bits of corpses at all, but bits of corpses which have been changed into rock by a slow translation and replacement. They are copies at second hand of the original writing. As the bones or other enduring fragments lie buried they slowly dissolve away and are replaced more or less completely by mineral substances. In a word they are “petrified.” In this process, of course, they undergo varying degrees of distortion, although in one or two exceptional cases the translation is astonishingly accurate, each little difference of texture in the original being faithfully reflected in the mineralized fossil. ‘The record may be even more indirect than that ; it may be a dried footprint or the hollow impress of a bit of skin or a shell. From these scattered and accidental remains the paleontologist patiently reconstructs his picture of the world as it used to be.

Occasionally a lucky chance makes us realize how scanty our information really is.

In California, for instance, a pool of water with sticky margins impregnated with tar proved a death-trap to thousands of creatures as they came down to drink, and to hordes of carnivorous mammals, like wolves and sabre-toothed tigers, which endeavoured to catch the drinkers when they stuck fast. Now the paleontologist finds a hoard of treasure in that one pool. In France, in the drought of 1911, it was noted that all the fish in a pool burrowed into the mud when the water dried up, and were eventually baked hard in their hundreds. Unless the geologists of future ages happen to hit on such a patch of trapped life, they are not likely to find more than isolated bones of these kinds of fish.

Almost the only complete skeletons of the extraordinary giant reptile, Iguanodon, are those of a whole troop, twenty-nine of them, old and young, which were found in a