Infusorial earths may therefore be composed either of microscopic vegetable or animal remains, or of both. The brackish and fresh-water deposits I have examined are siliceous and almost wholly diatomaceous: the marine calcareous strata composed of microscopic organisms, consist chiefly of various kinds of foraminifera, a large proportion belonging to the polythalamia, or chambered shells. I am not certain as to the animal or vegetable nature of some of the beautiful siliceous disks (Coscinodisci, Arachnoidisci, Actinocyclus, &c.) so abundant in the Richmond, Barbadoes, and Bermuda infusorial earths, and which occur in so splendid a state in the Guano deposits of Ichaboe.

With the corrections which the above remarks will enable the reader to make, I would refer to the account of Fossil Infusoria in the Medals of Creation, and Wonders of Geology. [129]

XV. The Mosasaurus, or Fossil Reptile of Maestricht. (Plate LXX.) The occasional discovery of bones and teeth of an unknown animal in the limestone of St. Peter's Mountain, near Maestricht, and the innumerable shells, corals, teeth of fishes, claws of crabs, echini, and other organic remains, had long since attracted the attention of naturalists, and rendered these quarries celebrated throughout Europe. In 1770, M. Hoffman, the surgeon of the Fort, who had for some years been assiduously collecting the fossils of this locality, had the good fortune to discover a specimen which has conferred an enduring celebrity on his name. Some workmen, on blasting the rock in one of the caverns of the interior of the mountain, perceived to their astonishment the jaws of a large animal attached to the roof of the chasm. The discovery was immediately made known to M. Hoffman, who repaired to the spot, and for weeks presided over the arduous task of separating the mass of stone containing these remains from the surrounding rock. His labours were rewarded by the successful extrication of the specimen, which he conveyed in triumph to his house. This extraordinary discovery soon became the subject of general conversation, and upon reaching the ears of the Canon of the cathedral which stands on the mountain, excited in that functionary a determination to claim the fossil, in right of being lord of the manor; and he unfortunately succeeded, after a long and harassing lawsuit, in obtaining this precious relic. It remained for years in his possession, and Hoffman died without regaining his treasure, or receiving any compensation. At length the French revolution broke out, and the armies of the republic advanced to the gates of Maestricht. The town was bombarded; but at the suggestion of the committee of savans who accompanied the French troops to select their share of plunder, the artillery was not suffered to bombard that part of the city in which the celebrated fossil was known to be preserved. In the mean time, the Canon of St. Peter's, shrewdly suspecting the reason why such peculiar favour was shown to his residence, removed the specimen, and concealed it in a vault; but when the city was taken, the French authorities compelled him to give up his ill-gotten prize, which was immediately transmitted to the Jardin des Plantes, at Paris, where it still forms one of the most striking objects in that magnificent collection.[130]

The beautiful model of this most interesting fossil in the British Museum, was presented to me by Baron Cuvier. It is four and a half feet long, and two and a half wide; it consists of the jaws, with teeth, palatal bones, and the tympanic bone, or os quadratum, a bone possessed by reptiles, as well as birds, and in which the auditory cells are contained. There are likewise some fine portions of jaws, with teeth, in the British Museum, presented by Camper. The original animal was probably a terrestrial reptile, holding an intermediate place between the Monitors and Iguanas. It was about twenty-five feet long.

I discovered, many years since (1820), some vertebræ in the chalk near Lewes, which closely resemble the corresponding bones of the Mosasaurus, and in all probability belong to another species. In the cretaceous strata of New Jersey, Dr. Harlan found and described, and my friend. Dr. Morton, of Philadelphia, sent me, in 1834, teeth which cannot be distinguished from those of Maestricht. Vertebræ, and other bones, have since been obtained from the same deposits by Professor Rogers, and described by Professor Owen in the Geological Journal.

XVI. Fossil Reptiles. Although when Mr. Parkinson's work was published many fossil bones and teeth of reptiles had been discovered in various parts of England, yet the abundance and variety, and the extraordinary modification of form and structure of this class of vertebrated animals, which prevailed throughout the secondary geological formations, were not for a moment suspected. The few examples of the remains of fossil reptiles described by Mr. Parkinson, serve to mark the degree of knowledge which then existed respecting a department of palæontology that rapidly acquired an importance and interest unsurpassed by any other branch of fossil osteology.

The announcement of the founder of palæontology,[131] that there was a period when the lakes and rivers of our planet were peopled by reptiles, and cold-blooded oviparous quadrupeds of appalling magnitude were the principal inhabitants of the dry land; when the seas swarmed with saurians, exclusively adapted for a marine existence, and the regions of the atmosphere were traversed by winged lizards instead of birds; was an enunciation so novel and startling, as to require all the prestige of the name of Cuvier, to obtain for it any degree of attention and credence, even with those who were sufficiently enlightened to admit, that a universal deluge would not account for the physical mutations which the surface of the earth and its inhabitants had, in the lapse of innumerable ages, undergone.

Subsequent discoveries have established the truth of this proposition to an extent beyond what even its promulgator could have surmised; and the "Age of Reptiles" is now admitted into the category of established facts.

During the incalculable ages which the formation of the various systems of secondary strata must have comprised, we find no evidence in the fossils hitherto observed, of the existence of birds and mammalia, as the characteristic types of the faunas of the dry land. On the contrary, throughout the immense accumulations of the spoils of the ancient islands and continents, amidst innumerable relics of reptiles of various orders and genera, a few jaws and bones of two or three kinds of extremely small marsupials, and the bones of a species of wader, are the sole indications of the presence of the two grand classes of Aves and Mammalia, which now constitute the chief features of the terrestrial zoology of almost all countries.

The earliest indications of air-breathing vertebrata in the ancient secondary formations are those of small saurian reptiles in the carboniferous strata; a few vestiges occur in the succeeding group, the Permian. In the next epoch, the Triassic, colossal Batrachians (Labyrinthodonts) appear; and on some of the strata of this formation are the footmarks of numerous bipeds, presumed to be those of birds; but at present the evidence of the bones of the animals that made those imprints is required to establish the hypothesis.

In the succeeding eras, the Lias, Oolite, Wealden, and Cretaceous, swarms of reptiles of numerous genera and species everywhere prevail; reptiles fitted to fly through the air, to roam over the land, to inhabit the lakes, rivers, and seas; and yet not one identical with any existing forms! These beings gradually decline in numbers and species as we approach the close of the secondary periods, and are immediately succeeded in the eocene epoch, by as great a preponderance of warm-blooded vertebrata—birds and mammalia—as exists at the present time; and an equal decadence in the class of reptiles. With the Cretaceous Formation the "Age of Reptiles" may be said to terminate.

XVII.—Fossil Reptiles of the Wealden. The Iguanodon. The fluviatile deposits (termed Wealden), which in the south-east of England, and in the north of Germany, are intercalated between the oolitic and cretaceous formations, abound in the bones of terrestrial, fresh-water, and marine reptiles, comprising some of the most colossal land-saurians which have hitherto been brought to light. These remains belong to various genera of Chelonians, Saurians, and Crocodilians; and with these are associated those of flying lizards (Pterodactyles), Plesiosauri, gigantic whale-like reptiles (Cetiosauri), and of other oviparous quadrupeds of unknown species and genera.

The occurrence of fossils of this nature in the strata forming the districts denominated the Wealds of Sussex and Kent, was first brought under the notice of geologists in 1822, in my work on the "Fossils of the South Downs," in which the remains of several unknown reptiles were described; and among them the teeth and bones of that extraordinary herbivorous lizard, the Iguanodon, on which I am induced to offer a few observations in this place; the recent discovery of some previously undetermined parts of the skeleton, having materially elucidated the structure and economy of the original.[132]

Since the first announcement of the discovery of the remains of the Iguanodon, vast quantities of bones belonging to a great number of individuals of all ages have been collected; but until a few years since, not a vestige of the jaws had been observed, notwithstanding the most diligent research. In the early part of the year 1848, I was surprised and highly gratified by receiving from Capt. Lambart Brickenden (at that time a personal stranger to me), who then resided at Warminglid, near Cuckfield, in Sussex, the greater part of the right side (or ramus) of the lower jaw, with several successional teeth in their natural position, of an adult Iguanodon.[133] See p. 202.

In the course of last summer I obtained a very instructive fragment of the middle part of the right ramus of the lower jaw of a much larger Iguanodon, found by Mr. Fowlestone, with some enormous bones of the extremities, in the Wealden strata of the Isle of Wight. A portion of the upper jaw (without teeth) was discovered some years since in Tilgate Forest, and is deposited, with the whole of the collection I formed at Brighton, in the gallery of organic remains of the British Museum. These three specimens are the only parts of the jaws of the Iguanodon, with the exception of a fragment of the angular bone, that I have had the opportunity of examining. The other portions of the skeleton hitherto discovered are the following: the tympanic bone;[134] cervical, dorsal, lumbar, and caudal vertebræ, and chevron bones; ribs; the iliac bones, and sacrum composed of six anchylosed vertebræ;[135] the coracoid, scapula and clavicles; humerus, radius? metacarpals; femur, tibia and fibula, metatarsals and ungueals. The cranium, carpals, and tarsals, have not been discovered.

With the exception of the assemblage of bones promiscuously grouped together in a block of Kentish rag (of the greensand formation), found in a quarry near Maidstone, by Mr. Bensted, [136] a few connected caudal vertebræ, and two or three instances in which a femur, tibia, and fibula and some metatarsals, were found in contiguity, all the bones were isolated. They have been obtained from the quarries in St. Leonard's and Tilgate Forests, near Loxwood, Rusper, Horsham, Cuckfield, and Battel; and from the cliffs at Hastings, and in Sandown, and Brixton, and Brook Bays, on the southern shore of the Isle of Wight.

So anomalous is the osteology of the Iguanodon compared with that of existing saurians, that from my discovery of the first vestige of this reptile—a fragment of a tooth—thirty years ago, to the recent important acquisition of the jaws, I have had to contend with the opposition of eminent naturalists, who have refused assent to the physiological inferences suggested by the specimens which were from time to time brought to light, because the modifications of structure in a colossal herbivorous reptile, essentially differed from the hypothetical archetype skeleton of the class to which it belonged. When the first discovered teeth were shown to Baron Cuvier, he pronounced them to be the incisors of a Rhinoceros; the metatarsals, those of a Hippopotamus; the fragment of a femur, with a medullary canal, that of some large mammalian. But the candour and liberality of the founder of Palæontology were worthy of his transcendent genius; upon receiving further evidence, he immediately acknowledged the error, and expressed his conviction that the teeth and bones belonged to an herbivorous reptile more extraordinary than any that had previously been brought under his notice.[137]

Even the lower jaw, which presents characters so peculiar as to admit, as I conceive, of but one interpretation—that enunciated in my memoir on the teeth and jaws of the Iguanodon,[138]—has been adduced as affording a signal instance of the incorrectness of my physiological deductions. And why? Because in the entire class of living reptiles there is not a single species that has cheeks and flexible lips, which, according to my view of the subject, the Iguanodon must have possessed. But I do not hesitate to affirm that the structure and arrangement of the teeth, and the mammalian character of the bones of the extremities, are in perfect accordance with my exposition of the probable structure and functions of the maxillary organs of the original. The naturalists who advance these objections, forget that among the existing mammalia there is one genus, the Ornithorhynchus, or Duck-billed Platypus, that exhibits as striking a deviation from the typical maxillary structure of its class, as does the Iguanodon. If before the discovery of New Holland the jaw-bones of the Ornithorhynchus had been found in a fossil state in the strata of Tilgate Forest, and I had ventured to infer that the original, though a true mammalian, and giving suck to its young, had the extremities of the jaws covered with flat horny beaks, like those of a duck, instead of with the fleshy lips and integuments which are the peculiar attributes of its class, what censures would not my temerity have called forth! We cannot too often be reminded of the profound remark of William Penn: "Experience, which is continually contradicting theory, is the only test of truth."

The following are the physiological inferences relating to the structure and habits of the Iguanodon, which Dr. Melville and myself conceive our investigations have established: the discovery of the cranium, and of perfect examples of the upper and lower jaws with both successional and mature molars in their natural position, may modify, but, we believe, will in no material respect invalidate these conclusions.

In instituting a comparison between the maxillary organs of the Iguanodon, and those of the existing herbivorous lizards, we are at once struck with their remarkable deviation from all known types in the class of reptiles. In the Amblyrhynchi (of the Galapagos Islands), the most exclusively vegetable feeders of the Saurian order, the alveolar process beset with teeth is continued round the front of the mouth: the junction of the two rami of the lower jaw at the symphysis presenting no edentulous interval whatever, the lips not being more produced than in other reptiles; but this creature only bruises its food; it cannot grind or masticate it. In fact, the edentulous, expanded, scoop-shaped, procumbent symphysis of the lower jaw of the Iguanodon, has no parallel among either recent or fossil reptiles; and we seek in vain for organs at all analogous, except among the herbivorous mammalia. The nearest approach is to be found in certain Edentata; as for example in the Cholæpus didactylus, or Two-toed Sloth, in which the anterior part of the lower jaw is destitute of teeth, and much prolonged. The correspondence is still closer in the extinct gigantic Mylodons, in which the symphysis resembles the blade of a turf-spade, and has no traces of incisor sockets; and were not this part of the jaw elevated vertically in front, and the two sides confluent, it would present the very counterpart of that of the Iguanodon. The great number and size of the vascular foramina distributed along the outer side of the dentary bone in the Wealden reptile, and the magnitude of the anterior outlets which gave exit to the vessels and nerves that supplied the front of the mouth, indicate the great development of the integuments and soft parts with which the lower jaw was invested.

The sharp ridge bordering the deep groove of the symphysis, in which there are also several foramina, evidently gave attachment to the muscles and integuments of the under lip; and there are strong reasons for supposing that the latter was greatly produced, and capable of being protruded and retracted so as to constitute, in conjunction with a long extensile tongue, a suitable instrument for seizing and cropping leaves and branches, which, from the construction of the teeth, we may infer was the food of the Iguanodon.

Thus we find the mechanism of the maxillary organs of the Wealden herbivorous saurian, as demonstrated by recent discoveries, in perfect harmony with the remarkable dental characters which rendered the first known teeth so enigmatical. In the Iguanodon we have a solution of the problem, how the integrity of the type of organization peculiar to the class of cold-blooded vertebrata was maintained, and yet adapted by simple modifications to fulfil the conditions required by the economy of a gigantic terrestrial reptile, destined to obtain support exclusively from vegetable substances; in like manner as the extinct colossal sloth-like Edentata of South America. In fine, we have in the Iguanodon the type of the terrestrial herbivora, which in that remote epoch of the earth's physical history—the Age of Reptiles—occupied the same relative station in the terrestrial fauna, and fulfilled the same general purposes in the economy of nature, as the Mylodons, Mastodons, and Mammoths, of the tertiary periods, and the large pachyderms of modern times.

Although some important data are still required to complete our knowledge of the structure of the Iguanodon, we are warranted in concluding that this colossal herbivorous reptile was as bulky as the elephant, and as massive in its proportions: for, living exclusively on vegetable substances, the abdominal region must have been largely developed. Its limbs must have been of proportionate size to support and move so enormous a carcass. The hinder extremities probably presented the unwieldly contour of those of the Hippopotamus, and were based on strong short feet, protected by broad horny ungueal phalanges, or nails. The fore-legs appear to have been less bulky, and adapted for seizing and pulling down plants and branches: the teeth and jaws demonstrate the nature of its food; and the fossil remains of coniferous trees, arborescent, ferns, and cycadeous plants, with which its relics are commonly associated, indicate the character of the flora adapted for its sustenance.[139]

XVIII. The Pelorosaurus.—The humerus of a terrestrial reptile of enormous magnitude, has lately been discovered by Mr. Peter Fuller of Lewes, in the quarry near Cuckfield, from which many remains of the Iguanodon and Hylæosaurus were obtained in my early researches. This bone more nearly resembles the humerus of the Crocodiles, than that of the Lizards. It Is four and a half feet in length, and of corresponding proportions; it has a large medullary canal. As to the size of the animal to which it belonged, while disclaiming the idea that any certain conclusion can be drawn from a single bone, I may mention, with the view of conveying some general notion, that in a Gangetic crocodile eighteen feet long, the humerus is one foot: according to this scale the fossil animal would be eighty-one feet in length. I have proposed the name of Pelorosaurus (from πἑλωρ—pelòr—monster), or Colossal-saurian, for this new genus of reptiles which Inhabited the country of the Iguanodon.[140]

XIX. Silicification, or petrifaction by flint.—The various forms in which silex occurs have depended on its state of fluidity. In quartz crystals the solution was 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 shells of Echinites In chalk are almost invariably filled with flint, while their crustaceous shells are changed into calc-spar, is probably in many instances to be attributed to the animal matter having undergone silicification; for the most organized 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 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 completely 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 mineralization has been effected simply by replacement: the original substance has been removed, atom by atom, and the silex substituted in its place. One of the most eminent naturalists and mineralogists of the United States, Mr. J. D. Dana,[141] 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,[142] consists in the ready solution of silex in water at a high temperature (a fact affirmed by Bergman[143]) under great pressure, whenever an alkali is present, as is seen at the present time in many volcanic regions, and its ready deposition again when the water cools. A mere heated aqueous solution of silica, under a 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."[144]

XX. Stigmaria, Sigillaria, &c.—The most remarkable peculiarity of the flora of the carboniferous period is the immense numerical preponderance of the vascular or higher tribes of cryptogamic plants, which amount to two-thirds of the species described. With these are associated a few Palms, Coniferæ, Cycadeæ, and dicotyledonous plants, allied to the Cacteæ and Euphorbiaceæ. The number and magnitude of the vegetables bearing an analogy to the Ductulosæ, but differing from existing species and genera, constitute therefore the most important botanical feature of the carboniferous flora. Thus we have plants allied to the Equisetum, or mare's-tail (Calamites), eighteen inches in circumference, and from thirty to forty feet high; Zamia-like coniferæ (Sigillariæ) fifty feet high; and arborescent club-mosses (Lepidodendra) attaining an altitude of sixty or seventy feet. Of this ancient flora, the fossil plants whose stems have been named Sigillaria (see Plate XXI.), and their roots Stigmaria, are especially remarkable in consequence of the peculiar circumstances under which upright examples of these trees are occasionally met with. Referring for details to "Wonders of Geology," Lecture VII., I purpose describing in this place the facts recently brought to light, which prove that certain species of Stigmaria are the roots of Sigillariæ; while others in all probability belong to Lepidodendra:—an opinion maintained more than thirty years ago by the Rev. H. Steinhauer.[145] To the late Mr. Binney we are indebted for the first confirmation of the inference of my friend, M. Adolphe Brongniart, (derived from an examination of the structure of those bodies,) that the Stigmariæ are the veritable roots of Sigillariæ. At St. Helen's, near Liverpool, Mr. Binney discovered, in 1844, an upright trunk of a Sigillaria, nine feet high, to which were attached ten roots, several feet long, that extended into the under clay in their natural position;[146] and these roots were unquestionable Stigmariæ, the tubercles with their attached rootlets being clearly displayed. In the floor of the Victoria Mine at Dunkinfold, near Manchester, at the depth of 1,100 feet from the surface, Mr. Binney also discovered a magnificent specimen of Sigillaria, which exhibited on its stem the respective characters of three supposed species, and had stigmaria-roots extending twenty feet.

In the Sydney coal-field at Cape Breton, Mr. Richard Brown has observed several upright stems of Sigillariæ and Lepidodendra, with stigmaria-roots attached; and the same fact has been noticed in the Picton coal, in Nova Scotia. The following figures and notes from Mr. Brown's description of these interesting phenomena, will place the subject before the reader in a clear point of view.[147]

The main bed of coal is six feet in thickness, and is overlaid, as usual, by a roof of shale abounding in foliage and fragments of branches. As the coal is dug out, large masses of the shale fall in, and occasionally hollow spaces, called by the workmen pot-holes, are left in the roof; the fallen masses being the roots and truncated stems of Sigillariæ and other trees, which separate at the parting formed by the coaly bark covering the roots, when the supporting coal is removed.

The following sketch represents one of the specimens of the base of a stem of a Lepidodendron, with the roots (stigmariæ) attached. This figure (1) shows the position of the tree above the bed of main coal, with the inclination and length of two of the principal roots, so far as they could be distinctly traced; and the following sketch (2) represents the trunk, with its branching roots, constructed from careful measurements of the dimensions and position of each root, drawn on the spot. The stem at the part marked A, was encrusted with a coaly bark, covered by the usual cicatrices of the Lepidodendra, and the roots at B, C, D, with a similar carbonaceous investment, impressed with the characteristic pits or areolæ of Stigmariæ.

In the Instance of the upright stems of Sigillariæ in the same coal-field, the roots were also unequivocally Stigmariæ. Fig. 3, represents one of these erect stems, sixteen Inches high and twelve inches in diameter at the top, which dropped from the roof of the bed after the coal had been removed. Part of the coaly bark remains at c. The decorticated part of the trunk is covered with minute scales as far as the point h, a few inches below the first ramification of the roots. The carbonaceous crust investing the roots was thick at the upper part, but gradually became thin, and at a, and b, was a mere friable pellicle, that fell off upon the slightest touch.

The exact position of the tree in relation to the underlying coal is shown in the above section. Fig. 4. Immediately over the coal there is a bed of hard shale, six inches in depth, in which no fossils are found; this is overlaid by a softer shale abounding in coal-plants; all the upright stems were rooted in the six-inch shale. Upon clearing the base of this tree, a complete set of conical tap-roots was discovered, arranged as in the annexed sketch. Fig. 5. There are four large tap-roots in each quarter of the stump, and five inches below these another set of smaller tap-roots; the total number amounting to eighteen. The horizontal roots are seen to branch off in a regular manner, the base being divided into four nearly equal parts by deep channels, extending from the centre to the points i, k, l, m.

Mr. Brown remarks, that these short thick tap-roots were evidently adapted only to a soft wet soil, such as we may conceive was the nature of the first layer of mud deposited upon a bed of peat which had settled down slightly below the level of the water. He supposes, from the presence of a layer of shale without fossil plants immediately over the coal, that the prostrate stems and leaves which occur in such large quantities in the next superincumbent bed, fell from trees growing on the spot, and were entombed in layers of mud held in suspension in the water, which at short intervals inundated the low marshy ground on which they grew; for had the plants been drifted from a distance, he conceives they would also occur in the first layer of shale, as well as in those higher up.

Having thus shown that the Sigillaria alternans was provided with roots adapted for a soft muddy soil, Mr. Brown next describes the specimen represented in Fig. 6, which is the stem of the same species of tree broken off near the roots; the hollow cylinder of bark (a) having been bent down and doubled over by the pressure of the surrounding mud, so as effectually to close up the aperture, leaving only a few irregular cicatrices, of three or four inches in length, converging at the apex; the structure, arrangement, and number of the tap-roots, as well as the horizontal ramifications, are similar to those in Fig. 5. This fossil clearly explains the nature of the "dome-shaped" plant figured in the "Fossil Flora of Great Britain."[148]

"The roots of the preceding fossils repeatedly ramify as their distance from the stem increases, and ultimately terminate in broad flattened points. The whole of the spreading roots of these trees (the Sigillariæ) cover only an area of thirty square feet each; whilst those of the Lepidodendron (Fig. 1), whose stem is only two or three inches larger in diameter, covered a space of two hundred square feet. Since it is well known, from numerous examples, that the Lepidodendra were lofty trees, with spreading branches, which therefore required wide bases for support, may we not conclude that Sigillariæ of the species described were, on the contrary, trees of low stature, without heavy branches?"

I cannot quit this subject without again adverting to the remarkable phenomenon mentioned In a previous note, namely, that in the bed of pulverulent earth—the under-clay—on which the coal invariably reposes, the roots (or Stigmariæ) of large trees are generally the only organic remains met with. The constant occurrence of these fossils in the under-clay, and their rarity in the coal and shale, was long ago pointed out by Mr. Martin, Dr. Macculloch, and other geologists; but the importance of the fact was not appreciated till Mr. Logan drew attention to it. In the Welsh coal-field, through a depth of 1,200 feet, there are sixty beds of coal, each of which lies on a stratum of clay abounding in Stigmariæ. In the Appalachian coal formation of the United States, the same phenomena occur.

Thus it appears that the under-clay is the natural soil in which the roots (Stigmariæ) of the Sigillariæ and Lepidodendra grew; the coal above it is composed of the carbonized stems and foliage of those trees; and the roof or coal-shale is formed by the leaves and branches of a forest overwhelmed and buried beneath the transported detritus of distant rocks. These phenomena may be explained by supposing that a plain, densely clothed with a luxuriant intertropical vegetation, was either inundated by an irruption of the sea, or overwhelmed by a flood, from the sudden breaking up of the barrier of an inland lake; or by the subsidence of the country on which the forests grew. But when we find an uninterrupted series, in which triple deposits of this character are repeated through many thousand feet, the solution of the problem is beset with difficulties, which the hypothesis of repeated periodical subsidences, however ingenious, does not, in my opinion, remove.[149]