Chapter II: English and American Literature -- Outline Charts of English and (3)

Like all other planets, the earth is a solid sphere that has undergone a slight flattening at the opposite extremities or poles of the axis of revolution. More accurately, it is an oblate spheroid generated by the rotation of an ellipse about its minor axis. Such a figure would be assumed by a sphere of liquid rotating about a diameter, centrifugal force acting most vigorously at the equator, and tending to overcome the internal forces that keep the molecules together.

SIZE AND DENSITY OF THE EARTH

The smallest diameter of the earth is that measured from pole to pole along the axis of rotation; this is 7,899.6 miles, or about 500,000,000 inches. The greatest diameters are those measured between opposite points on the equator; these are 7,926.6 miles, and, therefore, show that the eccentricity of the earth, or the extent of its departure from the perfect sphere, is very slight.

The circumference of the earth, measured along the equator, is 24,899 miles; the area is 197,000,000 square miles; and the volume is 260,000,000,000 cubic miles. Experiments on the comparative attraction of the earth show that its density is about five and one-half times that of pure water. Its mass is, therefore, approximately six thousand trillion tons.

HOW WE KNOW THE EARTH IS A SPHERE

The ordinary proofs of the sphericity of the earth are: (1) It can be circumnavigated; (2) the appearance of a vessel at sea always indicates a nearer convexity of the earth’s surface; (3) the sea-horizon is always depressed equally in all directions when viewed from an elevation; (4) the elevation of the pole star increases as we travel northwards from the equator; (5) the shadow of the earth on the moon during a lunar eclipse is spherical.

THE ROTATION OF THE EARTH

The earth rotates uniformly about its axis. The time taken to make a complete revolution of three hundred and sixty degrees is called a sidereal day, for it is the interval of time between consecutive transits of any distant star across any meridian of the earth. The time between consecutive transits of the sun across any meridian is called a solar day; the average of these throughout the whole year is called a mean solar day, and is the practical standard of time adopted by civilized nations. The ordinary proofs that the earth rotates are: (1) Bodies falling from a great height have an easterly deviation; (2) Foucault’s pendulum experiment; (3) a gyroscope delicately balanced so as to be free to change the direction of its axis in any way will, if rotated, exhibit an apparent deviation; (4) in northern hemispheres a projectile deviates to the right, in southern hemispheres to the left; (5) the trade winds; (6) Dove’s law of wind-change.

The speed of a body on the equator, due to the diurnal rotation, is about 1,000 miles an hour. The centrifugal force due to this speed diminishes the weight of bodies; if the earth rotated in an hour, they would be thrown off from the surface at the equator.

The axis of the earth is not perpendicular to the ecliptic, but at angle of 66° 32′ to it; the equator is, therefore, inclined to it at an angle of 23° 28′. This unsymmetrical placing of the bulging portions of the earth causes a slow wobbling, or precession of its axis, in the same sort of way as a spinning top will wobble when pushed over on one side. There is also a slight vibration or “nodding” motion of the earth’s axis, known as nutation. The period of each precession is about twenty-one thousand years; if the earth’s orbit occupied a constant position in its plane, the periods would be twenty-six thousand years each. These motions have considerable influence on climate, the modern theories of the Ice Age being connected with the known facts of precessional motion.

THE EARTH A SERIES OF SHELLS OF MATTER

The great bulk of the earth consists of the _lithosphere_, or solid globe of rocks, with which geology properly deals. It is on the part of this lithosphere, composing a little more than a quarter of the earth’s whole area--55,500,000 square miles--which rises above the seas and is called land, that mankind lives.

The central core is a globe of about 7600 miles in diameter, which is composed of iron and other elements, probably not forming compounds, in the gaseous state, but exposed to such tremendous pressure that it behaves as a solid and extremely rigid body. Outside this core is a shell of liquid matter which consists of all the rocks which we know at the surface in a state of fusion, perhaps one hundred miles in thickness. Upon this magma floats the solid crust, thirty or forty miles thick, which is composed of various rocks, breaking down at the surface into soil. Three-fourths of the surface of this crust are covered by the water of the oceans, the hydrosphere, the rest being dry land. Outside all comes the atmospheric mantle, chiefly composed of air, which supports life, acts as a blanket to keep the earth warm, and as a shield against the blows of meteorites.

HOW THE EARTH’S CRUST IS CONSTRUCTED

An examination of the Earth’s crust shows us that it is constructed of numerous strata of rocks, some of limestone, some of sandstone, and some of clay; and some are very hard, others soft and crumbling, and readily worn away by the action of running streams or the waves of the ocean. To these several substances which form the materials of the earth’s crust we give the name _rock_. Hence we see that while in ordinary language the word rock denotes a great mass of hard stone, in geology a rock is any mass of natural substance forming part of the earth’s crust. In this sense, loose sand, gravel, and soft clay are as much rocks as hard limestone and granite.

=Porphyry=

=Basalt=

=Hornblende=

=COMPOSITION AND TEXTURE OF STONE AS REVEALED BY THE MICROSCOPE=]

MATERIALS OF WHICH ROCKS ARE COMPOSED

Rocks are formed of various materials called minerals. If we take a piece of sandstone rock, or a piece of granite, we shall probably be able to notice that the rock is made up of different substances.

On looking at a piece of _sandstone_, for example, especially if we use a magnifying glass, we see that it is composed of little rounded grains of a glassy-looking substance cemented together. In some specimens these grains are larger than in others. This cementing material is not the same in all sandstones, but in our specimen it is formed of _calcium carbonate_, for when we drop a little diluted hydrochloric acid on the rock there is an effervescence. The cementing material is dissolved, but the little rounded grains, which consist of _quartz_, are not affected by the acid. The sandstone, then, consists of quartz grains cemented together by calcium carbonate. It is called a calcareous sandstone.

Now take a piece of granite, and break it with a hammer to get a clean-cut face. On looking at this face we see that the rock is made up of _three_ different substances.

One of these has a glassy appearance like the grains in the sandstone, and is so hard that we cannot scratch it with a knife. This is _quartz_. Another of the substances is of a dull white or pinkish color. It lies in long, smooth-faced crystalline patches, which easily break along a number of smooth parallel surfaces having a pearly lustre. It can be scratched with difficulty by the point of a knife. This substance is called _felspar_. The third substance consists of bright glistening plates, sometimes of a dark color, which can be easily scratched, and which readily split into transparent leaves. This is _mica_. Notice that these substances do not occur in any definite order, but are scattered about through the stone irregularly, the felspar occurring in some specimens in larger crystals than in others.

WHAT A MINERAL IS

Hence we see that granite consists of a mixture of three substances, called quartz, felspar, and mica, the felspar being in greatest quantity. Each of these substances possesses properties more or less peculiar to itself, such as hardness, solubility in acids, specific gravity, crystalline form, way of splitting, etc. Hence, each of these substances has a _definite chemical composition and constant physical properties_ which define them as _minerals_.

This definition may be understood to include such substances as coal and chalk, which are the mineralized remains of plants and animals respectively. Even water and gases of the atmosphere may be said to belong to the mineral kingdom of nature, as plants and their parts are said to belong to the vegetable kingdom, and animals and their parts to the animal kingdom.

CHIEF ROCK-FORMING MINERALS

The total number of rock-forming minerals is very large, but many of them are very rare, and form but a very small part of the earth’s crust.

The most abundant materials or earths of which rocks are composed are _silica_, _lime_ and _aluminum_. Silica or flint is very universally diffused. It is found almost pure in quartz, opal, chalcedony, rock crystal, and the flinty sand of the sea-shore. Lime is also a very generally distributed earth, and is usually found in the form of carbonate. Under the several names of marl, limestone, oolite, and chalk it constitutes mountains, and even ranges of mountains. Aluminum is likewise very abundant, and of great importance to mankind. It enters largely into the clayey or argillaceous earths, and forms part of various kinds of rock which possess the property of not permitting water to pass through its substance--a property which renders it of inestimable value both for natural and artificial reservoirs of water.

CHIEF CHEMICAL ELEMENTS WHICH FORM MINERALS

The larger number of elements play so small a part in the constitution of the earth that they may be neglected by the geologist. The following list includes the elements of which ninety-nine per cent of the earth’s crust, as known to us, is composed, with their relative proportions, as indicated by Clarke’s laborious analyses of a very large number of typical rocks:

=========+========+==============
ELEMENT |CHEMICAL|PERCENTAGE OF
| SYMBOL |EARTH’S CRUST
| |WHICH IT FORMS
---------+--------+--------------
Oxygen | O | 47.02
Silicon | Si | 28.06
Aluminum | Al | 8.16
Iron | Fe | 4.64
Calcium | Ca | 3.50
Magnesium| Mg | 2.62
Sodium | Na | 2.63
Potassium| K | 2.32
Hydrogen | H | 0.17
Carbon | C | 0.12
| | -----
| | 99.24
---------+--------+--------------
The ten elements given above form
99.24 of the earth’s solid crust.

HOW ROCKS ARE CLASSIFIED

The beds or layers which form the crust of the earth are divided into three classes: (1) _Sedimentary_, or stratified; (2) _Igneous_, or unstratified; (3) _Metamorphic_, or transformed.

SEDIMENTARY OR STRATIFIED ROCKS

Sedimentary rocks are such as give evidence of having been formed by successive deposits of sediment in water. They include sandstones or freestones, limestones, clays, etc. The material for these must have been derived from some original source, and in many instances this may be traced to the disintegration of older rocks. Thus gneiss appears to be formed by the disintegration of granite. The great class of sedimentary rocks may be divided into three smaller divisions. These divisions, with the chief rocks of each division, may be tabulated as follows:

(a) Mechanically formed rocks from detrital sediments:
Conglomerates, sandstones, clay, and shale.

(b) Organically formed rocks from animal and plant remains:
Limestones, chalk, coral, peat, and coal.

(c) Chemically formed rocks from material once in solution:
Limestones, stalactites, gypsum, rock-salt and sinter.

Most of the stratified rocks contain fossils; and since each group contains certain kinds peculiar to itself, it is by means of these organic remains that their relative ages have been determined.

Although the lowest stratified rocks are more ancient than those which have been deposited above them, the layers or beds do not always retain a horizontal position. Were such the case, it could only be by deep cuttings that we should arrive at the older strata. We however find that, owing to some convulsion of nature, stratified rocks have been thrown out of their original position, and thus crop out to the surface. Not only is facility thus afforded us to become acquainted with the nature of the lower rocks, but many of the most valuable products of the earth are by this means rendered accessible to man.

A million years ago, a little stream trickled down a mountain-side, carrying with it grains of sand and stones which fell to the bottom of the sea. In the sea swam a great and wonderful creature called an ichthyosaurus. One day the great creature died, or probably it was killed in battle with another strange monster, and its body fell to the bottom of the sea among the shells and seaweed. Meanwhile, the stones and sand brought down by the stream continued to fall upon the bed of the sea until at last the great reptile’s body was buried, and the lower layers became pressed into hard rock by the weight on top. One day an elephant going to the river to drink broke off his tusk, and this was carried down by the river and sank in the sea. Another day a bird was drowned, and this, too, fell upon the ocean-bed. Dead fishes and shells also sank, and all were buried by the never-ceasing shower of mud and earth and sand and stones. Ages after the ichthyosaurus died, men began to live on the earth, and one day a man who had made a boat went out to fish. Trying to spear a big fish, the head of his harpoon broke off and fell to the bottom of the sea. In course of time this also was buried in the mud. The bottom of the sea crept higher and higher, till at last it became dry land. Then one day men began to dig, and the world’s wonderful story was revealed as we read it here. First the spear-head was found, then the tusk, the bird’s skeleton, the shells, the fish, and at last the skeleton of the great sea reptile, all turned to stone and become _fossils_, a word that means “something dug up.”]

The greater number of these beds contain organic remains, i. e., the remains of animals and plants, which are termed fossils. Among these the most numerous are the remains of marine animals, and in some instances shells and corals occur in such abundance as to form the principal part of extensive beds. Every part of the earth exhibits similar, or nearly similar formations; and not only are marine fossils met with in the interior of continents, and at great elevations above the sea, but a vast variety of plants, corals, shells, fish, reptiles, etc., are found, of species dissimilar to any at present on the land or in the waters. Besides rocks, we meet with earthy formations on the surface. These include such loose materials as are disintegrated or worn away from rocks, and form, when combined with decayed animal and vegetable matter, the soil of meadows and arable lands.

IGNEOUS, OR UNSTRATIFIED ROCKS are such as appear to be of igneous origin, or to have been formed by the action of fire or intense heat. They are called unstratified, because instead of having been deposited in successive layers, like the stratified rocks, they seem to have been formed by the fusion or melting of the materials of which they are composed, and the subsequent cooling and hardening of the melted matter into one great mass. Granite, basalt, lava, etc., are examples of this class of rocks, and represent respectively the sub-classes of plutonic, trap, and volcanic rocks. Plutonic rocks are those which have cooled under the pressure of overlying rocks; trap rocks, those which have cooled under that of deep water; and volcanic rocks, such as have cooled in the air.

Though granite is the most useful of the igneous rocks, basalt is probably the most interesting because of the wonderful formations it discloses. It is a dense basic lava of a dark color, that breaks with a conchoidal or shell-like fracture, and shows a finely grained or hemi-crystalline texture in a glassy base. The basalt rocks are found both as intrusive masses and as sheets that have been poured out on the surface. Many of these lava sheets of basalt in slowly cooling and solidifying acquired a columnar structure, the columns often having a more or less hexagonal shape, though the number of sides varies. Fine examples of these columnar basalts occur at Fingal’s cave in the island of Staffa, at the Giant’s Causeway in the north of Ireland, and on the shores of Lake Superior.

METAMORPHIC, or Transformed rocks, include altered rocks of either sedimentary or igneous origin, in which the acquired are more prominent than the original characteristics. Igneous rocks have, in many cases, forced their way up through stratified rocks. These igneous formations, while still in a molten state, in coming in contact with the aqueous or stratified rocks, have usually changed the character of those portions immediately near them. The chief changes of structure effected by metamorphic action are crystallization and foliation. Examples of metamorphic rocks are marble, quartzite, slate, gneiss, and the schists.

HOW THE METALS ARE FOUND

In some localities fissures in rocks are found to contain metallic substances. Such fissures are frequently found partially filled with calcareous spar which forms the matrix in which the metals are inclosed.

Metallic veins are supposed to be partially filled by mechanical means, the particles of metallic substances being conveyed into them by the action of water or some other power, and partly by chemical action, or by sublimation or fumes rising from below.

Some metallic deposits appear to occur in situations where igneous rocks have intruded themselves. Gold is supposed to be found almost invariably under such circumstances. Such appears to be the case in the rich deposits near the Ural mountains, and also in California and in Australia. In all these places it is met with in quartz. It is in pebbles or sand of the same rock that it occurs in the beds of rivers, and in some cases is found spread over a large extent of country.

Copper, though frequently met with in veins, is also found in extensive masses or beds, interposed between layers of rock. The same remark applies to tin, lead, and silver. Iron is also met with in beds, and also in nodules or rounded masses, which occur in great abundance among some kinds of rock. The last-named is the most universally diffused of all metals, and the most useful.

A GEOLOGICAL VIEW OF THE GROWTH OF THE EARTH

Giving the geological ages, rock systems, strata and the development of life, with their relative positions and order of succession, according to the latest scientific knowledge. Many attempts have been made to compute from geological, physical, and other data the length of the period during which the earth has been in a solid state.

Geologists, however, are disinclined to accept any period much less than 100,000,000 years as sufficient for the elaboration of the present structure of the earth. It is indisputable that many millions of years, probably thirty or forty, must have elapsed while the great sedimentary rocks were being deposited. With respect to the larger features of the earth’s surface, it is likely that two different kinds of movement are responsible. Where the contraction of the earth has caused a lessening of the support below the surface, there has been a subsidence of great areas. In the second place, where the rigid crust has been able to contract into a smaller space, great mountain ridges and folds have been formed. The subsidences which caused the ocean took place at different ages. The Atlantic Ocean probably dates from middle Cenozoic times; the Indian Ocean may be older; the Pacific suffered great modifications in comparatively recent times.

+-------------+------------------+------------------+-------------------+
|=Life Ages | =Rock Systems= | =Series of | =Characteristic |
|of the Earth=| | Rock Strata= | Rocks= |
+-------------+------------------+------------------+-------------------+
| | |Recent, or Human. |Alluvium, sand, |
| | | |gravel, mud, clay, |
| | | |marl, loess. |
| | | | |
| | |=Pleistocene= |Drift, boulder |
| | |(_plīs´tŏ-sēn_), |clay, gravel, |
| |=Quaternary= |or “most recent.” |loess, silt, gla- |
| |(_kwa-ter´na-ri_) |Glacial Period. |cial deposits and |
| |or “fourth.” Once | |other formations |
| |supposed to be the| |formed during |
| |_fourth_ sedimen- | |glacial period. |
| |tary system. Age | | |
| |of man. |=Pliocene= |In East and West, |
| | |(_plī´ō-sēn_), or |land deposits pre- |
| | |“more recent.” |dominate. Marine |
| | | |sands, clays, marls|
| | | |on Atlantic and |
| | | |Pacific coasts. |
| | | |Igneous rocks in |
| | | |West. |
| +------------------+------------------+-------------------+
| | |=Miocene= (_mī´ō- |On Atlantic coast: |
|=Cenozoic= | |sēn_), or “less |sand, clay, shell |
|(_se´nō-zō´- | |recent.” |marl, diatomaceous |
|ik_), or | | |earth. In West: |
|“Recent | | |sandstone, shale, |
|life.” | | |and diatomaceous |
| | | |material. Extensive|
|_Estimated | | |volcanic formations|
|Age of | | |in Rocky Mountains |
|Period_, | | |and Great Basin |
|=3,000,000= | | |region. |
|_years_. | | | |
| | |=Oligocene= |Limestone in |
| | |(_ŏl´ĕ-gō-sēn_), |Caribbean region, |
| |=Tertiary= (_ter´-|or “a little more |and deposits in |
| |shi-a-ri_), or |recent.” |West. Marine and |
| |“third”. Once | |fresh water beds on|
| |supposed to be the| |west coast. Many |
| |_third_ sedimen- | |coal beds in Puget |
| |tary system, or | |Sound. |
| |Age of mammals. | | |
| | |=Eocene= (_ē´-ō- |In Eastern States: |
| | |sēn_), or “dawn of|clays, sands, |
| | |recent.” |greensand marls. |
| | | |In West: conglom- |
| | | |erate, sandstone, |
| | | |shale, diatomaceous|
| | | |shale and igneous |
| | | |formations are de- |
| | | |veloped. Many coal |
| | | |beds in Puget |
| | | |Sound. Fresh water |
| | | |beds in western |
| | | |interior. |
+-------------+------------------+------------------+-------------------+
| | |{Upper. |In East: sand, |
| | |{ |clay, and greensand|
| | |{ |marl. In West: |
| | |{ |sandstone, shale, |
| | |{ |limestone, chalk, |
| | |{ |extensive coal |
| |=Cretaceous= |{ |beds, various |
| |(_krē-ta´-she-us_)|{ |igneous rocks. |
| |or “bearing |{ | |
| |chalk.” |{Lower. |Clay, sand, gravel |
| | |{ |on Atlantic coast |
| | |{ |and Gulf. Sedi- |
| | |{ |mentary and igneous|
| | |{ |rocks on west |
|=Mesozoic= | |{ |coast. Some non- |
|(_mĕs-ō-zō´- | |{ |marine beds in |
|ic_), or | |{ |Texas. |
|“Middle +------------------+------------------+-------------------+
|life.” | |{Upper. |Probably not repre-|
| |=Jurassic= (_jȯȯ- |{ |sented in East. |
|_Estimated |ras´sik_), or like|{ |Sandstones, lime- |
|Age of |the mass of the |{Middle. |stones and shales |
|Period_, |Jura Mountains. |{ |in West. Some “red |
|=9,000,000= |Age of Reptiles. |{ |beds” in western |
|_years_. | |{Lower. |interior. |
| | | | |
| | |{Upper. |In East sediments |
| | |{ |formed in shallow |
| | |{ |troughs between re-|
| | |{ |cently formed moun-|
| |=Triassic= |{ |tains. Considerable|
| |(_trĭăs´ĭk_), or |{ |bodies of igneous |
| |in a triple |{Middle. |rock, traps, and |
| |series. |{ |other flows and |
| | |{ |dikes. “Red beds” |
| | |{ |in West with salt |
| | |{ |and gypsum. Some |
| | |{ |igneous rocks on |
| | |{Lower. |west coast. |
+-------------+------------------+------------------+-------------------+
| | |=Permian= (_per´- |In East fresh water|
| | |mē-ăn_), like |sediments including|
| | |those at Perm, |coal; in West “red |
| | |Russia. |beds” probably of |
| | | |continental origin.|
| | | |Some marine sedi- |
| | | |ments; salt and |
| | | |gypsum in red beds |
| | | |in Kansas. |
| | | | |
| |=Carboniferous= |=Pennsylvanian=, |In Eastern States |
| |(_kăr-bŏn-if´-er- |like those of |grits, sandstones, |
| |us_), or coal- |Pennsylvania. |shales, limestone |
| |bearing. Age of | |and coal. In |
| |Amphibians. | |Western States much|
| | | |limestone; no coal.|
| | | |Igneous rocks on |
| | | |west coast. |
| | | | |
| | |=Mississippian=, |Limestones pre- |
| | |or Lower Carboni- |dominate with sand-|
| | |ferous. |stones near base |
| | | |and shales near top|
| | | |of series. Igneous |
| | | |rocks in |
| | | |California. |
| +------------------+------------------+-------------------+
| |=Devonian= (_de- |{Upper. |Sedimentary rocks, |
|=Paleozoic= |vō´ni-an_) like |{ |limestones, sand- |
|(_pāl-æ-ô- |those of Devon- |{Middle. |stones, shales; |
|zō´ic_), or |shire, England. |{ |igneous rocks in |
|“Old life.” |Age of Fishes. |{Lower. |Maine, Nova Scotia,|
| | |{ |and New Brunswick. |
|_Estimated +------------------+------------------+-------------------+
|Age of | |{ |Sedimentary rocks |
|Period_, | |{=Ontarian= (_on- |predominate; con- |
|=24,000,000= |=Silurian= (_si- |{tā´rē-ăn_), place|glomerates, sand- |
|_years_. |lū´ri-an_), in the|{name. |stones, shales, |
| |land of the |{ |limestones, salt, |
| |Silures, England. |{=Champlainian= |gypsum. Igneous |
| |Age of In- |{(_shăm-plān´ē- |rocks in Nova |
| |vertebrates. |{ăn_), place name.|Scotia, New Bruns- |
| | |{ |wick, and Maine. |
| +------------------+------------------+-------------------+
| | |{=Cincinnatian= |Chiefly limestone |
| |=Ordovician= (_ŏr-|{(_sĭn-sĭn-năt´-ē-|with subordinate |
| |dŏ-vīsh´ăn_), a |{ăn_), place name.|sandstone and |
| |place name in | |shale. Rocks great-|
| |Wales. |{=Mohawkian= (_mō-|ly folded in New |
| | |{hŏk´ē-ăn_), place|York, in Taconic |
| | |{name. |Mountain region. |
| +------------------+------------------+-------------------+
| | |{ |Mainly sandstones |
| | |{=Saratogan= |with some shales, |
| | |{(_săr-ă-tō´găn_),|and in Western |
| | |{place name. |States considerable|
| |=Cambrian= (_kam´-|{ |limestone. At some |
| |bri-an_), from |{=Acadian= (_ä- |places rocks are |
| |Cambria, the old |{kād´ē-ăn_), place|changed by pres- |
| |name for Wales. |{name. |sure, especially in|
| | |{ |the Appalachian |
| | |{=Georgian= (_jōr´|Mountains. Upper |
| | |{gē-ăn_), place |Cambrian covered |
| | |{name. |larger area than |
| | |{ |lower Cambrian. |
+-------------+------------------+------------------+-------------------+
| | |{=Keweenawan=, |A great series of |
| | |{(_kē´wē-năh- |sandstones, lime- |
| | |{wān_), pertaining|stones and shales, |
| | |{to Keweenaw Pen- |in middle portion |
| | |{insula, Michigan.|of which are many |
|=Proterozoic=| |{ |enormous flows of |
|(_prō-ter-ō- | |{ |lava. |
|zō´ik_) or |=Algonkian= (_ăl- |{ | |
|“Former |gŏn´kē-ăn_), from |{=Huronian= (_hu- |Three great series |
|life.” |district of |{rō´nē-ăn_), |of sedimentary |
| |Algonquin |{rocks on borders |rocks, sandstone, |
|_Estimated |Indians, north of |{of Lake Huron. |shale and lime- |
|Age of |St. Lawrence. |{ |stone, and iron |
|Period_, | |{ |formation. Contains|
|=18,000,000= | |{ |also many great |
|_years_. | |{ |igneous bodies, |
| | |{ |acidic and basic. |
| | |{ |Lower members much |
| | |{ |metamorphosed by |
| | |{ |pressure. |
+-------------+------------------+------------------+-------------------+
| | |{=Laurentian= |Granitic rocks and |
| | |{(_law-ren´shi- |gneisses that are |
| | |{an_), pertaining |believed to be |
| | |{to rocks along |granitic rocks |
| | |{the St. Lawrence |metamorphosed by |
| | |{River. |pressure. Formerly |
| | |{ |supposed to be |
| | |{ |older than Keewatin|
|=Archaeozoic=| |{ |and regarded as the|
|(_ar´kē-o-zō´| |{ |“original crust of |
|ic_), “With- | |{ |the earth.” |
|out life.” |=Archean= (_är- |{ | |
| |kē´-ăn_), |{=Keewatin= (_kē- |A great schist |
|_Estimated |“oldest.” |{wā´tĭn_), rocks |series made up of |
|Age of | |{in a district of |lava flows, tuffs, |
|Period_, | |{Manitoba, Canada.|and volcanic ashes.|
|=18,000,000= | |{ |With these are sub-|
|_years_. | |{ |ordinate sedimenta-|
| | |{ |ry rocks; sand- |
| | |{ |stone, shale, lime-|
| | |{ |stone, and iron ore|
| | |{ |formations nearly |
| | |{ |everywhere greatly |
| | |{ |metamorphosed by |
| | |{ |pressure. Includes |
| | |{ |the oldest rocks |
| | |{ |known. |
+-------------+------------------+------------------+-------------------+

+-------------+------------------+------------------+-------------------+
|=Life Ages | =Rock Systems= | =Series of | =Forms of Life= |
|of the Earth=| | Rock Strata= | |
+-------------+------------------+------------------+-------------------+
| | |Recent, or Human. |Man predominant. |
| | | | |
| | |=Pleistocene= |Mammoth, mastodon, |
| | |(_plīs´tŏ-sēn_), |bear, bison, rein- |
| |=Quaternary= |or “most recent.” |deer, musk-ox. |
| |(_kwa-ter´na-ri_) |Glacial Period. |Possibly man was |
| |or “fourth.” Once | |living but that is |
| |supposed to be the| |uncertain. |
| |_fourth_ sedimen- | | |
| |tary system. Age |=Pliocene= |Plants and animals |
| |of man. |(_plī´ō-sēn_), or |much as today, |
| | |“more recent.” |aside from human |
| | | |and domestic |
| | | |species. |
| +------------------+------------------+-------------------+
|=Cenozoic= | |=Miocene= (_mī´ō- |Land animals in- |
|(_se´nō-zō´- | |sēn_), or “less |clude elephants, |
|ik_), or | |recent.” |camels, deer, oxen,|
|“Recent | | |horses, true apes, |
|life.” | | |etc. Marine animals|
| | | |much like those to-|
|_Estimated | | |day. Among plants, |
|Age of | | |grasses become im- |
|Period_, | | |portant; deciduous |
|=3,000,000= | | |trees increase. |
|_years_. |=Tertiary= (_ter´-| | |
| |shi-a-ri_), or |=Oligocene= |Ancient dogs, cats,|
| |“third”. Once |(_ōl´ĕ-gō-sēn_), |rabbits, squirrels,|
| |supposed to be the|or “a little more |camels, and horses |
| |_third_ sedimen- |recent.” |were represented. |
| |tary system, or | | |
| |Age of mammals. | | |
| | |=Eocene= (_ē´-ō- |Mammals flourished,|
| | |sēn_), or “dawn of|including rodentia,|
| | |recent.” |carnivera, eden- |
| | | |tates, lemuroids, |
| | | |birds, reptiles, |
| | | |etc. Flora included|
| | | |figs, palms, |
| | | |bananas; willows, |
| | | |chestnuts, oaks, |
| | | |etc. |
+-------------+------------------+------------------+-------------------+
| | |{Upper. |Reptiles predomi- |
| | |{ |nate: turtles, |
| | |{ |lizards, |
| | |{ |crocodiles, flying |
| | |{ |reptiles, etc. Many|
| | |{ |waterbirds. Angio- |
| |=Cretaceous= |{ |sperms predominate:|
| |(_krē-ta´-she-us_)|{ |larch, beech, |
| |or “bearing |{ |walnut, tulip |
| |chalk.” |{ |trees, etc. |
| | |{ | |
|=Mesozoic= | |{Lower. |Reptiles abound. |
|(_mĕs-ō-zō´- | |{ |Flora includes |
|ic_), or | |{ |cycadeous, coni- |
|“Middle | |{ |fers, horsetails; |
|Life.” | |{ |angiosperms appear.|
| +------------------+------------------+-------------------+
|_Estimated | |{Upper. |Ammonites, belem- |
|Age of |=Jurassic= (_jȯȯ- |{ |ites continue in |
|Period_, |ras´sik_), or like|{ |great variety. |
|=5,000,000= |the mass of the |{Middle. |Reptiles numerous |
|_years_. |Jura Mountains. |{ |and varied types. |
| |Age of Reptiles. |{ |Flying reptiles and|
| | |{Lower. |reptile-like birds |
| | |{ |appear. |
| | | | |
| | |{Upper. |Reptiles of enor- |
| |=Triassic= |{ |mous size dominate |
| |(_trĭ-ăs´ĭk_), or |{ |the land and sea. |
| |in a triple |{Middle. |Mammals appear. |
| |series. |{ |Ammonites and |
| | |{ |belemites dominate |
| | |{Lower. |invertebrate life. |
+-------------+------------------+------------------+-------------------+
| | |=Permian= (_per´- |Reptiles become |
| | |mē-ăn_), like |prominent in number|
| | |those at Perm, |and variety; in- |
| | |Russia. |habit fresh water, |
| | | |salt water and |
| | | |land. |
| | | | |
| |=Carboniferous= |=Pennsylvanian=, |Plants abound. |
| |(_kăr-bŏn-if´-er- |like those of |Marked development |
| |us_), or coal- |Pennsylvania. |of land animals, |
| |bearing. Age of | |including insects, |
| |Amphibians. | |spiders and scorpi-|
| | | |ons. Lizards become|
| | | |important. Amphibi-|
| | | |ans reach climax. |
| | | | |
| | |=Mississippian=, |Crinoids greatly |
| | |or Lower Carboni- |developed. Am- |
| | |ferous. |phibians appear. |
| | | |Plant life expands.|
| +------------------+------------------+-------------------+
| | |{Upper. |Rapid changes in |
| | |{ |animal kingdom; |
| |=Devonian= (_de- |{ |shifting habitat; |
| |vō´ni-an_) like |{ |extensive develop- |
| |those of Devon- |{Middle. |ment of fishes; |
|=Paleozoic= |shire, England. |{ |sharks flourish. |
|(_pāl-æ-ô- |Age of Fishes. |{ |Plants are mainly |
|zō´ic_), or | |{ |small leaf and reed|
| “Old Life.” | |{Lower. |types. |
| +------------------+------------------+-------------------+
|_Estimated | |{=Ontarian= (_on- |Vertebrates appear;|
|Age of |=Silurian= (_si- |{tā´rē-ăn_), place|low forms of |
|Period_, |lū´ri-an_), in the|{name. |fishes. First reef |
|=24,000,000= |land of the |{ |building corals. |
|_years_. |Silures, England. |{=Champlainian= |Crinoids and bra- |
| |Age of In- |{(_shăm-plān´ē- |chiopods, important|
| |vertebrates. |{ăn_), place name.|Cephalopods con- |
| | |{ |tinue to dominate. |
| +------------------+------------------+-------------------+
| | |{=Cincinnatian= |Much as in the Cam-|
| | |{(_sĭn-sĭn-năt´-ē-|brian. Remains are |
| | |{ăn_), place name.|more abundant. |
| |=Ordovician= (_ŏr-|{ |Species more numer-|
| |dŏ-vīsh´ăn_), a |{=Mohawkian= (_mō-|ous; insects were |
| |place name in |{hŏk´ē-ăn_), place|present. Verte- |
| |Wales. |{name. |brates appear. Low |
| | |{ |forms of fishes. |
| | |{ |Trilobites reach |
| | |{ |climax. |
| +------------------+------------------+-------------------+
| | |{=Saratogan= |All great divisions|
| | |{(_săr-ă-tō´găn_),|of animal kingdom |
| | |{place name. |except vertebrates |
| |=Cambrian= (_kam´-|{ |are represented; |
| |bri-an_), from |{=Acadian= (_ä- |trilobites, bra- |
| |Cambria, the old |{kād´ē-ăn_), place|chiopods, sponges, |
| |name for Wales. |{name. |graptolites, etc. |
| | |{ |Little evidence of |
| | |{=Georgian= (_jōr´|vegetation, but it |
| | |{gē-ăn_), place |must have abounded |
| | |{name. |as food for |
| | |{ |animals. |
+-------------+------------------+------------------+-------------------+
|=Proterozoic=| |{=Keweenawan=, |Fossils rare or |
|(_prō-ter-ō- | |{(_kē´wē-năh- |wanting. |
|zō´ik_) or |=Algonkian= (_ăl- |{wān_), pertaining| |
|“Former |gŏn´kē-ăn_), from |{to Keweenaw Pen- | |
|Life.” |district of |{insula, Michigan.| |
| |Algonquin |{ | |
|_Estimated |Indians, north of |{=Huronian= (_hu- |Rocks contain clear|
|Age of |St. Lawrence. |{rō´nē-ăn,_), |evidence of low |
|Period_, | |{rocks on borders |forms of life. |
|=18,000,000= | |{of Lake Huron. | |
|_years_. | |{ | |
+-------------+------------------+------------------+-------------------+
| | |{=Laurentian= |Since the rocks are|
|=Archaeozoic=| |{(_law-ren´shi- |of igneous origin, |
|(_ar´kē-o-zō´| |{an_), pertaining |they contain no |
|ic_), “With- | |{to rocks along |organic remains. |
|out Life.” | |{the St. Lawrence | |
| |=Archean= (_är- |{River. | |
|_Estimated |kē´-ăn_), |{ | |
|Age of |“oldest.” |{=Keewatin= (_kē- |No fossils found, |
|Period_, | |{wā´tĭn_), rocks |but carbonaceous |
|=18,000,000= | |{in a district of |schists and lime- |
|_years_. | |{Manitoba, Canada.|stones are believed|
| | |{ |to indicate the |
| | |{ |presence of life. |
+-------------+------------------+------------------+-------------------+

+-------------+------------------+------------------+-------------------+
|=Life Ages | =Rock Systems= | =Series of | =Chief Economic |
|of the Earth=| | Rock Strata= | Products= |
+-------------+------------------+------------------+-------------------+
| | |Recent, or Human. |Clay, peat, bog |
| | | |iron ore, marl, |
| | | |gold placers. |
| |=Quaternary= | | |
| |(_kwa-ter´na-ri_) |=Pleistocene= |Clay, gravel, |
| |or “fourth.” Once |(_plīs´tŏ-sēn_), |gold placers. |
| |supposed to be the|or “most recent.” | |
|=Cenozoic= |_fourth_ sedimen- |Glacial Period. | |
|(_se´nō-zō´- |tary system. Age | | |
|ik_), or |of man. |=Pliocene= |Gold (in part |
|“Recent | |(_plī´ō-sēn_), or |placers), coal, |
|life.” | |“more recent.” |oil, gas. |
| +------------------+------------------+-------------------+
|_Estimated | |=Miocene= (_mī´ō- |Silver, gold, coal,|
|Age of | |sēn_), or “less |oil, gas, phosphate|
|Period_, | |recent.” |rock, diatomaceous |
|=3,000,000= |=Tertiary= (_ter´-| |earth. |
|_years_. |shi-a-ri_), or | | |
| |“third”. Once |=Oligocene= |Copper, silver. |
| |supposed to be the|(_ōl´ĕ-gō-sēn_), | |
| |_third_ sedimen- |or “a little more | |
| |tary system, or |recent.” | |
| |Age of mammals. | | |
| | |=Eocene= (_ē´-ō- |Gold, zinc, lead, |
| | |sēn_), or “dawn of|coal, oil, gas. |
| | |recent.” | |
+-------------+------------------+------------------+-------------------+
| |=Cretaceous= |{ |Coal, oil, gas, |
| |(_krē-ta´-she-us_)|{Upper. |copper, gold, china|
| |or “bearing |{ |clay, fire clay, |
| |chalk.” |{Lower. |cement building |
|=Mesozoic= | |{ |stone. |
|(_mĕs-ō-zō´- | |{ | |
|ic_), or | |{ | |
|“Middle +------------------+------------------+-------------------+
|Life.” |=Jurassic= (_jȯȯ- |{Upper. |Oil, gold. |
| |ras´sik_), or like|{ | |
|_Estimated |the mass of the |{Middle. | |
|Age of |Jura Mountains. |{ | |
|Period_, |Age of Reptiles. |{Lower. | |
|=5,000,000= | | | |
|_years_. | |{Upper. |Salt, gypsum, a |
| |=Triassic= |{ |little coal in |
| |(_trĭ-ăs´ĭk_), or |{Middle. |Virginia, copper, |
| |in a triple |{ |building stone. |
| |series. |{Lower. | |
+-------------+------------------+------------------+-------------------+
| | |=Permian= (_per´- |Salt and gypsum; |
| | |mē-ăn_), like |some coal in |
| | |those at Perm, |Eastern States. |
| | |Russia. | |
| | | | |
| |=Carboniferous= |=Pennsylvanian=, |Coal, oil, gas, |
| |(_kăr-bŏn-if´-er- |like those of |iron ore, fire |
| |us_), or coal- |Pennsylvania. |clay, phosphate |
| |bearing. Age of | |rock. |
| |Amphibians. | | |
| | | | |
| | |=Mississippian=, |Oil, gas, lead, |
| | |or Lower Carboni- |zinc, building |
| | |ferous. |stone, cement rock.|
| +------------------+------------------+-------------------+
| |=Devonian= (_de- |{Upper. | |
| |vō´ni-an_) like |{ |Gas, oil, iron ore,|
| |those of Devon- |{Middle. |phosphate rock. |
|=Paleozoic= |shire, England. |{ | |
|(_pāl-œ-ô- |Age of Fishes. |{Lower. | |
|zō´ic_), or +------------------+------------------+-------------------+
| “Old Life.” | |{=Ontarian= (_on- |Iron ore, gas, |
| |=Silurian= (_si- |{tā´rē-ăn_), place|salt, gypsum, |
|_Estimated |lū´ri-an_), in the|{name. |cement rock. |
|Age of |land of the |{ | |
|Period_, |Silures, England. |{=Champlainian= | |
|=24,000,000= |Age of In- |{(_shăm-plān´ē- | |
|_years_. |vertebrates. |{ăn_), place name.| |
| +------------------+------------------+-------------------+
| | |{=Cincinnatian= |Oil, gas, lead, |
| |=Ordovician= (_ŏr-|{(_sĭn-sĭn-năt´-ē-|zinc, phosphate |
| |dŏ-vīsh´ăn_), a |{ăn_), place name.|rock, manganese, |
| |place name in |{ |marble. |
| |Wales. |{=Mohawkian= (_mō-| |
| | |{hŏk´ē-ăn_), place| |
| | |{name. | |
| +------------------+------------------+-------------------+
| | |{=Saratogan= |Lead, zinc, barite,|
| | |{(_săr-ă-tō´găn_),|copper. |
| | |{place name. | |
| |=Cambrian= (_kam´-|{ | |
| |bri-an_) from |{=Acadian= (_ä- | |
| |Cambria, the old |{kād´ē-ăn_), place| |
| |name for Wales. |{name. | |
| | |{ | |
| | |{=Georgian= (_jōr´| |
| | |{gē-ăn_), place | |
| | |{name. | |
+-------------+------------------+------------------+-------------------+
| | |{=Keweenawan=, |Copper, silver. |
|=Proterozoic=| |{(_kē´wē-năh- | |
|(_prō-ter-ō- | |{wān_), pertaining| |
|zō´ik_) or | |{to Keweenaw Pen- | |
|“Former |=Algonkian= (_ăl- |{insula, Michigan.| |
|Life.” |gŏn´kē-ăn_), from |{ | |
| |district of |{=Huronian= (_hu- |Principal iron ores|
|_Estimated |Algonquin |{rō´nē-ăn,_), |of Lake Superior |
|Age of |Indians, north of |{rocks on borders |region; also |
|Period_, |St. Lawrence. |{of Lake Huron. |copper, nickel, |
|=18,000,000= | |{ |silver, cobalt, |
|_years_. | |{ |gold. Building |
| | |{ |stone and ornamen- |
| | |{ |tal stone. |
+-------------+------------------+------------------+-------------------+
| | |{=Laurentian= |Iron ores, precious|
|=Archaeozoic=| |{(_law-ren´shi- |metals, gems, |
|(_ar´kē-o-zō´| |{an_), pertaining |apatite, rare |
|ic_), “With- | |{to rocks along |earths, graphite, |
|out Life.” | |{the St. Lawrence |asbestos. |
| |=Archean= (_är- |{River. | |
|_Estimated |kē´-ăn_), |{ | |
|Age of |“oldest.” |{=Keewatin= (_kē- |Emery, building and|
|Period_, | |{wā´tĭn_), rocks |ornamental stones. |
|=18,000,000= | |{in a district of | |
|_years_. | |{Manitoba, Canada.| |
+-------------+------------------+------------------+-------------------+

THE SURFACE OF THE EARTH

LAND FORMS OF THE WORLD

The proportion of land to water upon the earth is as 27 to 72, or roughly _one-fourth_ to _three-fourths_; the land covering fifty-three million square miles, the sea one hundred and forty-four million. The land consists of six great bodies called continents, and a multitude of small fragments called islands, which skirt the shores of the continents or dot the broad expanse of the sea.

THE DISTRIBUTION OF LAND AND WATER

By far the greatest proportion of land is in the northern hemisphere, and in temperate latitudes. Broadly speaking, the northern hemisphere is the hemisphere of land, and the southern hemisphere is the hemisphere of ocean. The earth could be bisected in such a way that one hemisphere contained almost no land, while the other was composed almost equally of land and water.

LOCATION OF THE CONTINENTS

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