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

THE EVIDENCE

and his calculations still adorn the margin of the Authorized Version of the Bible. Though they are wrong, they are at least wrong on a grand scale, since the age of the earth is somewhere about half a million times greater than he supposed. Even to-day the average man tends to think the six-thousand-year antiquity of Babylon or Egypt enormous. But just as astronomy is teaching us to think of cosmic space on a wholly different scale from geography, to be measured in terms of “ light-years ”? running into ten thousands of millions of miles, so geology is making it necessary to think of earth-history on a wholly different time-scale from human history, in terms of million-year periods, to which a decade bears almost the same proportion as an hour does to a century, and a century as a day does to a whole generation of human life.

To think in such magnitudes is not so dificult as many people imagine. The use of different scales is simply a matter of practice. We very soon get used to maps, though they are constructed on scales down to a hundred-millionth of natural size ; we are used to switching over from thinking in terms of seconds and minutes to some other problem involving years and centuries ; and to grasp geological time all that is needed is to stick tight to some magnitude which shall be the unit on the new and magnified scalea million years is probably the most convenient—to grasp its meaning once and for all by an effort of imagination, and then to think of all passage of geological time in terms of this unit.

§ 2

Defects and Happy Finds in the Record

The principles on which the geologist relies in his attempt to decipher the past history of the earth and the life upon it are clear and simple—so simple and so clear that at first sight it would seem that the task of the paleontologist, apart from the physical labour of finding and digging out fossils and the mental labour of studying them, should be easy. But Nature rarely reveals her secrets cheaply, and we have not yet told of the complications of the task.

In the first place, even when a large thickness of rock shows every evidence of having been laid down steadily and continuously, year after year, it may well change its character. For instance, clay, being composed of fine particles, will only be deposited farther out to sea or in quieter water than

OF THE ROCKS

the more coarse-grained sandstone ; but a layer deposited off a coast which happened to be slowly rising (as for instance Spitsbergen is rising to-day) may easily begin as mud and end as sand, the one deposit gradually hardening into clay, the other into sandstone. And if it change its character, the character of the animals and plants which live on and in it will change, too. Muddwellers will give place to sand-dwellers. If the change is too rapid for adaptation the fossils will not show a gradual evolution, but there will be an invasion of new creatures from other parts of the sea-bed as the conditions alter. The old forms are extinguished and drift off elsewhere, and the paleontologist is left with his story broken.

Still more frequently there is a break in deposition. The layer perhaps comes within the range of scouring currents, which prevent deposition ; or it is shoved out of water by some upward movement of the crust of earth and has its newly deposited sheets removed. Later it sinks into favourable conditions and deposition begins again. But now, whether the new material be the same or quite different from that laid down before, there is a gap, during which life has bequeathed no record of itself. In general no widespread deposition will occur off-shore except when the coast is sinking, and very few animals will be preserved except when deposition is rapid.

Difficulties in some ways more serious confront us in studying the sequence of the rocks in many mountainous regions. Anyone who uses his eyes and opportunities on a railway journey through hilly or mountainous country will see that it is rare for the layers of rock exposed in the cuttings to be horizontal; they are usually tilted, and sometimes tilted at sharp angles. This tilting is due to movements of the earth’s crust, such as its shrinkage and consequent bending and folding during periods of cooling. Over all of midland and south-eastern England the tilting is usually slight but definite. This was very favourable to William Smith, the British pioneer of geology, for, although it left no doubt as to which layer was above which, it caused new layer after new layer to come to the surface, open to investigation, as he passed across country upon his work of canal-making. His genius worked under the luckiest conditions.

But in mountain regions the disarrangement may be much more serious. In the Alps it is common enough to see layers of rock standing on edge, or even turned quite upside down; and in some places, as mu parts of the Scottish Highlands, the pressure

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