a horny substance called an operculum, that serves as a door to close the shell, when the animal withdraws into it. In many species of univalves, the animal can fold the mantle so as to form a tube which protrudes into the water, while the head and foot remain in the shell. Some species of univalves are carnivorous, others are herbivorous, and the nature of their food determines their residence either near the shore or in deep water.

Fig. 15. represents the shell and animal of a species of Buccinum, which agrees with the above description of the inhabitants of univalve shells. The foot on which it crawls is on the left hand, with the oval operculum near the end of it. On the right hand of the figure, at the top, the mantle is represented folded, to form a tube, as above described.

In some species, both of bivalve and univalve shells, the animals depart considerably from the general character of the class to which they belong. There are some bivalves which have the cavities of the shells divided by partitions, the uses of which are not known; and some univalves have an apparatus for swimming on the surface of the water.

The Hippurite, a remarkable fossil bivalve, with a deep conical under shell, and a fiat lid, is represented fig. 14. It is classed by Cuvier with the oyster family; and, by Parkinson, with chambered shells. The nature of the animal is unknown. The shell is divided by transverse septa, or partitions, on which account Mr. Parkinson places it among other species of chambered fossils. The existence of a lid seems to prove, that it was not an internal shell, but the habitation of the animal. A fossil hippurite has recently been found in the chalk hills of Sussex, by Mr. Mantell.

The Janthina is a beautiful purple-coloured univalve shell, nearly resembling in form the snail; Lamarck discovered, that it could not crawl on its foot, but that the foot is covered with air bladders, which enable the animal to rise and swim on the surface of the water. The janthina is common in the Mediterranean; when touched, it excretes a deep purple liquor, which tinges the surrounding water. (Cuvier, R. A. tom. iii.) There are other animals occupying univalve shells, that have the power of swimming. The Lymnea stagnalis, an inhabitant of ponds, swims on the surface of the water in a reversed position. It descends by compressing itself whithin the shell, and expelling the air, and thus sinks immediately to the bottom. Mr. Parkinson rightly conjectures, that the shells resembling the Helix, or snail, in the older strata, were constituted for swimming, like the janthina: they could scarcely have used a foot for crawling, at the bottom of a deep and agitated ocean.

We come now to another division of the animal kingdom, called by Cuvier Radiated. See Chap. II. Some of the animals comprised in this division have left abundant remains in the fossil state, particularly the encrinite and the pentacrinite. These animals had a stem, composed of numerous plates, and terminating in branches surrounding the mouth, resembling the stem and branches of a vegetable. Both these species were supposed to be extinct; but a living pentacrinus has been discovered in the West Indies, and a smaller species, more recently, in the Cove of Cork. This has been described by Mr. J. V. Thompson, of Cork. A drawing of this animal, taking by Mr. Thompson, is given (Plate VIII. fig. 17. A cut of a remarkable species

of fossil encrinite is given, p. 164.: it is named the Lily Encrinite, because the arms, when folded, resemble the head of the lily. Indeed, the whole class, of encrinites and pentacrinites are called crinoidea, from krinon, the lily, by Mr. Millar, in his valuable work on these fossils. The arms of part of a Briarean pentacrinite are represented, p. 180. In the encrinite, the stem is composed of numerous round plates, or vertebræ; the branches are also composed of numerous smaller, but similar plates, as may be seen by referring to fig. 17. and the cuts. The pentacrinite differed from the encrinite by the plates, or vertebræ, of the stem and branches being pentagonal. The stems of both were attached to rocks. They appear, like various polypi, to have increased by throwing out lateral stems (see the above fig.). The calcareous vertebræ that formed the stem and branches, were enveloped by a thin coat of animal matter, which must have possessed great muscular power, to have enabled the animal to move its arms with great facility, when seizing its prey.

In fig. 17. the expanded arms of the upper head of the pentacrinus expose the pentagonal aperture or mouth in the centre; and a little above this is a round tube or aperture, which serves for the excretion of the fæces. In fig. 18., which is a head with the arms removed, it will be seen, that the excreting tube projects a little above the mouth. One head of the pentacrinus is represented as folded, and another as partly collapsed. As these animals were enveloped in a thin fleshy covering, their calcareous remains may be regarded as portions of the skeleton. Some beds of mountain limestone, in Derbyshire, are almost entirely composed of broken stems and branches of encrinites, not uncommonly called entrochites. In a part of this work it was stated, on the authority of a letter sent to the Author, that the Lily Encrinite had been discovered in Ireland; but the cut subsequently given of it in Mr. Loudon's Magazine of Natural History, makes it doubtful, whether it is the true Lily Encrinite, or a species nearly resembling it.

The Author cannot conclude these remarks, without expressing a wish, that scientific voyagers and medical gentlemen, who visit tropical seas, would carefully examine the different species of sepia that may be caught. It is probable, that there are living species, with internal chambered shells, resembling more or less the figures in plate VIII. Cuvier says, that a little change in the structure of the oval internal shell of the cuttle fish, would convert it into the internal chambered shell of the spirula. It was with a view to excite the curi osity of voyagers, when near the coast of North America, that the Author has suggested the possibility of the ichthyosaurus visiting those seas, p. 213. Cuvier too hastily conjectured that no new living species of large terrestrial quadrupeds remained to be discovered. The gigantic tapir and new species of elephants have since been discovered in India. The Author considers it far from improbable, that the great mastodon may exist in some of the unexplored recesses on the western side of North America; and he would particularly recommend comparative anatomists to examine the structure of the Grisly bear, and compare it with the skeleton of the cavern bear, (ursus spelæus.) When Cuvier published the last edition of his Règne Animal, in 1829, he does not appear to have known any thing respecting the Grisly bear.

INTRODUCTION TO GEOLOGY.

CHAPTER I.

Objects of the Science denominated Geology.-The Shape and Density of the Earth.-Opinions respecting the internal Parts of the Globe.-Central Heat.Temperature of the Earth.-Sea and dry Land.-Proportion of the Earth's Surface habitable by Man.-On the Appearances which led to the first Division of Rocks into Primary and Secondary.-Classification of Rocks.-Districts in which the different Classes appear in England.-The present Islands and Continents formerly covered by the Ocean.-Existing Proofs of this in Great Britain and various Parts of the World.-Fossil Remains of marine Animals, Vegetables, and land Quadrupeds; the Strata in which they are imbedded formed in Succession at different Epochs.-On human Bones occasionally imbedded in Rock-Inferences respecting the former Condition of the Globe.-Remarkable Passage in the Institutes of Menu.

In this Chapter the author has endeavoured to give such an outline of the science, and its practical application to the knowledge of the Geology of England, as may be clearly and easily understood by the general reader, and prepare him for the perusal of the succeeding Chapters.

THERE are perhaps few persons possessed of much curiosity in early life, to whom the following question has not frequently presented itself What is the world made of? Now this question, with certain conditions, comprises the most important objects of geological research; namely, What are the substances of which the Earth is composed? What is the order in which they are arranged? What are the changes they appear to have undergone?—But how are satisfactory answers to these inquiries to be obtained?

When we examine the terrestrial globe, where the solid parts are uncovered and exposed to our view, we observe vast masses of rock or stone lying in apparent confusion on each other: or, should we perceive some regularity in their position and arrangement, we soon lose sight of it again by the intervention of other rocks. In this department of nature all seems vast, unshapen, and chaotic; but let us not be discouraged, for we may recollect that the grandest objects in the material universe, seldom present to the hasty view of the superficial observer, immediate proofs of order or design.

The shepherd who first discovered that the planets were not fixed in the heavens, and noticed their apparently intricate wanderings among the stars, could not possibly anticipate the regularity and harmonious simplicity of their movements, which subsequent observations have demonstrated.

Let us then endeavour to ascertain by what means we may become acquainted with the structure of the solid covering of our globe. Were these means bounded by the power of man to penetrate below the surface, our knowledge must ever remain very limited and imperfect; but natural operations have greatly facilitated our inquiries, and have broken the rocky pavement of the globe, and raised up or laid bare the mineral substances of which it is composed. By an attentive examination of the situations where the rocks and strata are thus exposed to our research, we lay the foundation of the science denominated Geology.

Geology is derived from two Greek words, ge "the earth," and logos "reason," and signifies the Science of the Earth. Werner and his disciples, and also some of the French geologists, have changed the term into geognosy; but for this change no sufficient reason can be assigned, and it is contrary to established analogies of language.* Philosophers, in former ages, neglected the examination of the earth, and contented themselves with vain speculations respecting its formation; whereas the only proper answer to the question, How was the world made? is briefly this-"By the almighty power of its Creator." We may however be permitted, and indeed we are almost irresistibly impelled, to inquire into the nature of the secondary causes, that have been operative in reducing the surface of our globe to its present state. This inquiry comprises what may properly be denominated Speculative Geology. Nor is this, as some assert, entirely useless the advocates of particular systems have engaged in an active examination of nature to support their opinions, and have "compassed sea and land to gain proselytes:" thus numerous facts have been discovered, with which we should not have been acquainted had they remained idle in their studies.

The earth is now well known to be one of those globular bodies called planets, that revolve round the sun in orbits nearly circular, and in stated periods of time, which bear a certain ratio to their respective distances from it. They turn round their axis with different degrees of velocity; and this motion appears to have had considerable influence on their external shape, by enlarging their equatorial diameters; they are not perfect spheres, but are more or less flattened at their poles.

In the planet Jupiter, the velocity of the equatorial parts is more than four hundred miles per minute, whilst in the same time the equatorial parts of the Earth have moved only seventeen miles. A difference between the polar and equatorial diameter of Jupiter is perceptible with a telescope that has a distinct magnifying power of a hundred times, and it is ascertained to be as 12 to 13. The equa

Nothing can be more unmeaning than the apologies that have been offered for substituting (gnosis) "knowledge," for (logos) "reason." By the same rule we ought to change meteorology, physiology, &c. into meteorognosy, physiognosy, &c.

torial diameter of the earth exceeds its polar about twenty-seven miles; the length of the equatorial diameter being 7927, that of the polar 7900 miles.

The relative density of the sun, the earth, and of the other planets, is estimated by the attractive force which they exert on each other, as they move round their common center of gravity. The absolute density or the quantity of matter contained in the earth, compared with an equal bulk of any known substance, may be nearly determined by the attractive force which any given mass of matter exerts upon a plummet (when suspended in its vicinity) to draw it from a vertical line. This will be proportional to the absolute quantity of matter in that mass compared with that of the earth. By this method, it has been found that the mean density of the earth is about five times greater than that of water, or nearly twice the average density of the rocks and stones on the surface.

Hence it has been inferred that the interior part of the earth is solid; or, if it be cavernous, that the solid matter must possess great density. It is not improbable that iron, nearly in a metallic state, may be one of the constituent parts of the central mass, and to this it may owe its magnetic polarity.

Dr. Halley supposed that the earth is a hollow sphere, containing within it a central magnetic globe, and that the revolutions of this globe on its axis, occasioned the variations of the magnetic needle. Laplace, the celebrated French astronomer, asserts, that the nutation of the earth's axis, and experiments on the vibration of the pendulum, indicate an increase of density of the mineral beds, as they approach nearer to its centre, at least to a certain depth from the surface. The rapid transition of motion to very distant parts of the earth during violent earthquakes, renders it probable that there are cavities filled with fluid or gaseous matter, which extend to different parts of the globe, at great depths under the surface.

An opinion has long been entertained, that our planet contains within it a mass of igneous matter, the source of central heat, which is supposed to be an important agent in maintaining the present temperature of the globe, nor are facts wanting to lend support to this opinion. The occurrence of numerous active volcanoes in both hemispheres, and in every degree of latitude; the existence of extinct ancient volcanoes, and of rocks of igneous origin in almost every country; and the numerous hot and warm springs that preserve an unvarying temperature for centuries,-all indicate the existence of a source of heat deeply seated beneath the surface. It seems also to be proved by observations made for the purpose in deep mines, that the temperature of the earth increases as we descend; though at a small distance from the surface, the temperature of the ground and of wells is the same in every season, but it varies in different latitudes. The animals and vegetables whose remains in a fossil state are found in northern climates, are, generally, analogous in structure