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CHAPTER IV.

Organic Remains of the Eastern Part of Yorkshire. That vegetable and animal remains should be enclosed in hard rocks, in prodigious abundance and of exquisite beauty, has been a subject of admiration from very early periods. The difficulty of conceiving how the rocks could be so softened and dissolved by the deluge, (to which all geological phenomena were attributed in the 17th century,) as to admit shells and plants into their substance, induced Plot and Llwyd, and even Ray and Lister, to deny that these fossil bodies had ever been living beings. This absurdity gradually yielded to the talent and industry of Woodward; and is remembered only to be ridiculed. It is now universally admitted by naturalists that fossils are the reliquiæ of beings once endowed with life ; and that all the difference in appearance between them and analogous recent objects, has been caused by circumstances attendant on their long sepulture in the earth.

The earth contains reliques of perhaps the most ancient plants and animals which existed on this globe, and they lie enclosed in rocks of different chemical composition, at various depths and of unequal antiquity. According to their original qualities, and the circumstances in which they were placed, fossils have undergone different changes of substance.

Few organic bodies are preserved in the earth, except such as were originally of a durable constitution. Remains of plants are common in coal districts, wood is found in many limestone rocks, nuts and hard fruits have been obtained from the Isle of Sheppey : zoophytes of many kinds fill our limestone and sandstone rocks; thus the horny substance of spongiæ, and the calcareous mass of corals is accurately preserved : the columns of crinoidal animals, and the hard crusts of echini are very plentiful : shells are innumerable: the crusts of lobsters and crabs, and the scales of fishes are scarce, but teeth and vertebræ of the latter are more abundant : aquatic reptiles have left us their bones. Now all these were originally durable; they are all capable of conservation in our cabinets; but the softer animal substances once connected to them, have entirely disappeared. Even the ligament which is placed at the hinge of bivalves to open the shell, is most rarely preserved in a fossil condition. Considerations of this nature render it extremely probable, that the process of mineralization, or (as it is commonly called) petrifaction, was slow and gradual.

Another general remark must be made to present misconception. Fossils are at some places found perfect, at other places in fragments. Now they must have been enveloped in these conditions respectively. From carefully observing these appearances, we may form pretty clear notions as to the tranquillity or agitation of the fluid in which they were deposited. In general, substances originally bound together by perishable ligaments are found in detached pieces, owing to the decay of those parts previous to their being enclosed in the rocks. Thus the shells of crabs and lobsters are commonly disintegrated, bivalves are often separated, and vertebræ and teeth of fishes scattered far asunder in the rock. Such instances as these occur daily in our streams and on the seacoast, and, therefore, in former periods may have happened without any particular agitation of the waters.

Some beds of shells, as the forest marble of the neighbourhood of Bath, appear to have been accumulated with violence and confusion : but generally the sharpness of their angles and perfection of their ornaments lead to the conclusion that they were quietly entombed near the spots where they lived.* The vegetable fossils are, however, a remarkable exception to this, and, being almost always in fragments, seem to

Consult the Preface of Mr. Smith's works, Strata identified by Organized Fossils, and Stratigraphical System of Organized Fossils; and Cuvier's Theory of the Earth, for illustrations on this point.

point out a general turbulence in the waters at the period of their deposition. I must not go further into the cause of this exception, than to state, that if, as is believed, nearly all these vegetables grew on land, and were thence transported to the sea, they would naturally be broken to pieces by that operation.

The chemical changes which fossil plants have undergone are various, and seem partly to depend on peculiarities in their original structure, and partly on the nature of the strata which enclose them. Thus the fibrous wood of dicotyledonous plants found in the limestone of Malton, appears as a brown carbonaceous mass, much traversed by calcareous spar : that which lies in the calcareous gritstone beneath, is sometimes impregnated with siliceous matter; but in the aluminous shale of Whitby, such wood is partly converted to jet, and partly filled with pyrites, or calcareous spar. The ferns and other monocotyledonous plants which lie in the sandstones and shales of our coal districts, are very differently preserved. Whatever be the kind of plant which is found in shale or fine-grained gritstone, all that remains of its substance is coal, often of the purest and most inflammable quality. In this case we may suppose the decomposition of the vegetable matter to have been slow and gradual; and being operated under a close covering of shale or gritstone, the resulting chemical substances were prevented from escaping, and made to combine into a new inorganic compound, coal. The same plants lying in coarse sandstone retain very little of their original substance, perhaps on account of the porosity of the rock, which might both favour the decomposition of the plant, and hasten the escape of the resulting gases, and soluble matter.

After investigating the changes which have happened to fossil plants, no reasonable doubt can be entertained as to the vegetable origin of all our beds of coal. Perhaps the different qualities of coal may be in a great degree owing to the nature of their constituent plants.

The hard parts of invertebral animals, which are preserved in the earth, are closely allied to each other in chemical composition. In all of them, glutinous or gelatinous matter forms the base, and is more or less hardened by admixture with carbonate of lime. Soft corallines, echini, and the coverings of crustaceous animals, contain likewise some phosphate of lime, but generally in small quantity. Four principal states of preservation may be distinctly observed among these fossils.

First, when the coralline or shell retains not only its external figure and appearance, but even its internal texture and almost all its original substance. Such specimens look as if obtained from the sea in a dead state, with no other loss than that of colour and brilliancy. This perfect state of preservation is well exemplified in the beautiful fossils which lie in the comparatively recent strata near London and Paris. The fossil shells of Speeton on the Yorkshire coast are very little altered except by the loss of their gelatinous matter, which causes them to be of a chalky or friable consistence.

In the second condition of fossil shells and corallines, the figure and general appearance is little or not at all altered, but the composition is completely changed by the insinuation of extraneous matter : thus the calcareous substance of shells and corals, and the horny fibres of sponge, are become flint. In such cases the new substance appears to have been introduced gradually, so as to fill the pores of the perishing original body. The same explanation probably applies to the petrifaction of wood.

The third condition is exemplified by those stony masses frequently found in limestone quarries, which have the general figure of shells, but not their structure nor texture. These are casts or moulds in the cavities of shells, which have been dissolved and carried away from the places they once occupied in the rock. In consequence, the cavity left retains the exact impression of the outside of the shell, and encloses a stony mass which was moulded within it. The same explanation applies to the flint moulds in the cavities of echini, and to the screw-stones which are casts in the central hollow of crinoidal columns.

The fourth condition of fossil shells, &c. is produced by a process in addition to that just described. The cavity left by the removal of the shell is, in this instance, filled again by crystals of carbonate of lime, introduced by water filtrating through the stone. When this process is but partially executed, the cavity is imperfectly lined with crystals, but when it is completed, the new substance takes exactly the form of the original shell, but displays no trace whatever of its internal structure. In the Yorkshire oolites, the thick shells of trigonia and gervillia exhibit this metamorphosis in a very striking manner.

That the peculiarities in the fossil shells and analogous reliquiæ, depend much on the original nature of the bodies, is evident from the following well-known facts; the shells and spines of echini, and the columns of crinoidea are almost invariably converted to a peculiar kind of calcareous spar, in whatever strata they may be found ; so the belemnite is always known by its radiated structure, and the gryphite has retained its original laminæ. But the nature of the imbedding substance is also of great consequence in the inquiry. Shells which lie in the green sand are generally converted to flint; those which lie in oolite are often changed to calcareous spar; but those which lie buried in clay seldom exhibit either of these characters.

The interesting subject of the conservation of fossils might be extended to great length; but as these explanations were introduced chiefly to facilitate the understanding of terms which will afterwards be employed, there is room only for another observation on the bones and teeth of vertebral animals, which are principally composed of phosphate of lime, united by a cartilaginous substance. Remains of this kind being much fewer than those of the preceding tribes, do not afford so great a variety of mineral appearances. On the contrary, their state of preservation is remarkably uniform, under whatever circumstances they are found. Fish teeth, for example, are always recognised by a peculiar polish and hardness, and are commonly of a black colour (except in chalk.) They retain the whole of their phosphate of lime, but part of the animal substance is generally replaced by an additional quantity of carbonate of

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