[13]See M. Pictet's "Palæontologie."
[13]See M. Pictet's "Palæontologie."
Tertiary Epochs.
An extensive series, comprising many isolated groups of marine and lacustrine deposits, containing fossil remains of animals and vegetables of all classes; the greater number of genera and species in the most ancient or lowermost beds belong to extinct types.
Subdivisions:—
1. ThePliocene[14](more new, orrecent.[15]Wond.p. 221); strata in which the shells are for the most part of recent species, having only about ten per cent of extinct forms. (Norwich Crag.)
2. TheMiocene(less recent,[16]Wond.p. 221); containing about 20 per cent of recent species of shells. (Suffolk Crag.)
3. TheEocene(dawn of recent,[17]in allusion to the first appearance of recent species—Wond.p. 226); the most ancient tertiary strata contain but very few existing species of shells; not more than five per cent. (London, Hants, and Paris basins.)
[14]In the present state of our knowledge, this arrangement is of great utility, but it will probably require considerable modification, and must, perhaps, hereafter be abandoned; for it cannot be doubted, that strata in which no recent species have yet been found, may yield them to more accurate and extended observations, and those in which only a few recent species are associated with a large number of extinct forms, may have these proportions reversed.[15]From πλειων,pleion, more; and καινος,kainos, recent.[16]From μεἱων,meion, less, and recent.[17]From ἠως,eos, the dawn or commencement, andrecent.
[14]In the present state of our knowledge, this arrangement is of great utility, but it will probably require considerable modification, and must, perhaps, hereafter be abandoned; for it cannot be doubted, that strata in which no recent species have yet been found, may yield them to more accurate and extended observations, and those in which only a few recent species are associated with a large number of extinct forms, may have these proportions reversed.
[15]From πλειων,pleion, more; and καινος,kainos, recent.
[16]From μεἱων,meion, less, and recent.
[17]From ἠως,eos, the dawn or commencement, andrecent.
Obs.—The marine are often associated with fresh-water deposits, and the general characters of the Tertiary system are alternations of marine and lacustrine strata. In England the most important Tertiary deposits are those of the London basin, the Isle of Sheppey, the south-western coasts of Sussex and Hampshire, the north of the Isle of Wight, and the eastern coasts of Essex, Norfolk, Suffolk. (Wond.p. 226.)
Secondary Epochs.
The CretaceousorChalk Formation. (Wond.p. 296). A marine formation, comprising a vast series of beds of limestone, sandstone, marl, and clay, &c.; characterized by remains of extinct zoophytes, mollusks, cephalopods, echinoderms, crustaceans, fishes, &c.; lacertians, crocodilians, chelonians, and other extinct reptiles; drifted coniferous and dicotyledonous wood and foliage, fuci, &c.
Subdivisions:—
[18]Another subdivision, withothernames (chiefly derived from French localities), has lately been proposed by M. D'Orbigny; which I notice with the more regret, since this eminent naturalist formerly repudiated the censurable practice of many modern systematists, of changing established names of strata and fossils, without any just cause. The British geologist will smile to see the Wealden Formation—so eminently distinguished in England and Germany by its extent, thickness, and remarkable fauna and flora,—ranked as a subordinate member of the "Formation néocomien," of France.
[18]Another subdivision, withothernames (chiefly derived from French localities), has lately been proposed by M. D'Orbigny; which I notice with the more regret, since this eminent naturalist formerly repudiated the censurable practice of many modern systematists, of changing established names of strata and fossils, without any just cause. The British geologist will smile to see the Wealden Formation—so eminently distinguished in England and Germany by its extent, thickness, and remarkable fauna and flora,—ranked as a subordinate member of the "Formation néocomien," of France.
Obs.—The Maestricht beds are chiefly composed of fawn-coloured limestones of friable texture; containing peculiar species of corals, shells, fishes, reptiles, &c. TheChalkis generally white, but in some localities is of a deep red, in others of a yellow colour; nodules, layers, and veins of flint occur in the upper, but are seldom present in the lower chalk. TheMarlis an argillaceous limestone, which generally prevails beneath the white chalk; it sometimes contains a large intermixture of green or chlorite sand, and then is calledFirestone, orGlauconite. TheGaltis a stiff, blue or blackish clay, abounding in shells which frequently retain their pearly lustre. TheGreensandis a triple alternation of sands and sandstones with clays; and beds of cherty limestone calledKentish Rag.
TheWealden; a formation, whose fluviatile character was first observed and established by the researches of the author (Wond.p. 360). A series of clays, sands, sandstones and limestones, with layers of lignite, and extensive coal-fields; characterized by the remains of several peculiar terrestrial reptiles, namely,Iguanodon,Hylæosaurus,Pelorosaurus,Megalosaurus;CrocodiliansandChelonians;Enaliosaurians;Pterodactyles, &c.; Fishes of fluviatile and marine genera; Insects of several orders; fresh-water mollusks and crustaceans; conifers, cycads, ferns, &c.
Subdivisions:—
Obs.—Clays and limestones, almost wholly composed of fresh-water snail-shells, and minute crustaceans, generally occupy theuppermost place in the series in Sussex; sands and sandstones, with shales, and lignite, prevail in the middle; while in the lowermost, argillaceous beds, with shelly marbles or limestones, again appear; and, buried beneath the whole, is a petrified pine-forest, with the trees still erect, and the vegetable mould undisturbed! The upper clay beds and marbles form the deep valleys or Wealds of Kent and Sussex, and the middle series constitutes the Forest-Ridge. The Purbeck strata are obscurely seen in some of the deepest valleys of eastern Sussex; they emerge on the Dorsetshire coast, form the Island or Peninsula whose name they bear, and surmount the northern brow of the Isle of Portland. On the southern coast of the Isle of Wight, the Wealden beds emerge from beneath the Greensand strata between Atherfield and Compton Bay on the western limit, and in Sandown Bay on the eastern; and their characteristic fossils are continually being washed up on the sea-shore.
The Jurassic or Oolitic Formation.(Wond.pp. 202, and 491). A marine formation of great extent and thickness, consisting of strata of limestone and clay, which abound in extinct species and genera of marine shells, Corals, Insects, Fishes, and terrestrial and marine Reptiles. Land plants of many peculiar types, and the remains of two genera ofMammalia.
Subdivisions:—
Upper Oolite of Portland, Wilts, Bucks, Berks, &c.
Middle Oolite of Oxford, Bucks, Yorkshire, &c.
Lower Oolite of Gloucestershire, Oxfordshire, and Northamptonshire.
Lower Oolite, of Brora in Scotland.
Lower Oolite of the Yorkshire coast.
Obs.—The difference observable between the lower beds of the Oolite in the midland counties, and those of Yorkshire and Scotland, is a fact of considerable interest. The fluvio-marine accumulations of vegetable matter in the state of coal, with the remains of land-plants at Scarborough and Brora, together with the presence of insects, fresh-water crustaceans, mammalia, and terrestrial plants, in the Stonesfield slate, attest the existence of neighbouring land, and the action of rivers and currents.
The Lias.(Wond.p. 521) A series of clays, shales, and limestones, with marine shells, cephalopoda, crinoidea, and fishes in great abundance; reptiles, (particularly of two extinct genera,Plesiosaurus, andIchthyosaurus,) in immense numbers. Drifted wood and land plants: coniferæ, cycadesæ &c.
Subdivisions:—
Obs.—The Lias is the grand depository of those tribes of marine reptiles, theIchthyosauriandPlesiosauri, whose remarkable forms, structure, and state of preservation, have excited the attention even of the most incurious. The collection of these remains in the British Museum, principally formed by Mr. Hawkins, is unrivalled.[19]
[19]See "Petrifactions," p. 337-367.
[19]See "Petrifactions," p. 337-367.
The Trias; orNew Red Sandstone Formation,[20](Wond.p. 533). This group of rocks consists of variegated marls, sandstones, and conglomerates, frequently of a red colour, with marine shells, crinoideans, fishes, and reptiles; marine and terrestrial plants. This series contains extensive deposits of rock-salt, and brine-springs.
[20]Called by some geologistsPoikilitic(variegated) group.
[20]Called by some geologistsPoikilitic(variegated) group.
This formation comprises theTrias, or triple group,viz.theKeuper,Muschelkalk, andUpper Bunter Sandstein, of the German geologists.
Subdivisions:—
Obs.—To this formation belong the principal deposits in Leicestershire and other midland counties of England. Fossils are not generally abundant, but some localities yield highly interesting remains. The shelly limestone of Germany, called Muschelkalk, which contains theLily Encrinite, &c. does not occur in England. Remains of Conifers allied to the Yew and Araucaria, are found near Coventry; and peculiar reptiles (Labyrinthodons) near Warwick.
Palæozoic Epochs.
ThePermian Formation. (Wond.p. 533). The separation of the strata now termedPermianfrom theTriassic, with which they were formerly classed, was first proposed by Sir Roderick Murchison, and is based on the fact that the fossils hitherto discovered are entirely distinct from any that occur in the Trias and subsequent formations; it is, therefore, inferred, that after the deposition of the so-called Permian strata, a complete change took place in the faunas and floras of the lands and seas, and the Trias isat presentregarded as the dawn of a new system of organic beings.
The strata comprised in this group are variegated blue and red marls and sandstones, like those of the Triassic; magnesian or dolomitic limestones; and conglomerates more or less coloured with peroxide of iron.
Subdivisions:—
Obs.—This group includes theLower Bunter,Zechstein, andRothliegendes[21]of the German geologists. ThePermiancomprises all the deposits that intervene between the Triassic above, and the Carboniferous below; and it is believed that this formation contains but one type of animal and vegetable life.
[21]SignifyingRed-dead-layer; it is a German mining term denoting that the copper of the upper bed has died out; this layer not being metalliferous.
[21]SignifyingRed-dead-layer; it is a German mining term denoting that the copper of the upper bed has died out; this layer not being metalliferous.
The Carboniferous, orCoal Formation. (Wond.pp. 660-748). Sandstones, grits, shales, layers of ironstone, and clay, with immense beds of coal; fresh-water limestones sparingly; marine limestones abundantly.
Subdivisions:—
Obs.—The strata comprised in the carboniferous (coal-hearing) system, consist of sandstones more or less felspathic, and of dark bituminous shales with innumerable ferns, and other vascular cryptogamiæ, and coniferæ, &c. The uppermost group is composed of numerous alternations of coal, clay, shale, ironstone, and sandstone; the middle, of sandstones, shales, clays, and quartzose conglomerates, generally of a dull red colour; and the lowermost, of crystalline limestones with occasional layers of chert, abounding in marine shells, corals, crinoidea, and other exuviæ. These lower limestones are the principal repositories of the lead ores of Derbyshire.
The DevonianorOld Red Formation. (Wond.pp. 204 and 751). Conglomerates, quartzose grits, sandstones, marls, and limestones; the prevailing colour is a dull red. Shells, corals, and ganoid fishes, of a very peculiar type. Reptiles, (Telerpeton;Batrachians?Chelonians?); the most ancient reptilian remains hitherto discovered.Ferns,Lepidodendrons, and other trees of the carboniferous flora; fluviatile plants with batrachian ova(?).
Subdivisions:—
Obs.—The term Devonian, by which the series of strata comprehended in this formation is now generally distinguished by geologists, was first proposed by Sir R. Murchison, as being more precise than the name formerly applied to this group. In Scotland, where the formation is of vast extent, it was first characterized by its peculiar ganoid fishes (Pterichthys, Coccosteus, Cephalaspis), and will probably always there retain the original name of Old Red.[22]In Devonshire it is marked by the presence of shells of a character intermediate between those of the Silurian and Carboniferous systems.
The sandstones are in various states of induration, and when slaty, are employed for roofing. The red colour predominates in the marls, and is derived from peroxide of iron. The formation of these rocks has manifestly resulted from the waste of ancient slate rocks, the detritus of which is cemented together into coarse conglomerates. In South Devonshire (at Torquay, Babbicombe, &c.), beautiful coralline marbles occur in this formation.
[22]See the charming volume of Mr. Hugh Miller, entitled "The Old Red Sandstone, or New Walks in an Old Field."
[22]See the charming volume of Mr. Hugh Miller, entitled "The Old Red Sandstone, or New Walks in an Old Field."
The Silurian System. (Wond.p. 765). Marine limestones, sandstones, shales, and calcareous flagstones, characterized by peculiar types of corals, crinoideans, mollusks, and crustaceans, constitute this important and extensive system of rocks; theGrauwacké, or Transition series of the earlier geologists.
Subdivisions:—
UPPER SILURIAN.
LOWER SILURIAN.
Obs.—TheSilurian System, (so named by Sir R. Murchison, from theSilures, the ancient Britons who inhabited those parts of our island in which the geological relations of these strata were first recognised by him,) occupies the border counties of England and Wales, and spreads over a vast area of both North and South Wales, intervening between the Carboniferous series and the Cambrian or ancient slate-rocks of that country.[23]The strata are entirely of marine origin, and many of the beds (as the well-known Dudley or Wenlock limestone) are composed of shells, corals, crinoideans, and remains of that remarkable family of crustaceans termed Trilobites, cemented together by carbonate of lime. A few remains of Fishes occur: Reptiles are unknown. No vegetable relics, exceptingFuci, have been found in Britain below theDevonianorOld Red formation.[24]
[23]"The Silurian System, founded on Geological Researches in the Border Counties of England and Wales." In two parts, royal 4to., with map, sections, &c., by Sir R. I. Murchison, G.C. St.S. &c. In studying the beautiful map which accompanies the work, it must be borne in mind that ten years have elapsed since Sir R. Murchison abrogated the boundary line that separates the Cambrian and Silurian rocks in this chart, from the conviction that those deposits constitute but one natural system (seeWond.p. 803). For an account of the Silurian rocks of other countries, see "Geology of Russia," by the same Author. A summary of the characters of the Silurian System, by Sir R. Murchison, is given inGeolog. Journal, vol. viii. pp. 173-183.[24]"The Silurian System" contains excellent figures of all the organic remains known at the period of its publication.
[23]"The Silurian System, founded on Geological Researches in the Border Counties of England and Wales." In two parts, royal 4to., with map, sections, &c., by Sir R. I. Murchison, G.C. St.S. &c. In studying the beautiful map which accompanies the work, it must be borne in mind that ten years have elapsed since Sir R. Murchison abrogated the boundary line that separates the Cambrian and Silurian rocks in this chart, from the conviction that those deposits constitute but one natural system (seeWond.p. 803). For an account of the Silurian rocks of other countries, see "Geology of Russia," by the same Author. A summary of the characters of the Silurian System, by Sir R. Murchison, is given inGeolog. Journal, vol. viii. pp. 173-183.
[24]"The Silurian System" contains excellent figures of all the organic remains known at the period of its publication.
TheCambrian Formation. This term is applied to a largely developed series of unfossiliferous slate-rocks and conglomerates, many thousand yards in thickness.
Obs.—Certain beds of dark-coloured schists containing a few corals, fuci, and shells, are referred to the uppermost part of this formation by some eminent geologists, but it is more consonant to the established system of classification to regard these fossiliferous beds as the lowermost of the Silurian rocks. The fineness of grain, general aspect, hardness, and texture of these strata, are well known, from the general employment of slate for economical purposes. These rocks extend over a great part of Cumberland, Westmoreland, and Lancashire, reaching to elevations of 3,000 feet, and giving rise to the grand scenery of the Lakes, and of North Wales.
Non-fossiliferous.
Metamorphic(transformed) or stratified crystalline rocks.
Subdivisions.—
Plutonic Rocks; unstratified crystalline masses.
Granitic System.(Wond.p. 844.) Granite—a rock composed of mica, quartz, and felspar; Porphyry; Serpentine; Trap. These rocks occur in amorphous or shapeless masses, and in dykes and veins.
>Obs.—No fossils have been detected in these rocks: but the intense igneous action which the masses appear to have undergone, may have obliterated all evidence of animal and vegetable structures, should any have been present, as well as the lines of stratification. By the aid of the microscope, we may yet perhaps solve the mystery which shrouds the origin of these rocks, and the student may take up the investigation with the certainty of obtaining much valuable information, even should the search for organic structures prove abortive. It is not, however, improbable that the siliceous frustules of diatomaceæ may have escaped destruction, and remain to reward the researches of some skillful and patient observer.
The products of subterranean fire or heat, erupted from profound depths through fissures in the Earth's crust, whether in ancient or modern times.
Subdivisions.—
Obs.—These igneous products are of all ages, and they traverse alike the hypogene rocks and the older and newer sedimentary deposits. Their characters, and the effects they have produced, are considered in the work to which reference is made.
By a reference to the geological map of England (Wond.pl. i. vol. i.), it will be seen, that the several formations appear on the surface in a somewhat chronological order, as we pass from the eastern or south-eastern part of the Island to the west or north-west. Thus the principal Tertiary deposits are situated in the eastern and south-eastern parts; and proceeding towards the north-west, we traverse successively theSecondary—theChalk,Oolite,LiasandTrias; then thePalæozoic—Permian,Carboniferous, andDevonian; next theSilurianandCambrian; and at lengthmetamorphicandprimaryrocks appear. It is this distribution of thestrata of the respective formations that has determined the characters of the physical geography of England. The Alpine or mountainous districts, which extend north and south along the western portion of England and Wales, from Cornwall to Cumberland, are formed by the elevated masses of the Silurian, Cambrian, and Metamorphic rocks. These are succeeded by a band of the Carboniferous and Triassic deposits, with a few intrusions of metamorphic and plutonic rocks, that stretches from the coast of Devonshire, through the midland counties, by Leicestershire and Derbyshire, to Newcastle. On the south-east of this tract, the Oolite and Cretaceous formations, chiefly made up of argillaceous and calcareous strata, constitute a diversified agricultural district, extending from the southern shores of Hants and Dorset to the coast of Yorkshire. The Wealden occupies the country lying between the Chalk Downs of Sussex, Hants, Surrey, and Kent. The Tertiary deposits lie in basins or depressions of the upper cretaceous rocks in the south-eastern and eastern maritime districts, and on an extensive area of these beds stands the metropolis of England; lastly, irregular accumulations of Drift, containing mammalian remains in some localities, are spread over the surface of the ancient formations, and form the immediate subsoil of the most fertile regions.
ON THE NATURE OF FOSSILS, OR ORGANIC REMAINS.
Fossils; Petrifactions.—It is very generally the case, that persons who are not conversant with the nature of organic remains, suppose that all fossils are petrifactions; and unless a specimen has the aspect and hardness of stone, they regard it as of modern origin, and devoid of interest. Hence they are surprised to find among the choicest treasures in the cabinet of the geologist, shells and corals as perfect in form, as if recently collected from the sea-shore; bones as little changed, as if they had been interred but for a short period; and teeth possessing their sharp edges and enamel unimpaired. In my early researches I fell into this error, and threw away many beautiful shells that were associated with casts of ammonites in the marl at Hamsey, supposing, from their perfect state, that they had been accidentally imbedded, and were not genuine fossils. But the state of preservation, and the degree of change which an organic body has undergone in the mineral kingdom, have no necessary relation to its antiquity. The shells in some of the ancient secondary strata are frequently as little changed as those in modern tertiary deposits. I have collected from the lowermost clays of the Wealden, fresh-water shells with traces of the epidermis, and the ligament by which the valves were held together, perfect; and bones of reptiles from the strata of Tilgate Forest, as light and porous as those of the bears and hyenas, from the Caverns of Germany. On the other hand, fossil remains from the newest tertiaryformations are often completely petrified, that is, permeated by, or transmuted into, stone.
The wordsfossilandpetrifactionare so commonly used as synonymous terms, even by educated persons, that it is necessary to define the sense in which they are employed in these volumes.
Fossilsare the durable parts of animal and vegetable structures imbedded in rocks and strata by natural causes at a remote period; thus wood in a state of lignite, bog-wood, and coal, or of siliceous or calcareous stone, isfossil wood; and bones or shells, whether in an earthy and decaying state, or permeated by calc-spar, flint, or iron, and converted into a hard mineral substance, are alikefossil bonesorshells.
Petrifactionsare the remains of animals and vegetables in which the original structure is converted into stone, or, in other words, ispetrified; such are the silicified stems of trees from Antigua, and Germany, and the calcified bones and shells in the Oolitic and Wealden limestones. Such petrifactions may be correctly termed fossil plants, bones, or shells; but similar organic remains, though of equal antiquity, which have not undergone such changes, are not petrifactions in the proper meaning of that term.
The process by which petrifaction is effected is still involved in obscurity; mineral solutions have permeated the original tissues, and the organic molecules have been replaced by mineral molecules, but how this transmutation is produced is not understood. Mr. Dana's observations and Mr. Jeffery's experiments have, however, thrown much light on the process of silicification.[25]
[25]SeeWond.p. 100.
[25]SeeWond.p. 100.
Incrustations.—Another prevalent error is that of considering Incrustations to be fossils or petrifactions; a mistake which is sanctioned by the custom of calling waters that arecharged with calcareous earth (lime), and deposit it in considerable quantity, petrifying springs; as those of Matlock, and other places in Derbyshire. (Wond.p. 76.) But incrustations are not petrifactions; stems and branches of trees, skulls, bones, shells, &c., are simply invested with a calcareous coating or crust, which is generally porous and friable, but often crystalline and compact. The inclosed bodies are not permeated by the stony matter; if the mass be broken, or the incrustation removed, we find the twig, or stem, either dry and shrivelled, as in the specimens,figs. 2, 3, 4, Plate III.; or tubular cavities are left by the decay and removal of the vegetable structure, as infig. 10, Plate III.
But although incrustations be not petrifactions, natural specimens, (not the so-called petrified nests and twigs, in which the bad taste of the guardians of the Derbyshire springs is embodied, and dispersed all over England,) are objects of considerable interest, as illustrative of a process, by which important changes are effected in the mineral kingdom. Thus springs as clear and sparkling as poet ever feigned or sung, may transform beds of loose sand and gravel into rock, and porous stone into solid marble, and cover extensive tracts of country with layers of concretionary and crystalline limestone. This process is effected in the following manner. Most fresh water holds in solution a certain proportion of carbonate of lime; and changes of temperature, as well as other causes, will occasion this calcareous earth to be in part or wholly precipitated. Thefur, as it is called, that lines a kettle or boiler which has been long in use, affords a familiar illustration of this fact. At the temperature of 60° lime is soluble in 700 times its weight of water; and if to the solution a small portion of carbonic acid be added, a carbonate of lime is formed, which is thrown down in an insoluble state. But if the carbonic acid be in such quantity as to supersaturate the lime, it is again rendered soluble in water: it is thus that carbonate oflime, held in solution by an excess of carbonic acid, not in actual combination with the lime, but contained in the water, and acting as a menstruum, is commonly found in all waters. An absorption of carbonic acid, or a loss of that portion which exists in excess, will therefore occasion the lime to be set free, and precipitated on the foreign bodies in the water, as stones, twigs, leaves, &c.
The substance thus deposited is termedtufa, or travertine;[26]and in some parts of Italy, and of our own Island, it constitutes beds of stone of great extent, in which bones, shells, and the impressions of leaves and stems, are preserved. The stalactites and stalagmites of caverns have a similar origin; many of these caves are of incalculable antiquity, and beneath their stalagmitic floors, the bones and teeth of extinct carnivorous animals are found in vast quantities.
[26]Travertine, so called from the riverTibur, whose waters are loaded with calcareous earth—Tiburtina, Ital.travertina.
[26]Travertine, so called from the riverTibur, whose waters are loaded with calcareous earth—Tiburtina, Ital.travertina.
Silicification, or petrifaction by Silex or Flint.—Silex, or the earth of flint, is held in solution in large proportions, in certain thermal or boiling springs, which, on cooling, deposit the siliceous matter (in the same manner as the travertine is precipitated from incrusting streams) on foreign substances, and produce exquisite chalcedonic infiltrations of mosses, &c. But this operation is now only known to be in activity in the immediate neighbourhood of foci of volcanic action, as in the celebrated Geysers of Iceland (Wond.p. 95), and the boiling springs of the volcano of Tongariro, in New Zealand (98). We have everywhere evidence that in former periods, the petrifaction, as well as the incrustation of organic bodies by silex, was carried on to an immense extent; and, doubtless, far beneath the surface, the same operation is at the present moment in constant progress, and effecting as important changes in the consolidation of loose materials, as in the earlier geological epochs.
The various states in which silex occurs have depended on its fluidity; in quartz crystals the solution appears to have been complete; in agate and chalcedony it was in a gelatinous state, assuming a spheroidal or orbicular disposition, according to the motion given to its particles. Its condition appears also to have been modified by the influence of organic matter. In some polished slices of siliceous nodules the transition from flint to agate, chalcedony, and crystallized quartz, is beautifully shown. The curious fact, that the cavities of echinites in chalk are almost invariably filled with flint, while their crustaceous cases are changed into calc-spar, is probably, in many instances, to be attributed to the animal matter having undergone silicification; for the soft gelatinous parts are those which appear to have been most susceptible of this transmutation. In some specimens, the oyster is changed into flint, while the shell is converted into crystallized carbonate of lime. In a Trigonia from Tisbury, formerly in the cabinet of the late Miss Benett, of Norton House, near Warminster, the body of the mollusk was completely metamorphosed into a pure chalcedony, the branchiæ or gills being as clearly defined as when the animal was recent. In specimens of wood from Australia (presented to the British Museum by Sir Thomas Mitchell), which are thoroughly permeated by silex, there are on the external surface some spots of chalcedony that have apparently originated from the exudation of the liquid silex from the interior in viscid globules filled with air, which burst, and then collapsed, and became solidified in their present form.
In silicified wood the permeation of the vegetable tissues by the mineral matter, appears to have been effected by solutions of silex of a high temperature. In some examples the mineralization is simply a replacement: the original substance has been removed atom by atom, and the silex substituted in its place.
One of the most eminent naturalists and chemists of the United States, Mr. Dana,[27]suggests that the reason why silica is so common a material in the constitution of fossil wood and shells, as well as in pseudo-morphic crystals,[28]is the ready solution of silex in water at a high temperature (a fact affirmed by Bergman[29]) under great pressure, whenever an alkali is present, as is seen at the present time in many volcanic regions, and its deposition again when the water cools. A mere heated aqueous solution of silica, under high pressure, is sufficient to explain the phenomenon of the silicification of organic structures. Mr. Dana states that a crystal of calc-spar in such a fluid being exposed to solution, from the action of the heated water alone, the silica deposits itself gradually on a reduction of temperature, and takes the place of the lime, atom by atom, as soon as set free. Every silicified fossil is an example of this pseudo-morphism; but there seems to be no union of the silica with the lime, for silicate of lime is of rare occurrence.[30]