beds are larger and more numerous than in the preceding group, indicating an increase of the causes which produced carbonic acid. Fossils are also more numerous in them than in some of the other beds. The grauwacke forms the immediate surface in many large districts in Scotland, England, France, Germany, and North America, showing that, at the time of its formation, "some general causes were in operation over a large portion of the northern hemisphere, and that the result was the production of a thick and extensive deposit enveloping animals of similar organic structure over a considerable surface."* Fossils of the Grauwacke Group. The fossils of the grauwacke (a few of which extend to the clayey and sandy slates immediately below) are of both plants and animals. Amongst the plants are algæ, or sea-weeds, showing that seas like the present now existed. Some land plants are also found, but of the simpler structures; as filices, or ferns; equisetacea, a class of plants of the character of the mare's tail of our common marshes; and lycopodiacea, a class of the character of our club mosses. All of these land plants are monocotyledons, that is, produced from seeds of a single lobe, and therefore endogenous, that is, growing from within-timber plants being, on the contrary, the produce of two-lobed seeds, and growing by exterior layers. The flora of this era thus appears of a very simple kind, indicating the existence only of marshy and damp grounds. The animals are also, in general, of a humble and simple kind. There is abundance of those creatures (Polypi) resembling plants, which fix themselves on the bottom of the sea by stalks, and send forth branch-like arms for the purpose of catching prey, which they convey into an internal sac, and digest. At present these creatures abound in the bottoms of tropical seas, where they live by devouring minute impurities which have escaped other marine tribes, and thus perform a service analogous to that of earth-worms and other land tribes, the business of which is to clear off all decaying animal and vegetable matter. But the class of creatures found in greatest numbers in the grauwacke series of rocks are shell-fish, possibly because the remains of these creatures are peculiarly well calculated for preservation. All over the earth, wherever grauwacke rocks are found, shell-fish are found imbedded in vast quantities, proving that shell-fish were universal at the time when that class of rocks were formed. In a work entitled "Remarks on the Geology and Mineralogy of Nova Scotia," by Abraham Gesner,§ it is stated that they abound to a surprising degree in the valuable iron ores which in that province accompany or form part of the grauwacke rocks. In reference to the beds at Nictau, the author says, "The impressions made by marine organic remains in the ore and slate are extremely beautiful and distinct. Millions of shell-fish, of the molluscous and crustaceous tribes, which once enjoyed a perfect animal existence, have been swallowed up by this ore, where their remains and perfect likenesses are yet to be seen in the same natural and symmetrical beauty they possessed when alive." At New Canaan, another of the places where these rocks are dug, the lily encrinite, a remarkable example of the radiated tribes, is * De la Beche. The mare's tail is an elegant plant, having a succulent erect Jointed stem, with attenuated foliage growing in whorls round the joints, the latter being protected by a distinct striated sheath; the parts of fructification constitute a scaly catkin at the apex of the stem.-Mantell. Herbaceous prostrate plants found in damp woods and bogs, having their leaves simple and imbricated, that is, lying over each other. genus Halifax, Nova Scotia. Gossip and Coade, 1836. "They are almost all bivalves," he adds, "of the anomia, although some were obtained resembling the nautilus disens and planorbis equalis." He elsewhere mentions that the encrinite and trilobite, which in Germany are said not to have been found in rocks earlier than the mountain limestone, are found in the iron ore and clay-slate of Nova Scotia. found. It is so called from its resemblance to a lily resting on its stalk; "it is supposed," says Mr Gesner, "that the animal resided in the bottom of the flower; and those portions of it which were moveable, stood stretched out like arms to seize its prey. In the grauwacke at New Canaan, this animal appears like the lily with its capsule and petals closed. It is often of large dimensions; some were procured during our last visit to their stony graves, as large as water-melons, although in general they are much compressed, and have been flattened by the weight of the rock resting above them when in a soft state. This species of radiated animals is now altogether extinct, and many ages have passed since a living species could be produced. It has never been discovered in any of the strata placed above the new red sandstone; and as it does not appear but in a few of the older strata, the whole race must have enjoyed but a short existence." Among the shell-fish of the early seas, a few of the most remarkable kinds are described by geologists as ammonites and nautili. These fishes have been found in great varieties of size; but one peculiarity pervades them all, that the greater part of the shell is a curve containing air-cells, while the animal itself resided in the outer portion, as if a human being were to have a house consisting of a long row of chambers, and live only in the front room. The ammonite receives its name from its resemblance to the curved horn on the head of the statue of Jupiter Ammon. It has been an animal of wonderful character and habits. Some of them have been of a minuteness scarcely visible, and others four feet wide. They are found over the whole surface of the earth. The economy of this animal destined it to live in general at the bottoms of deep seas, but to be able to rise occasionally to the surface. While it lived in the outer part of its wreathed shell, the interior curls were hollow, containing air, so as to make it of nearly the same weight with the element in which it lived. As the pressure of the water at the bottom of a deep sea would break in the plates of any ordinary shell, as it does a bottle when one is lowered to a considerable depth, the shell of the ammonite has been strengthened by a curious kind of internal arch-work, so as to be able to resist the weight of the incumbent fluid. This archwork so completely meets all human ideas of ingenious contrivance for the purpose which it was destined to serve, as to form one of the most striking examples of that adaptation of means to ends which is universal in the works of nature, and which is so well fitted to impress the conviction of a great designing First Cause. The weight of the ammonite was so nicely adjusted to an equality with the water, that its filling with air or water a small central pipe which runs through the whole extent of the curve, was sufficient to make it rise as high or sink as low as might suit its inclination. The Trilobites are another of the early species which deserve particular notice. Their remains, like those of the ammonites, are universal over the earth. It is curious that, while they have long ceased to live, other genera or kinds of the same class of creatures (Crustacea) still exist, and serve to afford some knowledge of their habits. The trilobite had a head and eyes, below which there was a body of no great length, covered with shelly plates in the manner of a lobster's tail, and terminating in a narrow rounded point. It is supposed that it had soft paddles to make way through the water, which have not of course been preserved. But the which several specimens have been obtained in a nearly most interesting feature in the trilobite was its eyes, of entire state. The eye of the trilobite has been formed the surface of a cornea projecting conically upwards, with 400 spherical lenses in separate compartments on so that the animal, in its usual place at the bottoms of waters, could see every thing around. As there are two eyes, one of the sides of each would have been useless, as it could only look across to meet the vision of the other; but on the inner sides there are no lenses, that nothing may, in accordance with a principle observ able throughout nature, be thrown away. It is found that in the serolis, a surviving kindred genus, the eyes are constructed on exactly the same principle, except that they are not so high, which seems a proper difference, as the back of the serolis is lower, and presents less obstruction to the creature's vision. It is also found that in all the trilobites of the later rocks, the eyes are the same. This little organ of a trivial little animal carries to living man the certain knowledge, that, millions of years before his race existed, the air he breathes, and the light by which he sees, were the same as at this hour, and that the sea must have been in general as pure as it is now. If the water had been constantly turbid or chaotic, a creature destined to live at the bottom of the sea would have had no use for such delicate visual organs. "With regard to the atmosphere," says Dr Buckland, "we infer that, had it differed materially from its actual condition, it might have so far affected the rays of light, that a corresponding difference from the eyes of existing crustaceans would have been found in the organs on which the impressions of such rays were then received. Regarding Light itself, also, we learn, from the resemblance of these most ancient organisations to existing eyes, that the mutual relations of Light to the Eye, and of the Eye to Light, were the same at the time when crustaceans endowed with the faculty of vision were placed at the bottom of the primeval seas, as at the present moment." A few bones of fishes have been found in the grauwacke; but some obscurity rests on the point. If such really have been the case, the remains of this era may be said to include specimens of all the four divisions of the animal kingdom-radiated, jointed, pulpy, and vertebrated animals, or radiata, articulata, mollusca, and vertebrata. SECONDARY. Carboniferous Group. This is a very comprehensive group, embracing not only the coal strata, and the beds of sandstone, limestone, and others alternating with these, but two great formations on which the proper coal group may be said to rest (though in a state of intimate connection), named the Mountain Limestone, and the Old Red Sandstone. The Old Red Sandstone is a rock composed of grains of sand, cemented by the oxide of iron (the same substance as common iron rust), which gives it its red colour. Its strata are sometimes thin, and sometimes of the thickness of several thousand feet. Mountain Limestone is an abundant rock. It forms the surface of a vast portion of the central counties of Ireland. Quick-lime, for the improvement of soil, and the preparation of the mortar used in building, is made from mountain limestone. It is also used in many countries as a building stone. Great caverns often occur in this rock, being probably owing to some chemical phenomenon in its formation. One of these at Mitchelston, in Ireland, comprehends passages several miles in extent. Mountain limestone is frequently traversed by beautiful veins of calcareous spar, at times appearing to be principally composed of organic remains, while at others not a trace of these can be detected. This rock is of various colours, but mostly grey, varying in intensity of shade. In some situations it affords good marble, which is susceptible of a considerable degree of polish. From its durable nature, it is likewise used in building. That stupendous work, the Breakwater at Plymouth, is composed of it. Many valuable veins of lead ore occur in this rock. mulated in certain situations, and afterwards been covered over and pressed into a hard consistence under other strata. Two suppositions have been formed respecting the circumstances under which coal was formed. According to one, the vegetable matter must have grown in a dense forest for many years; then the land must have sunk, and become the basin of a lake or estuary, in which situation rivers would wash into it mud and sand, which would cover over the vegetable mass, and form superincumbent beds of shale and sandstone respectively. Then, the ground would be once more elevated, or sufficiently shoaled up, to become again a scene of luxuriant vegetation. When the vegetation had again become accumulated, the land would be again sunk, and become once more the basin of a lake, in which case the beds of mud and sand might again be formed by rivers. And this alternating process is supposed to have taken place as often as there are beds of coal to be accounted for. The other theory is, that, into some great estuary or lake, rivers coming from different quarters would bring the various matters forming the strata of the carboniferous group, a river from one direction bringing the mud which would form shale, another from another direction the vegetable matter which would form coal, and so on, each deposit perhaps taking place through the efficacy of some local circumstances, while the causes for the other deposits were temporarily suspended. At present great difficulties beset both theories. Fossils of the Carboniferous Group. In this group of rocks, about 300 species of plants have been discovered, all of them now extinct. About two-thirds of them are ferns; the others consist of large conifera (allied to the pine), of gigantic lycapodiacea, of species allied to the cactee and euphorbiacea, and of palms. Most of these plants probably exist in the coal beds, forming in fact their sole composition; but the peculiar nature of this mineral renders it difficult to detect them by examination. Thin slices, however, have been examined by the microscope, and the vege table structure has then been detected, where no external trace of it was visible. In cannel coal, a kind peculiarly hard, the vegetable structure is observed throughout the whole mass, while the fine coal retains it only in small patches, which appear as it were mechanically entangled. Slate and cannel coal often bear.distinct impressions of plants. The plants are such as grow in hot moist situations; and it is therefore presumed, that a climate of that nature existed at an early period where coal is now found, even in Melville's Island, which is within the polar circle. Dr Hutton thought that the vegetables must have been carbonised (or charred) by heat; but Dr Macculloch contends, on good grounds, that the change has been effected solely by water and pressure, and that by these agents peat is capable of being converted into coal. Large fragments of trees are often found in the shale and sandstone beds of the carboniferous group, more frequently in the former than in the latter. As usual with fossil substances, they are converted into the material in which they are imbedded, but preserve all their original lineaments, except that they are generally changed from their original round to a flattened form, the result of the pressure they have sustained. In most instances, these fragments of trees appear to have been transported from a distance, and laid down horiThe superior group more particularly called Car-zontally in their present situation; but some have been boniferous, and variously termed the Coal Measures, is found with their roots still planted in their native soil composed of beds of that mineral, often very numerous, of mud, and the stems shooting upwards through several alternating with beds of sandstone, shale, limestone, superior beds of various substances. Even in some ironstone, and some other substances. As many as coal beds, there are found stems of trees in their origi forty beds of coal exist in the neighbourhood of the nal vertical position; the roots being imbedded in shale town of Newcastle. The great utility of this mineral as a domestic fuel, and in the arts, gives it a high importance, and happy is the country in which it exists in any considerable quantity. In a merely geological point of view, it is equally important. This rock is entirely a mass of vegetable matter, which has accu beneath. In these instances, we must suppose the fossil to be on the spot where the living tree was planted, grew, and died. In the Bensham coal seam, in the Jarrow coal-field, a few years ago, there was found an upright tree of the kind called lepidodendra, thirteen * Maclaren's Geology of Fife and the Lothians. and a half feet wide at the base, and thirty-nine feet | high, the branches at the top being also entire: the lepidodendron, a common plant in this group, is so called from the scaly appearance of its stem, the scales being the roots of the leaf stalks. Various fossil trees have been discovered in the sandstone beds of the carboniferous group, at Craigleith and Granton, in the county of Edinburgh. One found in Craigleith quarry was twenty feet long, three feet in diameter, with scars where the branches had been torn off, and was ascertained, by microscopic inspection of slices of the trunk, to have been a conifera of the genus Araucaria, of which living species exist in New Holland. The animal remains of the carboniferous group are much the same as those of the grauwacke-zoophytes, mollusca, crustacea, and a few fishes. New Red Sandstone Group. This group of strata, lying above the carboniferous group, comprehends rocks called The Red Conglomerate, formed of pieces of earlier rocks, some rough, some smoothed by rolling, all caked together; Zechstein, a kind of limestone, abounding in Ger many; Red or Variegated Sandstones, a group of many varieties of colour, and principally of argillaceous and siliceous consistence, much used for building in England and other countries; Muschelkalk, a limestone varying in texture, but most frequently grey and compact; not found in Britain or France, but occurring in Germany and Poland; Variegated Marls-beds of rock of different colours, red, blue, and grey, composed of the remains of shellfish. To this group also belong beds of rock salt, of which many exist in England, particularly in the county of Chester. Rock salt is a crystalline mass, forming regular strata, sometimes of the thickness of many feet. The substance is rarely pure, but generally contains some portion of oxide of iron, which gives it a red colour. It is dug like coal and other minerals, and when melted and subjected to proper purification, is sold for domestic purposes. Fossils of the New Red Sundstone Group. The vegetable remains of this group are much the same as those of the preceding; but in the department of animal life, when we arrive at the Muschelkalk, or Shell Limestone, we find a great difference, leading to a supposition that, at this era of geological chronology "circumstances had arisen changing the character of marine life over certain portions of Europe; that certain animals abounding previously, and for a great length of time, disappeared never to reappear, at least as far as we can judge from our knowledge of organic remains" and that certain new forms of a very remarkable kind continued to flourish while a great succession of other rocks was forming: throughout the whole of the Secon dary Formation, there were few other land animals. In fact, the world must have been in the possession of reptiles for a many thousand times longer period than it appears to have yet been in the possession of man. "When we see," says Dr Buckland," that so large and important a range has been assigned to reptiles among the former population of our planet, we cannot but regard with feelings of new and unusual interest, the comparatively diminutive existing orders of that most ancient family of quadrupeds, with the very name of which we usually associate a sentiment of disgust. We shall view them with less contempt, when we learn, from the records of geological history, that there was a time when reptiles not only constituted the chief tenants and most powerful possessors of the earth, but extended their dominion also over the waters of the seas; and that the annals of their history may be traced back through thousands of years antecedent to that latest point in the progressive stages of animal creation, when the first parents of the human race were called into existence."* The Reptiles of this early age were peculiar both in size and in structure. Some, which inhabited the seas, resembled lizards, but were of gigantic size; others, designed for land as well as sea, resembled the crocodiles which still exist in warm climates. One of the most remarkable kinds (genera) has received the name of Ichthyosaurus (Fish Lizard), of which seven species or varieties have been discovered. The head is like that of the crocodile, composed of two long slender jaws, provided with a great number of teeth (in some cases 180), and eyes of great size (in one instance, the cavity for the eye has been found to measure fourteen inches), while the nostril, instead of being near the snout, as in the crocodile, was near the anterior angle of the eye. The body was fish-like, arranged upon a long spinal column, which consisted of more than a hundred joints, and to which a series of slender ribs was attached, and terminating in a long and broad tail, which must have possessed great strength. The whole length of some specimens of the Ichthyosaurus was about thirty feet. Instead of the feet, with which the lizard and crocodile are furnished, the Ichthyosaurus had four paddles like those of the whale tribes, fitting it to move through the waters in the manner of those animals. It had also a construction of the sternum or breast-arch, and of the fore paddles, similar to that found in the Ornithorynchus, an aquatic quadruped of New Holland, and evidently designed, as in the case of that animal, to enable it to descend to the bottoms of waters in search of food. While the Ichthyosaurus, then, is mainly allied to the lizard tribes, it combined in itself the additional characters of the fish, the whale, and the Ornithorynchus. "As the form of the ver tebrae by which it is associated with the class of fishes seems to have been introduced for the purpose of giving The new creatures were of such a class as we might rapid motion in the water to a lizard inhabiting the expect to be the first added to the few specimens of fish element of fishes, so the further adoption of a structure which had hitherto existed: they were of the class of in the legs, resembling the paddles of a whale, was Reptiles, creatures whose organisation places them next superadded, in order to convert these extremities into in the scale of creation to fish, but yet below the higher powerful fins. The still further addition of a furcula class of animals which bring forth their young alive and clavicles, like those of the Ornithorynchus, offers and nourish them by suck (mammalia). The earth was a third and not less striking example of selection of as yet only fit to be a partial habitation to creatures contrivances, to enable animals of one class to live in breathing its atmosphere and living upon its productions. the element of another class."+ Such deviations canIt is supposed to have been under so high a temperature not be considered as monstrosities; they are perfect as to be unsuitable for mammalia: the lands which adaptations of a creature to its purposes in the theatre existed were probably low and marshy, with a hot, moist of being. Only the spine of the Ichthyosaurus as yet atmosphere, so as to present an appropriate field of existed in other animals. Its head, its paddles, and its existence only for lizards, crocodiles, and creatures of breast-arch, were all detached parts of future animals. similar character. It is also to be supposed that the How strange to reflect, that some of these contrivances land was at this period undergoing frequent changes were allowed to become extinct, and, as it were, lost to and convulsions, so that only a class of creatures to nature, and ultimately, after a long interval, were rewhich submersions and deluges were matters of indif-vived in connection with new creatures! ference, could reside upon it without a greater waste of life than was part of the Great General Design. The Reptiles, which first begin to appear in the Muschelkalk, * De la Beche's Manual, 408. were added. The internal structure and the modes of living of the * Bridgewater Treatise, i. 167. Ichthyosaurus, have been in a most unexpected manner Of the Crocodile family found in abundance in this made clear by the discovery of the half-digested remains class of rocks, the Iguanodon, of which remains have of animals found within them or in their neighbour- been found in the fresh-water formation at Wealden in hood. It appears that the creature possessed a large England, may be cited as a specimen. It was a huge stomach, extending throughout nearly its whole body, animal, resembling the present Iguana of South Ame and that it lived upon fish and other reptiles, including rica, which chiefly lives upon plants and seeds. The its own kind. It must have occasionally devoured crea- smallest part of the thigh-bone of an Iguanodon was tures several feet in length. Masses of the refuse of found to be twenty-two inches in circumference, and the Ichthyosaurus, petrified as hard as the finest marble, much larger than that of any existing elephant. Species and well known to geologists under the name of copro- resembling the present Gavial of the Ganges have also lites, are found to be marked spirally, like the refuse of been found. It may fairly be inferred from the present certain species of sharks and dog-fish, the intestinal gut habits of the Gavial and other kinds of Crocodiles, that of which winds greatly, in order that it may take up the at the time when the extinct species flourished, the least possible room. We thus obtain a distinct idea of world must have contained many low shores and sathe nature of a very important part of the bodily eco-vannahs, fitted for the residence of such creatures. nomy of this long extinct race of animals. The stomach Some parts of England are thus proved to have had at occupied so large a space in their bodies, for the recep- one time shores of lakes and estuaries resembling those tion of large quantities of food, and it was at the same of the Ganges, the Nile, and other waters in hot countime so necessary that the speed of the animal in pur-tries, and consequently a much higher temperature than suit of prey should not be clogged by a very large or at present. long body, that the smaller intestines had been, by a But perhaps the greatest wonder of the Reptile Age, wise arrangement of nature, reduced nearly to the state was the creature called the Pterodactyle. Mainly a of a flattened tube, coiled like a cork-screw around reptile of the lizard kind, its body possessed some of the itself; "their bulk being thus diminished," says Buck-characteristics of the mammalia; it had the wings of a land, "while the amount of absorbing surface remained nearly the same as if they had been circular." bat, the neck of a bird, and a head furnished with long jaws full of teeth, so that in this last part of its organisation it bore some resemblance to the crocodile. Eight species of the Pterodactyle which have been found, vary from the size of a snipe to that of a cormorant. The eyes were of enormous size, apparently enabling it to minated by long hooks, like the curved claw on the thumb of the bat. These must have formed a powerful paw, wherewith the animal was enabled to creep or climb, or suspend itself from trees. It has been con jectured that the Pterodactyle would chiefly live on flying insects, of which, it is important to notice, several varieties existed at the same time, their remains being found in the same rocks. And it is likely, from the size of the eyes, that it searched for prey by night as well as by day. But it has also been argued, from the great length and strength of the jaws, and the length of the neck, that the Pterodactyle did not live solely upon flies, but likewise sought for fish in the manner of our own present sea-birds. The name Plesiosaurus is applied to another highly remarkable reptile of gigantic size, which inhabited the world before the days of mammalia. A particular species has been described as having a body and paddles which bore some resemblance to those of the Ichthyo-fly by night. From the wings projected fingers tersaurus, the former being more bulky, and the latter longer and more powerful. At the end of a long neck, like the body of a serpent, was a head resembling that of a lizard, but also partaking of the characters of the head of the crocodile and Ichthyosaurus. The tail was short. The backbone of this creature, and the neck and tail continuing it, contained in all about ninety vertebral pieces, thirty-three of which composed the neck; and the vertebrae are found to be of a less fish-like structure than those of the Ichthyosaurus, and not nearly so well calculated for rapid motion. The ribs describe a large circle, and, being formed in four parts, seem to have been designed to contain a capacious set of lungs, and to rise and fall as the lungs were inflated or emptied in this respect they resemble the ribs of the chamelion, whose changes of colour are now known to be occasioned by the varied depth of its inspirations. It is therefore surmised, though with little confidence, that the Plesiosaurus was also capable of changing its colour -a power which must have been highly necessary to a creature of its unwieldy character, both to enable it to elude the quicker and equally voracious Ichthyosauri, and that it might more readily ensnare and seize the creatures designed to be its prey. The Plesiosaurus probably lived chiefly on or near the surface of the water, breathing the air, and dabbling for prey like a duck or swan, but might also be able to descend to the bottom, and even to move, though awkwardly, upon land. One part of its organisation is peculiarly striking, as foreshadowing a structure of a more important kind. The paddles, which may be considered an advance or improvement upon the fins of fishes, are at the same time the type of the legs of quadrupeds and of the arms and limbs of man. The fore-paddle consists of scapula (shoulder blade), humerus (shoulder), ulna (upper bone), and radius (lower bone), succeeded by the bones of the carpus and metacarpus, and the phalanges, equivalent to those which compose the palm and fingers of a human being. The hind-paddle presents femur, tibia, and fibula, succeeded by the bones of the tarsus and metatarsus, and five toes. Thus "even our own bodies, and some of their most important organs, are brought into close and direct comparison with those of reptiles, which at first sight appear the most monstrous productions of creation; and in the very hands and fingers with which we write their history, we recognise the type of the paddles of the Ichthyosaurus and Plesio saurus."* Buckland's Treatise, i. 213. Tortoises also existed during this age, as is proved by the marks of their feet on sheets of sandstone, and by their remains. But as yet no animals of a higher class had appeared upon earth-for the remains of certain creatures of the Opossum family, found in the oolite at Stonesfield, near Oxford, stand as yet so solitarily, that we cannot consider them as proving that mammalia were added to reptiles. With, then, flocks of Pterodactyles flying in the air in pursuit of huge dragon-flies; gigantic crocodiles and tortoises crawling amidst the jungles of low, moist, and warm shores, and such monsters as the Ichthyosaurus and Plesiosaurus swarming on the surface of the sea, while its depths were peopled by infinite varieties of fish, shelled and vertebrated; we can form some faint idea of what sort of world it was while the strata between the coal and the chalk were in the course of being deposited. Oolitic Group. Next in order is a group which derives its name from a kind of limestone conspicuous in it, called oolite. Oolite, again, is named from its resembling the eggs or roe of fish. The oolite group comprehends, besides oolite itself, various alternating clays, sandstones, marls, and limestones. Oolite is a carbonate of lime, intermixed with other ingredients. The oolites found at Bath, Portland, and Purbeck, are much esteemed in building. In the oolite which occurs over a considerable part of western Europe, there is a general uniformity of structure. In other parts of the world it differs very considerably, especially in its mineralogical character; and when this is the case, in order to determine whether certain rocks belong to the group or not, recourse has been had to the organic remains contained in them. In some parts of the term, could scarcely have caused the effects required, we may perhaps look to a greater exertion of the power which now produces thermal waters for a possible explanation of the observed phenomena." Mr Lyell states, that chalk must have originated in the sea, in the form of sediment, from tranquil water; and that, before the existence of the rocks above, it must have been raised in large portions above the water, and exposed to the destroying power of the elements. of Europe these are very abundant, and in other places | solution of carbonate of lime and silex, covering a conthe reverse. To account for this difference, it has been siderable area." M. de la Beche goes on to state, that supposed that in those parts of western Europe where no springs, or set of springs, could have produced the they are abundant, shallow seas existed; while in those great deposits of chalk which cover immense surfaces. places, such as Italy and Greece, where few remains" But," says he, "although springs, in our acceptation are found in the formation, the waters were deep. With respect to the deposition of the oolitic group, nothing very satisfactory can be said. Whence came the immense quantity of carbonate of lime, is a question not easily answered. To account for it by springs, similar in size and saline contents to those we now see, appears to be unphilosophical. Many limestones are nearly altogether composed of organic remains; and this has led to a theory, that these animals extracted lime from the water, leaving their shells, produced through millions of generations, to be gradually converted into limestone. Notwithstanding all that we In this group, confervæ and naides were added to the can suppose was deposited from springs and organic vegetables; to the animal remains some fishes are bodies, "there remains," says de la Beche, "a mass of added, but the number of the saurian reptiles is dimilimestone to be accounted for, distributed generally over a very large surface, which requires a very gene-beneath the lower green sand of the English series, A species of rocks, called the Wealden rocks, occur ral production, or rather deposit, of carbonate of lime and are characterised by the presence of terrestrial contemporaneously, or nearly so, over a great area.” In the oolitic group is comprehended by some geologists an important subordinate group, named the lias, which may be generally described as an argillaceous and calcareous deposit, sometimes the clayey material predominating, and sometimes the limey. Fossils of the Oolitic Group. In this group are found, as in some of the preceding, alga (sea-weeds), equisetacea (mares' tails), filices (ferns), and conifere (allied to the pine). The animal remains are nearly the same as in the preceding group, but of a greater variety of species. Cretaceous or Chalk Group. nished. Fossils of the Cretaceous Group. and fresh-water remains in abundance. TERTIARY. The cretaceous group was at one time thought to be the uppermost; but it was in time discovered that, in several places, and particularly under the sites of the cities of London and Paris, there existed a still higher group or series, to which the name supra-cretaceous was therefore at first given, afterwards changed to the Tertiary Formation or Tertiary Series. in great hollows or basins in the former surface: hence 2. Marine formation, This group, the uppermost of the secondary series of rocks, is so named from the beds of chalk of which it is mainly formed. Chalk is a carbonate of lime. It is very plentiful in England; and at Dover and other places, it runs along the coast in cliffs and mountains of considerable size. Nodules or small masses of flint, sometimes containing remains of shells and animals, are abundant in chalk, and it is extremely difficult to account for their presence there. In the lower parts of the English chalk deposits, the flints disappear, becoming gradually more rare in the passage downwards. From this circumstance, the group has been sometimes divided into upper, or chalk with flints, and lower, or chalk without fiints. But this characteristic does not universally prevail. Beneath the chalk there is a rock called greensand, which in Normandy is used as a build- 5. Third fresh-water foring stone. An argillaceous deposit called gault also occurs; it is of a bluish-grey colour, and is frequently composed of clay in the upper, and marls in the lower part. The cretaceous group, taken as a mass, may in England, and over a considerable portion of France and Germany, be considered as cretaceous in its upper part, and sandy and clayey in its lower part. The group is extensively distributed over Europe; and M. de la Beche makes the following observations upon its mineralogical character in general :-" Throughout the British islands, a large part of France, many parts of Germany, in Poland, Sweden, and in various parts of Russia, there would appear to have been certain causes in operation, at a given period, which produced nearly, or very nearly, the same effects. The variation in the lower portion of the deposit seems merely to consist in the absence or presence of a greater or less abundance of clays or sands, substances which we may consider as produced by the destruction of previously existing land, and as deposited from waters which held such detritus in mechanical suspension. The unequal deposit of the two kinds of matter in different situations would be in accordance with such a supposition. But when we turn to the higher part of the group, into which the lower portion graduates, the theory of mere transport appears opposed to the phenomena observed, which seem rather to have been produced by deposition, from a chemical 3. Second fresh-water for mation, First sandstone. 4. Second marine forma- Gypscous marine marls, mation, Upper marine sands and sandstone. Upper fresh-water marls. Calcaire Grossier, as its name implies, is composed of a coarse limestone, which is employed for architectural purposes. It is frequently separated from the plastic clay beneath by a bed of sand, and it alternates with argillaceous or clayey beds. The animal and vegetable remains enclosed in it are numerous, and generally the same in corresponding beds, presenting considerable differences when the beds are not identical. Siliceous Limestone is sometimes white and soft, sometimes grey and compact, and penetrated by silex. It is often full of cells, which are occasionally large, and communicate with each other in all directions. |