In the Middle Ages, distinguished writers held curious views regarding fossils. Thus Avicenna (980-1037) believed that fossils represented unsuccessful attempts on the part of nature to change inorganic materials into organisms within the earth by a peculiar creative force (_vis plastica_). About two centuries later, Albertus Magnus held a somewhat similar view. Leonardo da Vinci (1452-1519), the famous artist, architect, and engineer, while engaged in canal building in northern Italy, saw fossils imbedded in the rocks, and concluded that these were the remains of organisms which actually lived in sea water which spread over the region. During the seventeenth and eighteenth centuries, many correctly held that fossils were really of organic origin, but it was commonly taught that all fossils represented remains of organisms of an earlier creation which were buried in the rocks during the great Deluge (Noah's Flood). William Smith (1769-1839), of England, was, however, the first to recognize the fundamental significance of fossils for determining the relative ages of sedimentary rocks. This discovery laid the foundation for the determination of earth chronology which is of great importance in the study of the history of the earth. (See discussions below.) Organic remains, dating as far back as tens of millions of years, have been preserved in the rocks of the earth in various ways. A very common kind of fossilization is the preservation of only the hard parts of organisms. Thus the soft parts have disappeared by decomposition, while the hard parts, such as bones, shells, etc., remain. In many cases practically complete skeletons of large and small animals which lived millions of years ago have been found intact in the rocks. Fossils which show none of the original material, but only the shape or form, are also very abundant. When sediment hardens around an imbedded organism, and the organism then decomposes or dissolves away, a cavity or fossil mold only is left. Casts of organisms or parts of them are formed by filling shells or molds with sediment or with mineral matter carried in solution by underground water. Only rarely have casts of wholly soft animals been found in ancient rocks. In other cases both original form and structure are preserved, but none of the original material. This is known as petrifaction which takes place when a plant or hard part of an animal has been replaced, particle by particle, by mineral matter from solution in underground water. Not uncommonly organic matter, such as wood, or inorganic matter, such as carbonate of lime shells, has been so perfectly replaced that the original structures are preserved almost as in life. The popular idea that petrified wood is wood which has been changed into stone is, of course, incorrect. It is doubtful if flesh has ever been truly petrified. In many cases mainly the carbon only of organisms has been preserved. This is also true of plants where, under conditions of slow chemical