When we were speaking of the coal-mines I told you something about the remains of giant ferns, sedges, reeds, and mare's-tails of far larger growth than any now known, which have been found there. You are familiar with fossil-plants, but I do not think we have spoken much of fossil-animals, which are found in all except the oldest layers of rock—the first pages of the "Stone Book."
The children had been with me to the Museum in the town in which we lived, and had looked with wonder at the huge creatures whose skeletons have been built up bone by bone, after being taken from their rocky tomb—for this earth of ours which has seen so many changes has been rifled of her treasures; not the gold and silver, coal and iron with which she is so richly stored, but the wonderful specimens of God's work in bygone ages which He has allowed us to see; so that we cannot doubt that such creatures once existed, though we may know nothing with certainty as to the time of their first appearance in the sea and on the dry land, and can only guess at the kind of life they lived.
You remember that we spoke, in the chapter about the earth's crust, of the "fire-made rocks," which were once in a liquid state from intense heat (we could not expect to find any remains of plants or animals there, and none have been found), and of the "water-made rocks," which have been gradually accumulated by the action of water in wearing down the land. These rocks lie in layers, and fossil shells, plants, and bones of animals have been found in them, as we have already seen.
But how did these fossils get into the rocks? And how is it that they have been found in all countries and at all heights above the sea?
Before I try to answer these questions, I must tell you that when geologists speak of "rock" they mean everything which has gone to form the crust of the earth, whether clay, or loose sand and gravel, or the hard heavy granite which some of us had seen crowning the Dartmoor tors.
It is thought that the huge creatures whose bones have been found at different depths in the earth's strata were buried there when the "rock" which formed the layers was soft; perhaps in the mud of lakes, or in peat or sand at the mouths of rivers. Then, as time went on, their softer parts perished, but the harder turned to stone, thus forming the "letters" in the stony pages from which those who study the earth try to read something of its history. Then, as sea-shells are found inland, deeply buried in the hills, it is thought that the land in which they were buried has been raised by earthquakes, or thrown out by volcanoes: or was altered in position at the time when the earth's foundations were overflowed with a Flood, and "the waters stood above the mountains." As geologists read the Stone Book, like the writing of Eastern lands, backwards—as they search deeper and deeper into the crust of the earth, they speak of its Old life, Middle life, and New life: but we must remember that they do read backwards, calling the older life what is really the younger. And we must also bear in mind that many of the words used in what is called science—especially those relating to the study of the earth—betray our ignorance rather than prove our knowledge. The marking off stages in the life-history of the earth, and speaking of its Old, Middle, and New Age has been done to help in the study of its crust. Nothing is known, however, with certainty about these different periods or where one ends and another begins, and no one knows whether the first, or oldest, layer has yet been discovered. One geologist says, "I have found it," and presently another penetrates a little deeper, goes a little farther back, and finds one lower still. Nor can anyone say certainly where a fossil-fern or the mummy of an old-world fish appeared for the first time, and though many plants and animals which are found in a fossil state have long been extinct, yet there are many more which appear at a very ancient date and have continued unchanged to the present time.
There is a famous cliff in Dorsetshire upon which may be read, almost as upon a map, the record of the changes which have passed over it during its life-history.
On examining the strata, or layers which lie one above the other, geologists find the first, or lowest of all, to be Portland stone, which was formed by the accumulation of lime at the bottom of the sea.
The second layer shows that this sea-bed in time became dry land, and was covered with soil—what had once been the seashore gradually giving place to a forest.
But how do we know that such a wonderful change was wrought in process of time?