Part 10

The greater part of the land on the earth’s surface is grouped into two great _hemispheres_, the Old and the New World. The former and far larger of these consists of Eurasia in the north, separated by ill-defined boundaries from Europe to the west and Asia to the east, and of Africa in the south, united to Eurasia by the narrow neck of the isthmus of Suez. The hemisphere of the New World is divided into North America and South America, united by the long, narrow isthmus of Central America. The island of Australia is also reckoned as a continent. It is believed that an island continent, Antarctica, surrounds the South Pole. Of islands not reckoned as continents, the largest is the polar island of Greenland.

CERTAIN RESEMBLANCES OF THE CONTINENTS

In comparing the continents, we at once notice certain resemblances. The first is the tapering to the south, which is seen in Greenland, North and South America, Africa, and Australia (Tasmania). Another is the southward-running peninsulas which characterize Europe and Asia. We may notice, too, that the general lines of the Old World, broad in the north, tapering in the south, resemble those of the New World, especially if we include Australia (Tasmania), and compare its position with that of South America. There is also a certain uniformity in the distribution of relief. Notice the so-called Mid-World and Pacific Mountain systems, which may be traced in the mountains of Central Europe, North Africa, Central Asia, the islands of the Pacific from Japan to New Guinea, and the lofty mountains of North, Central, and South America.

COMPARISON OF THE CONTINENTS

+---------------------+------+------+------+------+------+-----+------+ | |=Asia=|=Afri-|=North|=South| =Eu- |=Aus-| =All | | =Continent= | | ca= |Ameri-|Ameri-|rope= |tra- |Land= | | | | | ca= | ca= | |lia= | | +---------------------+------+------+------+------+------+-----+------+ |Area (million square | | | | | | | | |miles) | 16.4| 11.1| 7.6| 6.8| 3.7| 3.0| 55.0| |Average Height (feet)| 3,000| 2,500| 1,900| 2,000| 940| 800| 2,100| |Highest Point (feet) |29,000|18,800|18,200|22,400|18,500|7,200|29,000| | | | | | | | | | |PERCENTAGE AT VARIOUS| | | | | | | | | ALTITUDES (feet) | | | | | | | | | | | | | | | | | |Below Sea-Level | 1.4 | 0.1 | 0.05| 0.0 | 1.8 | 0.0| 0.6| | 0 to 600 feet| 23.3 | 12.5 | 32.25| 40.0 | 53.8 | 29.8| 26.7| | 600 to 1,500 feet| 16.0 | 34.8 | 32.1 | 26.8 | 27.0 | 64.3| 27.8| | 1,500 to 3,000 feet| 21.7 | 27.6 | 13.3 | 16.8 | 10.0 | 4.1| 19.3| | 3,000 to 6,000 feet| 21.8 | 21.8 | 13.2 | 7.0 | 5.5 | 1.5| 17.0| | 6,000 to 12,000 feet| 10.0 | 2.8 | 8.4 | 5.0 | 1.7 | 0.3| 6.0| | Above 12,000 feet | 5.8 | 0.4 | 0.7 | 4.4 | 0.2 | 0.0| 2.6| +---------------------+------+------+------+------+------+-----+------+

THE SHAPING OF THE COAST

The coast line, or margin of sea and land, is an area rapidly wearing away under the ceaseless influence of the waves, and of the sand and rock, they are perpetually hurling to and fro. Coasts may be either flat or high, composed either of hard or soft rock, and either submerged or raised. A submerged coast is one where the land has sunk or the sea has risen, so that the low grounds and valleys are flooded. A raised coast is one where the land has risen or the sea has retired, and what was formerly the sea bottom is bared.

A flat coast is usually sandy, often bordered by sandhills and lagoons. It may be carved into cliffs, as in the clay cliffs of Norfolk, England. A raised coast is usually flat from the long-continued action of the waves during the period when it was submerged. Flat coasts have no good harbors.

A submerged coast differs according to the nature of the submerged region. If this was hilly or mountainous, with valleys running parallel to the shore, the coast will be ironbound and harbor-less unless the sea-level has risen sufficiently to give access to the valleys behind the first range of heights. If this happens, T-shaped gulfs are formed. Where the valleys open at right angles to the sea, they become bays, usually with excellent harbors. The hills between the valleys rise as peninsulas, or islands. If the land was flat before submerging took place, a flat coast is the result.

Where the land is composed of soft rocks, a more uniform coast-line results than where it is composed of harder rocks, or of hard and soft rocks mixed. The waves, in eating out the softer rocks, often form magnificent sea-caves, natural arches, and pinnacles.

THE COASTLINE OF THE VARIOUS CONTINENTS

EUROPE surpasses all the other continents in the magnitude of its indentations and projections. Three great peninsulas--the Balkan peninsula, Italy, and Spain, project into the Mediterranean; while Brittany, Denmark, and Scandinavia jut into the shores of the Atlantic. Even the British Isles are scarcely more than a projection of the continent.

ASIA is a second in the relative extent of its peninsula. Asia Minor on the west, Arabia, India, and Indo-China on the south, and China, Manchuria with Corea and Kamchatka, advancing into the waters of the Pacific, form a wide border of projecting lands, containing the richest regions of the continent.

NORTH AMERICA is considerably less indented. Florida, Nova Scotia and Labrador are more prominent on the Atlantic coast, and California Peninsula and Alaska on the Pacific.

The southern continents on the contrary, are nowhere deeply penetrated by the waters of the ocean. The Gulf of Arica in South America, the Gulf of Guinea in Africa, and the Great Australian Bight, are merely gentle bends in the coast line.

LOCATION OF THE GREAT PLAINS OF THE WORLD

Plains occupy nearly one-half of the surface of the continents. They are most extensive and unbroken on the Arctic slopes of the Old World, and in the interior of the two Americas.

Treeless plains, whose vegetation consists of grasses and other herbaceous plants, or stunted shrubs, occur in every continent, and are designated by a variety of terms. Wherever treeless plains are subject to periodical rains, they lose their verdure in the season of drought, and assume the aspect of a desert; but they resume their freshness on the return of the rain, and many are adorned with a great variety of beautiful flowers.

PLAINS OF THE OLD WORLD. The great Siberian plain extends from the northeastern extremity of Asia to the Ural Mountains and Caspian Sea; and the European plain stretches from the Ural westward, through Russia and North Germany, to the lowlands of Holland.

The plains of the Caspian Sea and western Siberia are dreary steppes, covered with coarse grasses, often growing in tufts, alternating with patches of heather, furze, dwarf birch, and other stunted shrubs; or old sea bottom, covered with salt efflorescence. Immense reaches of flat country, near the Arctic shores of Asia and Europe, consist of frozen marshes, called tundras, where mosses and lichens are almost the only vegetation. Those of eastern Europe and Asia are denominated steppes; while more limited treeless regions in western Europe are called landes and heaths.

On the alluvial plains of the Old World, civilization began and developed; and their inexhaustible fertility supplied the wants of the most populous nations of antiquity. The great centers of ancient civilization in Egypt, China, India and Babylonia, all had their growth in alluvial plains, built up and fertilized by the mighty rivers which traverse those countries.

PLAINS OF THE NEW WORLD. In North America the great _Central Plain_ extends, with but slight interruptions, from the Arctic shores to the Gulf of Mexico. The fertile, treeless plains are termed “prairies” (meadows), while the sterile ones, east of the Rocky Mountains, are known as “the plains.” There are vast cane fields and forests in the lower Mississippi Valley.

In South America the plains of the Orinoco basin, the _Selvas_ of the Amazon, and the _Pampas_ of the La Plata, form an uninterrupted series of lowlands which, continued by the plains of Patagonia to the southern extremity of the continent, extend over a distance of three thousand five hundred miles from north to south. The Spanish term “llano” (plain), and the Peruvian “pampa,” designate the treeless plains of the Orinoco and La Plata basins. The Llanos of the Orinoco, during one-half of the year are covered by the richest pasturage, bright with flowers, but during the other half are a parched waste. The Selvas of the Amazon, a luxuriant forest, cover more than a million square miles; and the treeless Pampas, with their tall grasses and thickets of clover and thistles, illustrate the endless richness and variety of nature.

Alluvial and marine plains generally have but a slight altitude, while the undulating plains are sometimes considerably elevated. The Mississippi Valley, at St. Louis, one thousand miles from the ocean, is hardly four hundred feet above the sea-level; and the Amazon, at an equal distance from the sea, does not reach two hundred and fifty feet. The marine plains adjacent to the Caspian and Aral seas are still lower, the larger portion being below the sea-level.

SITUATION, ELEVATION AND SOIL OF PLATEAUS

Plateaus are situated either between two lofty mountain chains, which form their margins, or descend by successive terraces to the nearest seas; or they pass, by gradations, from the base of high mountains to the low plains in the interior of the continents.

The Great American Basin, between the Rocky and Sierra Nevada Mountains, and the plateau of Tibet, between the Himalaya and Kuenlun mountains, are examples of the first position; and the table-land of Mexico, of the second. The third is seen in the high plains at the eastern foot of the Rocky Mountains, which descend from an altitude of five thousand or six thousand feet, at the foot of the mountains, to the low plains of the Mississippi basin.

The plateaus most remarkable for their elevation are, Tibet, from ten thousand to eighteen thousand feet above the sea; and the elongated valley-like highlands, from ten thousand to thirteen thousand feet high, between the two chains of the Andes, in South America. East Turkestan and Mongolia, in central Asia; the plateau of Iran, in western Asia; Abyssinia, and the vast plateau which occupies all the southern part of Africa; and the broad table-land which fills the western half of North America with a continuous mass of high land, range in height from four thousand to eight thousand feet.

The great peninsulas of Deccan, Arabia, Asia-Minor and Spain, the central plateau of France, and those of Switzerland, Bavaria, and Transylvania, vary from one thousand to four thousand feet in elevation.

SOIL AND CLIMATE OF PLATEAUS

The nature of the soil and climate of great plateaus is in general such as to render them the least useful portions of the continents. Sahara, with an average altitude of 1,000 feet, and the higher plateaus of Mongolia, Iran and parts of the American Basin, may serve as types.

Their surface consists of hardened sand and rock; of hillocks and plains of loose sand constantly shifting by the wind; and of immense tracts, as in Mongolia, covered with pebbles varying from the size of a walnut, or even less, to a foot in diameter: all indicating the original transporting, grinding and depositing of these materials by water.

Salt lakes without outlet occur in each, and salt efflorescence often covers the ground. A lack of rain to wash from the soil substances injurious to vegetation, and supply the water necessary for the growth of plants, leaves these plateaus generally sterile, and some of the most extensive are in part, if not wholly, deserts.

MOUNTAINS AND THEIR STRUCTURES

Mountains rise in long and comparatively narrow lines or ridges, the tops of which are often deeply indented, presenting to the eye the appearance of a series of peaks detached one from another. As each of these peaks or distinct elevations is called a mountain and often receives a separate name, the common designation chain or range of mountains is naturally applied to the whole.

The top of the ridge, from which the waters descend on opposite sides, is called the crest; and the notches between the peaks, from which transverse valleys often stretch like deep furrows down the slopes of the chain, are called passes.

HOW MOUNTAIN CHAINS FORM SYSTEMS

Mountain chains are seldom isolated, but are usually combined into _systems_, consisting of several more or less parallel and connected chains, with their intervening valleys,--as the Appalachian system, the Alps, and the Andes.

Most mountain chains seem to have been produced by tremendous lateral pressure in portions of the Earth’s crust, causing either long folds, or deep fissures with upturned edges rising into high ridges, the broken strata forming ragged peaks.

TWO TYPES OF MOUNTAIN CHAINS

Mountains by folding are generally of moderate elevation, while mountains by fracture include the highest chains of the globe. The Appalachian Mountains in North America, and the Jura in Europe, are examples of the first; the Rocky Mountains, Andes, Alps and Himalayas, of the second.

Folded mountains are curved into long arches, either entire or broken at the summit and forming a system of long, parallel ridges, of nearly equal height, separated by trough-like valleys. Here and there, however, deep gaps, or gorges, cut the chains allowing the rivers to escape from one valley to another.

In systems of mountains produced by fracture, there is usually one main central chain, with several subordinate ranges. They have, however, less regularity and similarity among themselves than the parallel chains of mountains by folding.

The crests are deeply indented, cut down one-third or one-half the height of the range, forming isolated peaks and passes which present to the eye the appearance of a saw, called in Spanish Sierra; in Portuguese, Serra. Such ranges are frequently distinguished by these terms, as the Sierra Nevada, in North America; and the Serra do Mar, in Brazil.

HOW VALLEYS ARE FORMED

Valleys among mountains owe their existence primarily to folds or fissures in the Earth’s crust, produced in the upheaving of the ranges; but they are subsequently deepened, widened and otherwise changed in form and extent, by the action of rains and frosts, and the streams to which they furnish a pathway. Most of the Alpine lakes, celebrated for their picturesque beauty, occupy deep basins at the outlet of transverse valleys.

Valleys in plains and plateaus are mainly, if not entirely, the result of the erosion, or wear of the surface, by running water.

Little rills, formed by the rains or issuing from springs, set out on their course down the slope of the ground, each wearing its small furrow in the surface. Uniting they form a rivulet which wears a broader and deeper channel; and the rivulets in turn combining, form rivers which produce still greater effects.

The great basin of the Mississippi for example, is one grand central valley, cut by the main stream in the line of lowest level, towards which the valleys of the Missouri, the Arkansas, the Ohio, and a multitude of smaller streams, all converge.

REMARKABLE CANONS OF THE ROCKY MOUNTAIN PLATEAUS

Wonderful examples of valleys by erosion occur in the plateaus adjacent to the Rocky Mountains. The Grand Canon of the Colorado, three hundred miles long, has a depth of from three thousand to six thousand feet below the surrounding country. The sides of this tremendous gorge, which are nearly or quite precipitous, exhibit the successive geological strata down to the oldest rocks. A similar formation exists in the upper course of the Yellowstone, one of the main tributaries of the Missouri, and to a less extent in all the streams flowing through the high barren plateaus.