Araucarioxylon Daintreei, Chapm. = Dadoxylon australe, Arber; × 28. Carbopermian: Newcastle, New South Wales. (_Nat. Mus. Coll._) ] =Structure Preserved.--= Petrifaction does not necessarily destroy the structure of a fossil. For example, a piece of wood, which originally consisted of carbon, hydrogen, and nitrogen, may be entirely replaced by flint or silica: and yet the original structure of the wood may be so perfectly preserved that when a thin slice of the petrifaction is examined under a high power of the microscope, the tissues with their component cells are seen and easily recognised (Fig. 3). =Early Observers.--= Remains of animals buried in the rocks were known from the earliest times, and frequent references to these were made by the ancient Greek and Roman philosophers. Xenophanes.-- Xenophanes, who lived B.C. 535, wrote of shells, fishes and seals which had become dried in mud, and were found inland and on the tops of the highest mountains. The presence of these buried shells and bones was ascribed by the ancients to a plastic force latent in the earth itself, while in some cases they were regarded as freaks of nature. Leonardo da Vinci.-- In the sixteenth and seventeenth centuries Italian observers came to the fore in clearly demonstrating the true nature of fossils. This was no doubt due in part to the fact that the Italian coast affords a rich field of observation in this particular branch of science. The celebrated painter Leonardo da Vinci (early part of the sixteenth century), who carried out some engineering works in connection with canals in the north of Italy, showed that the mud brought down by rivers had penetrated into the interior of shells at a time when they were still at the bottom of the sea near the coast. Steno.-- In 1669, Steno, a Danish physician residing in Italy, wrote a work on organic petrifactions which are found enclosed in solid rocks, and showed by his dissection of a shark which had been recently captured and by a comparison of its teeth with those found fossil in the cliffs, that they were identical. The same author also pointed out the resemblance between the shells discovered in the Italian strata and those living on the adjacent shores. It was not until the close of the eighteenth century, however, that the study of fossil remains received a decided impetus. It is curious to note that many of these later authors maintained the occurrence of a universal flood to account for the presence of fossil shells and bones on the dry land. [Illustration: =Fig. 4.--William Smith (1769-1839.)= "The Father of English Geology," at the age of 69. (_From Brit. Mus. Cat._) ] =Fossils an Index to Age.--=