PALEONTOLOGY.

CHAPTER I.

INTRODUCTION.

DEFINITION OF PALEONTOLOGY.

PALEONTOLOGY (Gr. palaios, ancient; onta, beings; logos, discourse) is the science which treats of the living beings, whether animal or vegetable, which have inhabited this globe at past periods in its history. It is the ancient life-history of the earth, and if its record could ever be completed, it would furnish us with an account of the structure, habits, and distribution of all the animals and plants which have at any time flourished upon the land-surfaces of the globe or inhabited its waters. From causes, however, which will be subsequently discussed, the palæontological record is most imperfect, and our knowledge is interrupted by gaps which not only bear a large proportion to our solid information, but which in many cases are of such a nature that we can never hope to have them filled.

As Zoology, then, treats of the animals now inhabiting the earth, and as Botany treats of the now existing plants, Paleontology may be considered as the Zoology and Botany of the past. Regarding it from this, the only true point of view, some knowledge of Zoology and Botany is essential to a prosecution of the study of Palæontology, and such details of these sciences as may be deemed requisite will be introduced in the proper place. The materials, again, which fall to be studied by the paleontologist, are derived entirely from the proper province of the geologist. Fossils are derived from rocks. It will therefore be necessary to trespass to some ex

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tent upon the peculiar domain of the geologist, and to obtain some knowledge of the origin, composition, and mode of occurrence of the rocks from which Paleontology derives its materials. Lastly, Paleontology, apart from its own importance as an independent science, is employed by the geologist to assist him in his determination of the chronological succession of the materials which compose the crust of the earth. Palæontology, therefore, must be separately studied in its relation to historical Geology.

DEFINITION OF FOSSILS.

All the natural objects which come to be studied by the palæontologist are termed "fossils" (Lat. fossus, dug up). In most cases, fossils, or, as they are often termed, "petrifactions," are actual portions of animal or vegetable organisms, such as the shells of Molluscs, the skeletons of Corals, the bones of Vertebrate animals, the wood, bark, or leaves of plants, &c.; and these may be preserved very much in their original condition, or may have been very much altered by changes subsequent to their burial. Strictly speaking, however, by the term "fossil" is understood "any body, or the traces of the existence of any body, whether animal or vegetable, which has been buried in the earth by natural causes" (Lyell). We shall find, therefore, that we must include under the head of fossils objects which at no time themselves formed parts of any animal or vegetable, but which, nevertheless, point to the former existence of such organisms, and enable us to reason as to their nature. Under this head come such fossils as the moulds or "casts" of shells and the footprints left by various animals upon sand or mud.

In the great majority of cases fossils are the remains of animals or plants which are now extinct—that is to say, which no longer are in existence, but have entirely disappeared from the earth's surface. In some cases, however, fossils are the remains of recent animals-that is, of animals which are still found in a living condition upon the globe. The term "subfossil," sometimes applied to these, has been more appropriately applied in another sense, and is best discarded in this connection. The terms" fauna" and "flora" are employed in Paleontology much as they are by the naturalist, to mean the entire assemblage of the animals or of the plants respectively belonging to a particular region or a particular time. Thus we may speak of the "fauna" of the Carboniferous Period, or the "flora" of the Tertiary Epoch, or the fauna of the Chalk, or of any other set of beds.

FOSSILISATION.

Fossilisation may be applied in a general sense to all the processes through which an organic body passes in order to become a fossil. Here we need only consider the three leading forms in which fossils present themselves. In the first instance, the fossil is to all intents and purposes an actual organic remain, being itself a fragment of an animal or plant. Thus we may meet with fossil bones, shells, or wood, which may have undergone certain changes, such as would be produced by pressure, by the deprivation of organic matter originally present, or by more or less complete infiltration with mineral matter, but which, nevertheless, are practically the real bodies they represent. As a matter of course, it is in the more modern formations that we find fossils least changed from their primitive condition, but all formations almost contain some fossils in which the original structure is more or less completely retained.

In the second place, we very frequently meet with fossils in the state of " casts or moulds of the original organic body. What occurs in this case will be readily understood if we imagine any common bivalve shell, as an Oyster, or Mussel, or Cockle, embedded in clay or mud. If the clay were sufficiently soft and fluid, the first thing would be that it would gain access to the interior of the shell and would completely fill up the space between the valves. The pressure, also, of the surrounding matter would insure that the clay would everywhere adhere closely to the exterior of the shell. If now we suppose the clay to be in any way hardened so as to be converted into stone, and if we were to break

up the stone, we should obviously have the following state of parts. The clay which filled the shell would form an accurate cast of the interior of the shell, and the clay outside would give us an exact impression or cast of the exterior of the shell (fig. 1). We should have, then, two casts, an interior and an exterior, and the two would be very different to one another, since the inside of a shell is very unlike the outside. In the case, in fact, of many univalve shells, the interior cast is so unlike the exterior or unlike the shell itself, that it may be difficult to determine the true origin of the former.

It only remains to add that there is sometimes a further complication. If the rock be very porous and permeable by water, it may happen that the original shell is entirely dissolved away, leaving the interior cast loose, like the kernel of a nut, within the case formed by the exterior cast. Or it may happen that subsequent to the attainment of this state of things, the space thus left vacant between the interior and exterior castthe space, that is, formerly occupied by the shell itself—may be filled up by some foreign mineral deposited there by the infiltration of water. In this last case the splitting open of the rock would reveal an interior cast, an exterior cast, and finally a body which would have the exact form of the original shell, but which would be really a much later formation, and which would not exhibit under the microscope the minute structure of shell.

In the third class of cases we have fossils which present with the greatest accuracy the external form, and even sometimes the internal minute structure, of the original organic body, but which, nevertheless, are not themselves truly organic, but have been formed by a "replacement" of the particles of the primitive organism by some mineral substance. The most elegant example of this is afforded by fossil wood which has been "silicified" or converted into flint. In this case we have a piece of fossil wood, which presents the rings of growth and fibrous structure of wood, and which under the microscope exhibits even the minutest vessels which characterise ligneous tissue. The whole, however, instead of being composed of the original carbonaceous matter of the wood, is now converted into pure flint. The only explanation which can be given of this by no means very rare phenomenon, is that the wood must have undergone a slow process of decay in water holding silica or flint in solution. As each particle of the wood was removed by decay, its place was taken by a particle of flint deposited from the surrounding water, till ultimately the entire wood was silicified. The replacing substance is by no means necessarily flint, but may be iron-pyrites, oxide of iron, sulphur, &c. ; and it is not uncommon to find many other fossils besides wood preserved in this way, such as shells, corals, or sponges.

DEFINITION OF Rock.

The crust of the earth consists of various different materials, produced at different successive periods, occupying certain definite spaces, and not confusedly mixed together, but, on the contrary, exhibiting a definite and discoverable order of arrange

ment. All these materials, however different in appearance, texture, or mineral composition, are called "rocks" by the geologist. The term "rock," then, is to be understood as applying to all the materials which compose the crust of the earth. In the language of geology, the finest mud, the loosest sand, and the most incoherent gravel, are just as much rocks as are the hardest and most compact granites or limestones.

CLASSIFICATION OF ROCKS.

For the purposes of the palaeontologist all the rocks which enter into the composition of the solid exterior of the earth may be divided into two great classes:-1. The Igneous Rocks, which are formed within the body of the earth itself, and which owe their structure and origin to the action of heat; and 2, the Aqueous or Sedimentary Rocks, which are formed at the surface of the earth, and which owe their structure and origin to the mechanical action of water. The Igneous Rocks are formed below the surface of the earth, are as a general rule destitute of organic remains or fossils, and are mostly in the form of unstratified masses. The Aqueous and Sedimentary Rocks are formed at the surface by the disintegration and reconstruction of previously existing rocks, are mostly fossiliferous, and are stratified-i.e., are arranged in distinct layers or "strata." The Sedimentary Rocks, as containing fossils, are the only rocks which it is essential for the paleontologist to be acquainted with, and we shall very briefly consider their leading physical characters, their chief varieties, their mode of origin, and their historical succession.

CHAPTER II.

SEDIMENTARY ROCKS.

THE Sedimentary or Fossiliferous Rocks form the greater portion of that part of the earth's crust which is open to our examination, and are distinguished by the fact that they are regularly "stratified," or arranged in distinct and definite layers or "strata." These layers may consist of a single material, as in a block of sandstone, or they may consist of different materials. When examined on a large scale, they are always found to consist of alternations of layers of different mineral