wholly a marine deposit. The land we know little or nothing of; sufficient, however, to shew that it was clothed with vegetable life, as in other periods, but little to picture its appearance. Over it, huge Wealden reptiles sought their prey, birds flew, and great apes swung from tree to tree. But the ocean swarmed with varied life, mild sea-breezes blew, and smiling sunbeams sparkled upon its waters; for the climate was warm, as shewn by the corals, reptiles, and monkeys. In the tepid waters lived numberless fishes and shells, and on their surface the nautilus spread its coloured sail. The origin of chalk is a problem not yet satisfactorily settled, but the generally received opinion is, that the shores were fringed by coral reefs, which the dashing waves gradually wore down into fine powder, as they still do in tropical seas; while millions of shell-fish teemed in its waters, and left their white shells as an impalpable sand, that, under the microscope, shews the tiny houses of the old inhabitants as perfect as on the day they died. Flint seems mostly to consist of concretions round sponges, corals, and other substances. It may be found at any epoch, and occurs in many other formations besides the Chalk, though there in greatest abundance.

X. TERTIARY SYSTEM.

We have now arrived at a new epoch in the history of the rocks, known as the Epoch of Recent Life. Henceforward, the plants and animals bear not only a close resemblance to those now existing, but a great proportion of them are identical. We discover real exogenous trees, the same corals, crustaceans, and shells, equal-lobed fishes, birds, and mammals of existing familiesand all these not only more numerous than hitherto, but also more perfectly preserved. The name given to this system is a relic of the names used in early geology, when all rocks were divided into Primary, Secondary, and Tertiary. In the Tertiary System, two great periods are easily distinguishable: 1. The Warm Period; 2. The Cold Period.

1. THE WARM TERTIARY PERIOD. This system exhibits clays, sands loose or hard, gypsum or plaster of Paris, and marls. The only true rock is limestone, made up of innumerable little shells, so numerous, that the rock is named, from its coin-like shells, nummulitic, and is extensively found throughout the world. The limestone is burned for various purposes; the clays are extensively used; the harder sands are employed for building; and amber is also found. The strata occur exclusively in patches known as basins, the London and Paris basins being the most important.

Fossil Remains.-The remains are both numerous and important. Of plants there are few marine specimens, as these seem to have been too tender to be preserved. We find, however, mosses, palms, ferns, leaves, fruits, seeds of different kinds, and whole pods of pea-plants. We have real exogenous timber, with specimens of palm, cypress, and fir.

The animals resemble or are identical with existing species, and the Tertiary System has been divided into periods according to the percentage

From Latin nummus, a coin, and Greek lithos, stone.

of life-remains. We have corals, star-fish of the same species as those existing, and the shells are very beautiful, finely preserved, and scarcely distinguishable from those to be gathered on our present shores.

Mam

Among the fishes, we find various species of the shark, ray, sturgeon, sword-fish; and of freshwater kinds, the perch and the carp. Among the reptiles there are the crocodile and alligator, and the turtle. Birds are numerous, one specimen found in the Paris basin being gigantic. mals are found of every existing order, amongst others, the whale, elk, stag, antelope, camel, llama, tapir, hog, rhinoceros, hippopotamus, beaver, hare, squirrel, monkey, elephant, horse, tiger, and hundreds of others. So numerous are these remains in some parts, that one rock in Norfolk is known as the Mammaliferous Crag. But the most remarkable of the ancient animals are the huge monsters whose skeletons, carefully reconstructed, may be seen in the British Museum, some of them above 10 feet high, and 20 feet long. The most wonderful is the mammoth, with two great tusks like an elephant. Others are the dinotherium1 or fierce beast,' the megatherium or 'great beast,' and the mastodon.3 In different parts of the world, including many places in England, remarkable caves are found filled with bones of various animals in clay or sands, known as 'bone-caves.' These caves have some of them been the dens of savage tigers and other brutes, the bones of their prey being still found; some have formed the abodes of different creatures at different times who have lived and died there; while others have had their contents washed into them by floods.

seas

Scenery. During this period shallow rolled under a genial sun, in which low islands rose crowned with palms, while the savage shark and sword-fish swam in the surrounding waters. The elephant ranged through the tall groves on shore; the hippopotamus wallowed in the freshwater lakes; the rhinoceros crashed through rank jungles; the mastodon, mammoth, and tapir trod in forests of palm; and the wild ox and buffalo roamed over wide grassy prairies.

2. THE COLD TERTIARY PERIOD.-Immediately above the strata just described, with these strange organisms that speak of a warm climate, are found remarkable accumulations of sand, often found pure in hillocks; gravel, and clay interspersed with rounded worn boulders, known under the general title of the 'Boulder-clay,' some of the boulders being of enormous size. From various phenomena, it has been proved that the climate became arctic in character, and our country and others were enveloped from shore to shore in a vast icesheet, like the Greenland of to-day. From the ends of this huge ice mantle, immense masses broke off and floated away as icebergs. By and by, however, the climate became milder, and Britain looked like Norway and Iceland, with glaciers on the higher grounds, reaching here and there to the sea. Gradually the great ice-fields melted away under the rays of the genial sun, and our country looked like the present Switzerland, till at length the last glacier disappeared from the highest hills. The effects of all the wear and movement of these

1 From Greek deines, terrible, and therion, beast. 2 From Greek megas, great, and therion, beast.

3 From Greek miastos, a nipple, and odous, odontos, a tooth, from nipple-like projections on its grinders.

ice-masses, whether grinding down the land or grating on the floor of the ocean, or dashing against opposing islands, are seen in the thick clay and sand deposits everywhere around us, inclosing worn stones and gravel; the scratched and rounded rock-surfaces, often bright and smooth as polished marble; the 'erratic boulders,' perched on our hill-tops and plains; and the general wavy outline of all higher ground throughout our land. This glaciation has been ascertained to extend over the whole of Northern Europe and America, and round the shores of the Antarctic Ocean.

XI. QUATERNARY OR RECENT PERIOD. We have now reached the last of the great geological epochs, during which sea and land, plants and animals, were much the same as they are now. This last system has been variously named the Post-tertiary, Quaternary, or Recent. The whole system may be divided into two chief periods:

1. The Prehistoric, or that before history was written.

2. The Historic, or that since history was

written.

During the whole epoch, there is little or no solid rock, the whole deposits consisting of clay, sand, gravel, mud, peat, and the like.

Of prehistoric deposits, we find such remains as these plants of all kinds, all common shells and corals, and common animals, with a few now extinct, such as the long-fronted ox, the gigantic Irish deer-a creature ten feet high to the top of its horns-the elephant, rhinoceros, hyena, bear, and mammoth, besides human remains in the shape of bones, canoes, ashes, dwellings, and weapons.

In historical times, we find similar remains, but the deposits are comparatively small, and the plant and animal remains are almost solely those now existing in each country. Men have left traces of themselves in buildings, coins, implements, weapons, and works of art. While peatbogs have been formed, forests have been submerged or cut down, and considerable changes in sea-level have taken place.

Since the Glacial Period, in the Tertiary, a series of changes has been going on without intermission,

accomplished by various causes and in various ways. The land has changed its level several times both by elevation and depression, the sea thus alternately encroaching on and retiring from the land. Whole countries have been gained from the ocean, such as the Fens in England and the greater part of Holland. Old beaches may be distinctly traced, with their cliffs, shores, and shells far above sea-level. Whole forests have been submerged, whose old trunks and fruits are thrown up by every storm. Huge accumulations of sand, blown or washed, have been found along its shores. Rivers have been depositing new matter under the ocean at their mouths, increasing the land by the formation of deltas, sometimes hundreds of miles in extent, laying down fine carse-land along their banks, that now forms the richest soil of the farmer, and leaving terraces far above their present level, to mark their former beds. Many lakes have been formed, others have been gradually silting up from the earthy matter brought down by rivers, while some have become dry, their beds being now waving with corn. Animals have been busy forming new islands and continents, as in the Pacific, where the coral insect leaves its skeleton to form the nucleus of future islands. Igneous agencies have been and are as active as in the olden times in changing the land and throwing out vast deposits of lava and ashes.

Man.-It is an interesting question how far back man extends into these geologic eras, and this important inquiry has of late years received great attention. Striking results have also been arrived at. It seems to be indisputably proved that man existed as far back as the great Glacial Period, at the close of the Tertiary epoch, and that he was contemporary with the hairy rhinoceros, mammoth, woolly elephants, and other gigantic creatures now long extinct, which he no doubt hunted, as he now does the fox and the deer; at which period Britain was united to France, where now the sea flows in the Strait of Dover. But this inquiry is yet in its infancy, and it would, therefore, be unwise to make positive statements where our data are insufficient. Enough has, however, been discovered to shew that in our own quiet land men lived for many ages with strange denizens, and under conditions very different from the present.

regulate weather, seasons, and climates. It involves particularly the consideration of the atmosphere-its pressure; its moisture; the alterations made upon it by heat and cold; and its electrical condition. It is by an accurate knowledge of these that we are enabled to understand the nature and causes of the incessant movements and changes going on in the air around us, and of the ever-varying appearances of the sky over our head; including the phenomena of winds and storms, of rain, dew, hail, snow, mists, and clouds, and their connections with one another, and with seasons and places.

THE ATMOSPHERE.

Its Composition-General Structure-Density-Pressure. The atmosphere is a vast ocean of invisible gaseous matter, enveloping the terraqueous globe, and extending to a considerable height. Although it cannot be seen by the eye, it is yet felt to be an inert, material mass, which resists bodies in their motion through it, and when set in motion itself, possesses momentum or impetus, like a flying ball or a running stream. Another property of the atmosphere, which proves its genuine materiality, is its weight or pressure. It presses upon the earth, exactly as the sea does, under the influence of gravitation. At the sea-level, the average pressure is 14 pounds on every square inch; it is nearly the same as the pressure of a lake of water 33 feet deep, or a lake of mercury 2 feet The construction of the barometer, or deep. instrument for constantly measuring this weight, and shewing the degrees of its fluctuation from day to day, is explained under PNEUMATICS.

The fluid of which the atmosphere consists, is found to be not a single substance, but a mixture of several substances, totally distinct in their properties, and serving quite different offices in the economy of nature. The two chief ingredients-nitrogen and oxygen-which make up more than 8ths of the whole mass, are as different in their character as water and alcohol. The proportions of these two are very nearly 77 of nitrogen to 23 of oxygen by weight. The nitrogen is therefore the principal element in point of quantity; but the oxygen performs the greatest variety of functions: it is the supporter of life, and the indispensable agent in combustion, in putrefaction, and in many other natural pro

cesses.

Deferring in the meantime the consideration of the other ingredients of the atmosphere, we have to study in the first place the mode of mixture of these two gases, and the general structure of the mass they compose. Although the mechanism and constitution of a gas are not apparent to our senses, yet we can infer with certainty that it is made up of atoms or molecules which keep one another at a distance by a repulsive force. From the fact that any gas can be compressed into a very small fraction of its ordinary bulk, we are sure that its particles are usually at a great distance

Now, suppose two flasks connected by a tube having a stop-cock, by which communication can be cut off, and suppose the one filled with nitrogen, and the other empty; on turning the stop-cock, part of the nitrogen will rush into the empty flask until it is equally spread over the whole space; for there is no stable balance of the repulsion if a particle is nearer its neighbour on one side than on the other. This uniform diffusion takes place almost instantaneously when the one flask is empty. But now, suppose both filled, the one with nitrogen, and the other with oxygen; when the communication is opened, each gas spreads itself uniformly through the whole space of both flasks, precisely as the nitrogen alone did, only in this case time is required. The gases mutually the progress of those of the other, but do not interpenetrate, and the molecules of the one retard otherwise affect the ultimate result. Each gas is independent, and distributes itself, and exerts its It is owing to this important principle that the pressure precisely as if the other were not there. proportion of oxygen to nitrogen is the same in all parts of the world and at all heights. The same law of uniform diffusion, and independence of the presence of other gases, holds with regard to the carbonic acid gas and the aqueous vapour existing in the atmosphere, although, in the case of aqueous vapour, its constant liability to condensation prevents its ever attaining to a state of actual uniformity.

sidered as an ocean of gaseous or elastic fluid, its Taking the entire mass of the atmosphere, condensity must diminish as we ascend from the earth. The rate at which the density of the air height, are complicated by the decrease of temdiminishes upward, and the estimation of its entire perature that is found to take place as we ascend (see page 41). It may be stated here, in a general way, that one-half of its material mass lies within three miles from the earth, and three-fourths of it within less than six miles. From the observation of luminous meteors, it is inferred that it is at least one hundred miles high, and that, in an extremely attenuated form, it may even reach two hundred miles.

MOVEMENTS OF THE ATMOSPHERE. All the movements and changes to which the air is subject, originate, some way or other, in the unequal distribution of heat.

If two adjoining spots of ground are of unequal temperature, and communicate an unequal temperature to the columns of air lying upon them, the column which is most heated will be expanded, so as to overtop the other, and will also be made rarer or lighter. Two effects will arise from this : a lateral or horizontal movement of the air from the cold to the warm column will take place below, according to the general law of hydrostatics, that a heavy fluid buoys up a light one. But if we consider the condition of the two columns above, or at their upper ends, we will find that the

When a difference of temperature exists between two spots that lie near one another, the effect of the aerial currents which take place between them is to equalise the temperature. Its lateral currents are the effect of unequal heating, and in some measure the remedy. These lateral currents are felt and recognised by us under the name of winds. If the differences of warmth that originate them were only limited and temporary, | the carrying of heat to and fro would bring about an equality, and then the air would come to a perfect repose; but these differences of warmth are perpetually kept up; hence the lateral currents go on for ever without ceasing. The middle band of the earth, or the equatorial zone, is exposed to the burning radiance of a direct sun, while the two polar regions are so faintly heated, that snow never melts upon them; and this great standing inequality keeps up two grand sets of movements, which encompass the globe. The equatorial air, being warmer and lighter below than the air on each side, is buoyed up by the cold masses moving in upon it from north and south, and thus two great under-currents are maintained between the poles and the equator. The equatorial mass being expanded far upwards, overtops and overpresses the upper portions of the colder columns on each side, and therefore flows in upon them on both sides; thus causing two great upper-currents towards the poles, while the under-currents are moving from the poles. The inequality of the equatorial and polar regions in respect of heat is thus in some degree mitigated; but it is by far too great to be entirely done away. We have supposed, for simplicity's sake, that the atmosphere is divided into an equatorial and two polar masses; but in reality there is a constant gradation from the equator to the polar regions, and the movements will take place between every two adjoining portions of atmosphere of which one is farther north or south than the other.

Although, as a general rule, the temperature of the earth decreases from the equator to the poles, this is not strictly or at all times true. Various causes occur to render a high latitude as warm as, or warmer than a lower, and in such a case a contradiction will arise to the general movement, which will have peculiar local consequences.

VAPOUR OF THE ATMOSPHERE. Vaporisation-Dew-Mist-Clouds-Rain-Hail-Snow. Next in quantity to nitrogen and oxygen, although very small compared with these, is the vapour of the atmosphere; that is to say, the gaseous water or steam that is constantly present in it. Although comparatively small in amount, this ingredient of the atmosphere is of immense importance in its effects. Unlike the other gases, it is easily reduced from the aërial to the liquid

It is essential to bear in mind that true gaseous water, steam, or elastic vapour of water, which all mean the same thing, is invisible, like the other portions of the atmosphere; and that the white cloud that appears at the chimneys of steamboats and locomotives, and at the spout of a kettle, is not gas or elastic vapour, but vapour partially condensed, whose particles only require to be brought together to become drops of water. It is, in fact, water in the form of something like dust. Mist, fogs, and clouds are of the same character: they are not gaseous steam, but precipitated watery particles destitute both of mutual repulsion and of the latent heat of steam. When the atmosphere contains nothing but true steam, it is transparent and cloudless.

The formation of steam out of water is most conspicuous in the process of boiling, where the surface is kept in an intense bubbling state, each bubble containing a mass of steam, which forces its way up into the air. This boiling takes place at 212° of the thermometer, called for that reason the boiling-point. The steam thus formed has an elastic force equal to the pressure of the atmosphere, or a force of 143 pounds to the square inch. The reason for the violent escape of steam at 212° is, that it has attained a force equal to the weight of the atmosphere pressing on the water, and is therefore able to set aside this pressure, or make its way in spite of it. But even at temperatures below boiling, water passes into steam slowly and invisibly. It is well known that a wet surface soon becomes dry; in other words, that the water upon it disappears. Water, however, cannot be annihilated; in the drying process, the liquid water becomes gaseous invisible water or steam, and mixes with the other steam contained in the air.

The rapidity of the process of drying up, or of the passing of water into steam, depends, in the first place, on the temperature of the water. We have seen that it is abundant and violent at 212°; and it is less and less for every degree downwards. The elastic force of the steam produced also depends upon the temperature. The elastic force of steam at the boiling-point is equal to the pressure of the atmosphere; while the elastic force of steam produced silently at 80° is only th of the elasticity of the atmosphere, or equal to one inch of the barometer.

Now, the entire quantity of steam that can rise is limited by the temperature in the same manner as the elasticity is limited. Water at 80° will give forth vapour, until as much has been produced in the atmosphere as would counterbalance one inch of mercury; evaporation then ceases, and the air is said to be saturated with steam. Whether the steam rise freely into the atmosphere, or rise into a vacuum, no more will be produced at 80° than this quantity. If the temperature were raised to