by distinct assemblages of organic remains-are of less universal occurrence, and frequently vary in their composition in different localities. Allowing, however, for these local diversities, a general correspondence is observable in the structure and arrangement of rocks in all parts of the earth, and indications everywhere present themselves, that similar agency has been employed in the formation of the whole.

A layer of alluvial soil, or of soil deposited by water, &c. and of recent formation, usually constitutes the upper coating of the earth's surface, especially in plains and valleys. This deposit is chiefly formed of particles abraded, or worn from older rocks, by the action of water, of the atmosphere, or of electricity; the whole surface of the earth being thus in a greater or less degree, in a state of perpetual change.

Within this wide great universe

Nothing doth firme and permanent appeare,

But all things tost and turnéd by transverse.

Water is a very powerful and active agent in altering and variously modifying the surface of the earth. The atmosphere, also, effects great changes on rocks; the differently shaped summits of mountains and hills, owing much of their form to the destroying influence of the weather, which acts with varied effects on differently constituted rocks. Electricity, again, both directly and indirectly, produces an infinity of changes. When we speak of electricity, however, we must not be understood as referring solely to the more striking effects of the electric fluid, but rather to its silent and unobserved operation; which in the economy of nature, is far more important than the former, though vast effects are occasionally produced by lightning. Thus, at Cayamba, in Colombia, about forty years ago, the lightning reft a cliff in its entire length, and precipitated a mighty mass into the river Huallaga, whose course was at the time obstructed; and the river now forces its way, with much violence and uproar, through the massive fragments of the shattered cliff.

But it will be evident that the changes produced by the

above-mentioned agents (at least in their external operation,) are limited to the wearing away, and consequent lowering of the more elevated parts of the earth's surface, and that all appear to have a tendency to reduce the whole to one general level. We must, therefore, look for some other agent, some opponent force, to counteract these effects, and accomplish the more remarkable work of elevating mountain ranges, and of raising land above its former level. Such is subterranean heat, displayed in the phenomena of earthquakes and volcanos: and probably also, although less obviously, in the gradual rise which is taking place in some regions. That a connexion exists between earthquakes and volcanos, has been proved by several well-authenticated facts; and that both owe their origin to subterranean heat, is generally admitted, though the mode of its operation may be regarded as still involved in obscurity. The disturbances it produces on the earth's surface, may, however, be regarded as some of the most important phenomena which it belongs to physical geography to describe*.

Some of our readers may, nevertheless, not be aware of the magnificent scale, on which the energy of subterranean heat is at present developed on the surface of the globe; for these commotions in the earth's crust, though, on account of the dreadful catastrophes to which they occasionally give rise, almost unavoidably universally regarded as appalling phenomena, are perhaps very generally considered as detached events of rare occurrence; remarkable, it is true, from the destruction of property, and awful loss of life, with which they have, in many instances, been accompanied, but of no further account in the natural world. Nay, in this highly favoured island, we are so far removed from the theatre of volcanic action, that the very circumstance of these powerful agents being continually at work

*The consideration of these phenomena is included in the department of physical geography; the application of their effects, as illus trations of changes on the earth's surface, belongs to geology.

in great force, in various parts of the globe, is perhaps altogether unheeded by many individuals. And thus

Some souls there are that live, and breathe, and die,

Scarce knowing more of Nature's potency,

Than what they learn from heat, or cold, or rain,

Or sad vicissitude of weary pain.

Volcanos and the movements of great earthquakes are confined to certain districts of the earth, a circumstance which has led an eminent geologist to designate the tracts where they more especially occur, as volcanic regions, or volcanic bands. These volcanic regions are continuous tracts, marked by the occurrence, at intervals, of active volcanic vents, often constituting mere spots, thinly scattered over the surface of vast districts, but presenting abundant evidence that subterranean fire is at work in the intermediate spaces, by the earthquakes which from time to time take place. Gaseous vapours also are plentifully disengaged from the soil; petroleum, and other bituminous substances, sometimes issue from the ground in great abundance; whilst thermal or hot springs often occur, possessing a very high temperature, and it not unfrequently happens that the temperature of springs is raised after earthquakes, even in districts distant from active volcanos. These volcanic regions, or bands, appear to have a general correspondence with the direction of the most elevated land in the districts they traverse. Thus, in Europe and Asia, the volcanic band runs parallel to the grand mountain ranges of those regions; and in America, it extends along the line of the mighty Andes. In more scattered volcanic groups, such as the Polynesian volcanic region, we usually find one island forming a central focus; of this the island of Hawaii forms an instance.

The principal volcanic regions, or bands, are those extending from the Caspian Sea to the Azores; from the Aleutian Isles to the Moluccas; and the volcanic region of the Andes. To these may be added, though of inferior importance, the African volcanic region, and the Icelandic volcanic region.

15

CHAPTER III.

GENERAL DISTRIBUTION OF LAND AND WATER.-MOUNTAINS, VALLEYS, PLAINS.-TABLE OF HEIGHTS OF MOUNTAINS, ETC.

On referring to the map of the world, we cannot fail to be struck with the unequal distribution of land and water on its surface, and with the large proportion covered by the waters of the ocean. The whole superficies of the globe (speaking in round numbers) is equal to 197,000,000 square miles, nearly; of this, the land does not occupy much more than 52,000,000 square miles, or little above one-fourth part ; whilst the ocean covers an area of more than 144,000,000 square miles. We shall further observe, that the proportion of dry land is much greater in the northern than in the southern hemisphere. The Antarctic regions are, however, too little known for any certain conclusion to be drawn respecting the relative proportions.

The sea occupies the cavities or depressions on the surface of the globe, separating and surrounding the more elevated portions of the earth's crust. The dry land may be arranged under two principal heads,-continents and islands. The term Continent has been applied to four portions of the earth, and it is common to speak of the continents of Europe, Asia, Africa, and America; but more correctly, there are only two continents,-two vast and detached portions of the globe,—the Old World and the New World; nay, in fact, these are but larger islands, though, on account of their vast extent, and the different natural phenomena presented by continental and insular surfaces, it will be highly desirable to distinguish these divisions of the earth by the designation of Continents.

The remaining portion of the dry land consists of islands of various dimensions, and of various elevation above the sea. Australia so greatly exceeds in size all other islands, that it has been ranked as a continent, but is, perhaps, with greater propriety included among the islands; it may however, in conjunction with the numerous smaller islands with which the Pacific is studded, be regarded as a distinct division of the globe. This is sometimes distinguished by the appellation Oceanica.

The general disposition of the land will be observed to differ greatly in the Old and New Continents. In the Old World, especially in that portion which comprehends the divisions of Europe and Asia, the principal extension is from east to west; whilst in the New World, it is from north to south. In both cases, this extension is in the direction of the principal mountain ranges. Thus, in the Old Continent, a continuous, or nearly continuous line of mountains, extends from the borders of Europe, on the Atlantic, to those of Asia, on the Pacific Ocean. In Africa the mountain ranges are less known. The Atlas range, which stretches in the same direction as the great European and Asiatic line, is considered to form a portion of that vast range. The greatest length, however, of Africa is from north to south; and although the interior of this portion of the globe, is still too much involved in obscurity for any certain conclusions to be formed, yet, from the general direction of the rivers, and from the notices of travellers, there is much reason to infer, that a range of mountains more or less interrupted, extends from Abyssinia towards the southern extremity of Africa.

This general correspondence between the form of the land and the direction of mountain ranges, is more strikingly exemplified in America, where the Rocky Mountains, and the elevated districts of Mexico, in the northern, and the stupendous Andes in the southern, hemisphere, pursue in a remarkable manner the general direction of the New Continent. The same character is observable in smaller portions