Pagina-afbeeldingen
PDF
ePub

divide, nations; but in general the boundaries are arbitrary, so that the natural geography of a country may be regarded as a sequel to the science, which is chiefly occupied in describing the diversities of nations, and the conditions of the various races of mankind.

The ancients considered the globe under the three grand divisions of Asia, Europe, and Africa. Here the distinctions were arbitrary, as they often included Egypt under Asia, and they had not discovered the limits of Europe toward the north-east. Modern discoveries have added a fourth division, that of America, which exceeding even Asia in size, might have been admitted under two grand and distinct denominations, limited by the isthmus of Darien. Till within these last thirty years it was supposed that a vast continent existed in the south of the globe; but the second navigation of captain Cook dispelled the idea, and demonstrated, that if any continent existed there, it must be in the uninhabitable ice of the south pole. The vast extent of New Holland rewarded the views of enterprise; this, which seems too large to be classed among islands, has been ranked as a fifth division of the globe by various geographers of the present day, and distinguished by the name of Australasia, which term, however, includes also the encircling islands.

Of the grand divisions of the earth, Asia has ever been esteemed the most populous; and is supposed to contain five hundred millions of souls, if China, as has been averred by the latest writers, comprises three hundred and thirty millions. The population of Africa may be estimated at thirty millions, of America at twenty millions, and one hundred and fifty millions may perhaps be assigned to Europe.

Modern discoveries have evinced that more than two-thirds of the globe is covered with water, which is contained in hollow spaces, or concavities, more or less large. But the chief convexities or protuberances of the globe consist of elevated uplands, sometimes crowned by mountains, sometimes rather level, as the extensive protuberance of Asia. In either case, long chains of mountains commonly proceed from those chief convexities in various directions, and the principal rivers usually spring from the most elevated grounds.

The grandest coucavity of this globe is filled by the Pacific Ocean; occupying nearly half its surface from the eastern shores of New Holland, to the western coast of America, and diversified with seve.

ral groups of islands, which seem in a manner the summits of vast mountains emerging from the waves. This ocean receives but few rivers, the chief being the Amur from Tartary, the Hoan Ho and Kian Ku from China, while the principal rivers of America run towards the east.

Next to this in magnitude is the Atlantic, between the Old and New Continents; and the third is the Indian Ocean. The seas be tween the arctic and antarctic circles and the poles, have been sometimes styled the Arctic and Antarctic Oceans; but the latter is only a continuation of the Pacific, Atlantic, and Indian Oceans; while the Arctic Sea is partly embraced by continents, and receives many important rivers. Besides these, there are other seas more minute, as the Mediterranean, the Baltic, and others still smaller, till we come by due gradation to inland lakes of fresh water.

The courses of rivers are sometimes marked by oblong concavi. ties, which generally at first intersect the higher grounds, till the declivity becomes more gentle on their approach to their inferior receptacles. But even large rivers are found sometimes to spring from lowland marshes, and wind through vast plains, unaccompanied by any concavity, except that of their immediate course; while on the other hand, extensive vales, and low hollow spaces, frequently occur destitute of any stream. Rivers will also sometimes force a passage where nature has erected mountains and rocks against it, and where the concavity would appear to be in another direction, which the river might have gained with more ease. In like manner, though the chief mountains of Europe extend in a south-easterly and north-westerly direction, yet there are so many exceptions, and such numerous and important variations in other parts of the globe, as to render any attempt at a general theory vain.

From the vast expanse of oceanic waters, arises in the ancient hemisphere, that wide continent, which contains Asia, Europe, and Africa; and in the modern hemisphere, the continent of America, which forms a kind of separate island, divided by a strait of the sea from the ancient continent. In the latter many discoveries of great importance to geography, are of very recent date, and it is not above sixty years since we obtained an imperfect idea of the extent of Siberia and the Russian empire, nor above twenty-five since ample, real, and accurate knowledge of these wide regions began to be diffused. So that, in truth, America may be said to have been discovered by Europeans before many parts of Asia; and of Africa

our knowledge continues imperfect, while the latest observations, instead of diminishing, rather increase our idea of its extent, at least in regard to its insular appendages.

But the grandest division of the ancient continent is Asia, the parent of nations, and of civilization: on the north-east and south, surrounded by the ocean; but on the west, divided by an ideal line from Africa; and from Europe by boundaries not very strongly impressed by the hand of nature. The Russian and the Turkish empires, extending over large portions of both continents, intimately connect Asia with Europe. But for the sake of clearness and precision, geographers retain the strict division of the ancient continent into three parts, which, if not strictly natural, is ethical, as the manners of the Asiatic subjects of Russia, and even of Turkey, differ considerably from those of the European inhabitants of those empires.

CHAP. VI.

SUBTERRANEAN PHENOMENA OF THE GLOBE.

:

Ir is only the mere surface or solid crust of the globe of which the ingenuity of man has hitherto been able to obtain any degree of knowledge of the centre, or even the parts below the surface, we are totally ignorant. So far, however, as we have been able to examine into the nature of its contents and structure by means of rivers, ravines, mines, earthquakes, and other causes, it consists of immense masses of rocks and veins; of volcanoes and other caverns; of wells, streams, and peculiar exhalations; of metallic mines and petrified fossils; and of whole towns and forests subverted and in a state of ruins. To each of these we shall direct the reader's attention in the subsequent chapters of the book before us, and nearly in the order in which we have thus enumerated them, and shall at the same time take a glance at whatever else may incidentally occur to us, whether of natural or artificial origin, that from its form, rarity, or any other circumstance, may be an object worthy of general attention.-Editor.

CHAP. VII.

ROCKS AND VEINS.

THE stony masses of which the earth, as far as we know it, is

composed, are numerous, and they are found laid one above another; so that a rock of one kind of stone is covered by another species of rock, and this by a third, and so on. Now in this superposition of rocks it has been observed, that their situation is by no means arbitrary; every one occupies a determinate place, so that they follow each other in regular order from the deepest part of the earth's crust, which has been examined, to the very surface. Thus there are two things respecting rocks which claim our attention; namely, their composition, and their relative situation. But besides the rocks which constitute almost the whole of the earth's crust, there are masses which must also be considered. These traverse the rocks in a different direction, and are known by the name of veins, as if the rocks had split asunder in different places from top to bottom, and the chasm had been afterwards filled up with the matter which constitutes the vein.

Thus it appears, that when we consider compound minerals, or rocks, the subject naturally divides itself into three parts; namely, 1. The structure of rocks; 2. The situation of rocks; 3. Veins. These shall form the subject of the three following sections.

SECT. I.-Of the Structure of Rocks.

Rocks may be divided into two classes; viz.

I. Simple, or composed of one mineral substance.

II. Compound, or composed of more than one mineral sub

stance.

Compound rocks are of two kinds; namely,

1. Cemented; composed of grains agglutinated by a cement, as sand-stone.

II. Aggregated; composed of parts connected together without a cement, as granite.

The aggregated rocks are likewise of two kinds; namely,

I. Indeterminate.

Only one instance of this kind of aggregation has hitherto occurred, namely in the older serpentine, where limestone and serpentine are so conjoined, that it is difficult to say which predominates.

II. Determinate.

The determinate are either, I. Single aggregated; or, II. Double aggregated.

There are four kinds of single aggregated rocks; namely,

1. Granular; composed of grains whose length, breadth, and thickness are nearly alike, and which are of contemporaneous for. mation. As granite, sienite.

2. Slaty; composed of plates laid above each other; as mica slate. 3. Porphyritic; composed of a compact ground, containing in it crystals which appear to have been deposited at the time the rock was formed; as common porphyry.

4. Amygdaloidal; composed of a compact ground, containing in it vesicles which appear to have been afterwards filled up; as amygdaloid.

There are five kinds of double aggregated rocks; namely, 1. Granular slaty; composed of slaty masses laid on each other. Every individual slate is composed of grains cohering together; or it is slaty in the great, and granular in the small; a gneiss. 2. Slaty granular; composed of large granular masses cohering together; each grain is composed of plates; or the rock is granolar in the great, and slaty in the small; as topaz rock. 3. Granular porphyritic; granular in the small, and porphyritic in the great; as granite, greenstone frequently.

4. Slaty porphyritic; slaty in the small, porphyritic in the great; as mica slate frequently.

5. Porphyritic and amygdaloidal; a inass porphyritic and amygda

loidal at the same time; as amygdaloid and basalt frequently. Such are the different kinds of structures of rocks hitherto observed and described. The following Table will give the reader a synoptical view of these different kinds of structure:

I. Simple rocks

II. Compound rocks

A. Çemented

« VorigeDoorgaan »