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? We have clear proof that it was so from the vegetable soil still remaining, and the numbers of trees the remains of which are embedded in the rock, many of them standing upright as when growing.