The science of GEOLOGY, of which PALAEONTOLOGY or the study of fossils, forms a part, is concerned with the nature and structure of the earth, the physical forces that have shaped it, and the organic agencies that have helped to build it. =Nature of Fossils.--= The remains of animals and plants that formerly existed in the different periods of the history of the earth are spoken of as fossils. They are found, more or less plentifully, in such common rocks as clays, shales, sandstones, and limestones, all of which are comprised in the great series of Sedimentary Rocks (Fig. 1). According to the surroundings of the organisms, whether they existed on land, in rivers, lakes, estuaries, or the sea, they are spoken of as belonging to terrestrial, fluviatile, lacustrine, estuarine, or marine deposits. [Illustration: =Fig. 1.--Fossil Shells Embedded in Sandy Clay.= About 3/4 nat. size. Of Cainozoic or Tertiary Age (Kalimnan Series). Grange Burn, near Hamilton, Victoria. (_F.C. Coll._) (G = Glycimeris. L = Limopsis. N = Natica).] [Illustration: =Fig. 2.--Tracks probably of Crustaceans (Phyllocarids).= About 3/4 nat. size. Impression of a Slab of Upper Ordovician Shale. Diggers' Rest, Victoria. (_F.C. Coll._) ] The name fossil, from the Latin 'fodere' to dig,--'fossilis,' dug out,--is applied to the remains of any animals or plants which have been buried either in sediments laid down in water, in materials gathered together by the wind on land as sand-dunes, in beds of volcanic ash, or in cave earths. But not only remains of organisms are thus called fossils, for the name is also applied to structures only indirectly connected with once living objects, such as rain-prints, ripple-marks, sun-cracks, and tracks or impressions of worms and insects (Fig. 2). =Preservation of Fossils.--= In ordinary terms, fossils are the durable parts of animals and plants which have resisted complete decay by being covered over with the deposits above-named. It is due, then, to the fact that they have been kept from the action of the air, with its destructive bacteria, that we are able to still find these relics of life in the past. =Petrifaction of Fossils.--= When organisms are covered by a tenacious mud, they sometimes undergo no further change. Very often, however, moisture containing mineral matter such as carbonate of lime or silica, percolates through the stratum which contains the fossils, and then they not only have their pores filled with the mineral, but their actual substance may also undergo a molecular change, whereby the original composition of the shell or the hard part is entirely altered. This tends almost invariably to harden the fossils still further, which change of condition is called petrifaction, or the making into stone. [Illustration: =Fig. 3. Thin Slice of Petrified or Silicified Wood in Tangential Section.=