taneously in another. Even the crystalline rocks called schists and gneisses (Chap. X) may belong to very

ages.

different

Thus we say that, in the case of any rock, "lithological characters," i.e., the nature and structure of the stone itself, are useless in assigning it to its place in the annals of the earth. We have now, however, another and a remarkable guide, which we can employ in any quarter of the globe.

We are all acquainted with fossils (Plate X). By this term we now mean the remains of any plant or animal that are found included in a rock.1 We must use our discretion as to how far we should apply the word "fossil to modern cases of entombment. A goat might become involved in a recent mud-flow on the flanks of a mountain; the rootlets of living plants, again, penetrate a long way down into the crevices of rocks; but neither of these cases gives rise to fossils in the generally accepted sense. Some fossil shells have preserved even their lustre, while the calcium carbonate of others has actually become changed, molecule by molecule, into some other chemical substance. Silica, iron carbonate, iron sulphide, and gypsum, are thus found actually replacing the original shells or hard parts of many organisms; and silicified tree-stems are well known from many parts of the world. Very often, all that is left us is a cast, formed by the fine particles of the rock itself, which have penetrated the hollows of the shell or skeleton and have taken an accurate mould of them. Clay forms excellent internal casts, and the fine limestone-mud associated with shell-banks has a similar effect. Often the substance of the fossil itself has become dissolved away, after its entombment, and a hollow space has arisen; but the enveloping and consolidated rock has at the same time taken an external cast of the fossil, from which many of its characters can be ascertained. Moreover, an internal cast may have been formed as well, which is generally fixed to the surrounding rock at the points where the material penetrated into the hollow of the shell. In some cases, however, the solution of the shell leaves the internal cast to rattle loosely

1 Up to the earlier years of the present century, a "fossil " meant anything dug up (fodio, fossum) out of the earth, and the objects to which the term is now restricted were called “organised fossils."

inside the external one, and to fall away when we break open the block of stone.

Excellent internal casts can be seen in the rougher beds of Portland limestone, known as the "Roach bed." The objects called the "Portland screw are the internal casts (fig. 18) of a long spiral gastropod shell, a Cerithium; and casts of a bivalve mollusc, Trigonia, are also frequent. In the latter, the depressions where the muscles were attached to the shell are of course represented by raised oval patches, and the teeth of the hinge, which are beautifully ribbed with fine ridges, are recorded in reversed pattern on the cast.

Casts are most common in sandstones, owing to the permeability of the material, and are

frequently the only indications of fossils throughout the rock. In clays, on the other hand, even the most delicate fossils, such as foraminifera decorated with fine spines, are generally well preserved.

Internal casts are often formed of marcasite, so that we have a mould of the fossil apparently modelled out of shining brass. These handsome specimens are unsatisfactory for collections, since so many, in the course of years, FIG. go to pieces through decomposition of the iron sulphide (see p. 17).

INTERNAL

18. CAST OF A CERITHIUM, PORTLAND STONE.

Fossils, we should also note, often serve as a centre round which chemical substances present in the rock collect. Concretions are thus formed, which are commonly harder than the rock itself, and which are seen sticking out on quarry-sections, or on cliffs, along the lines of bedding. At Whitby, for instance, where every visitor learns to be interested in geology, the concretions of calcium carbonate in the shales are pretty certain to reward the fossil-hunter. They can be broken open by the hammer along the beddingplanes that pass through them, and most frequently a perfect specimen of an ammonite is found lying, brown. and lustrous, in the middle.

While dealing with fossils, we may perhaps note how even the tracks and footprints of animals (p. 104) may be preserved by the gentle deposition of a stratum above that

in which the impressions were made. Similarly ripplemarks, water-groovings, sun-cracks, and rain-prints are often exquisitely "fossilised." Usually the footprints can be best studied on the under-surface of the covering bed, on which they stand out as casts. Worm-burrows, again, may be filled up by fresh sand, perhaps of a different colour to that in which the animals bored; and such traces of worms are sometimes the only evidence left to show us that life existed in the area in which the beds were being laid down.

Mr.

Fossils must long ago have been observed by the primitive peoples of the earth, and may have been regarded, like meteorites, with a certain amount of reverence. Worthington Smith1 records a case where the body of a girl, in a tumulus of the bronze-age near Dunstable, was found surrounded by 158 sea-urchins collected from the Chalk, more than half of which were perfect specimens. It is probable, then, that some mystery attached to these strangely shaped objects in the earth.

The ancient Greeks recognised that fossils were the remains of creatures which once lived in the places where they now are found. They saw that, from one cause or another, land and sea had changed places, and that marine shells had often been left high and dry. It seemed, however, difficult to conceive that a sufficient number of changes had gone on to account for the great variety of beds of rock crammed full with animal remains. Aristotle, in the fourth century B.C., thought to help on matters by suggesting that the earth itself contained a mysterious force within it, whereby animals grew in moist rocks, which subsequently consolidated and enclosed them. Such animals, we may presume, were like the toads that are sometimes found in hollow stones, living and breathing, but never seeing the world around them. We now know, however, that such toads have entered the hollows of the rocks in an infant form, and subsequently have grown to larger size. This idea of the development of animals in the earth itself spread very widely, and formed one of the chief errors that early

1 Nature, vol. xliii (1891), p. 320.

2 See Sir C. Lyell, "Principles of Geology," opening chapters; and O. C. Marsh, "History and Methods of Paleontological Discovery," Nature, vol. xx, pp. 494 and 515.