HOW THE MOON THROWS ITS SHADOW ON THE EARTH, SHUTTING OFF THE LIGHT OF THE SUNHOW THE MOON COMES BETWEEN THE EARTH AND SUN, CAUSING THE SHADOW SHOWN ABOVEHOW THE EARTH THROWS ITS SHADOW ACROSS THE MOONOn its way through space the moon passes sometimes between the sun and the earth, shutting off the sunlight from the earth, as shown in the top picture. The drawing in the middle shows us that the moon does not hide the sunlight from the whole of the earth, but only from a part of it. But in the part from which the sun is hid the moon’s shadow makes day so dark that we can see the stars. We call this an eclipse of the sun. Sometimes, too, the earth passes between the moon and the sun so as to cut off all sunlight from the moon, as shown in the bottom picture. We call this an eclipse of the moon.
HOW THE MOON THROWS ITS SHADOW ON THE EARTH, SHUTTING OFF THE LIGHT OF THE SUN
HOW THE MOON COMES BETWEEN THE EARTH AND SUN, CAUSING THE SHADOW SHOWN ABOVE
HOW THE EARTH THROWS ITS SHADOW ACROSS THE MOON
On its way through space the moon passes sometimes between the sun and the earth, shutting off the sunlight from the earth, as shown in the top picture. The drawing in the middle shows us that the moon does not hide the sunlight from the whole of the earth, but only from a part of it. But in the part from which the sun is hid the moon’s shadow makes day so dark that we can see the stars. We call this an eclipse of the sun. Sometimes, too, the earth passes between the moon and the sun so as to cut off all sunlight from the moon, as shown in the bottom picture. We call this an eclipse of the moon.
Solar Eclipses.The shadow cast by the moon is also conical, and extends over a slightly varying distance of about a quarter of a million miles from the moon’s surface. This being the approximate distance of the moon from the earth, it is seen that when the moon is between the earth and the sun the shadow may reach the earth. The extreme limit of the shadow may range from twenty-three thousand miles short of the earth, in which case an entire eclipse of the sun is impossible, to fifteen thousand miles beyond the earth. In the latter case a circular shadow will be projected on the surface of the globe, travelling onwards slowly in the direction of the motion of the moon. Within this shadow orumbrathe body of the sun cannot be observed, and a total eclipse prevails. A circular region exists round this shadow, in which only part of the sun is visible; this region is therefore partly in shadow, and is called thepenumbra. Outside the penumbra the whole sun may be viewed; the moon’s shadow is not nearly large enough to render a solar eclipse co-existent over all parts of the earth’s face towards the sun.
To the Greeks the starry heavens were an illustrated mythological poem. Every constellation was a picture, connected with some old fable of gods or heroes.
The two Bears have one story. Callisto was a nymph beloved by Jupiter, who changed her into a she-bear to save her from the jealous wrath of Juno. But Juno learned the truth, and induced Diana to kill the bear in the chase. Jupiter then placed her among the stars as Ursa Major, and her son Arcas afterwards became Ursa Minor. Juno, indignant at the honor thus shown the objects of her hatred, persuaded Tethys and Oceanus to forbid the Bears to descend, like the other stars, into the sea.
According to Ovid, Juno changed Callisto into a bear; and when Arcas, in hunting, was about to kill his mother, Jupiter placed both among the stars.
Ursa Minor was also called Phœnice, because the Phœnicians made it their guide in navigation, while the Greeks preferred the Great Bear for that purpose. It was also known as Cynosura (dog’s tail) from its resemblance to the upturned curl of a dog’s tail. The Great Bear was sometimes called Helice (winding), either from its shape or its curved path.
Boötes (the Herdsman) was also called Arctophylax and Arcturus, both of which names mean the guard or keeper of the bear. According to some of the stories, Boötes was Arcas; according to others, he was Icarus, the unfortunate son of Dædalus. The name Arcturus was afterwards given to the chief star of the constellation.
Cepheus, Cassiopeia, Andromeda, Perseus, and Pegasus are a group of star-pictures illustrating a single story.
Cepheus and Cassiopeia were the king and queen of Ethiopia, and had a very beautiful daughter, Andromeda. Her mother boasted that the maiden was fairer than the Nereids, who in their anger persuaded Neptune to send a sea-monster to ravage the shores of Ethiopia. To appease the offended deities Andromeda, by the command of an oracle, was exposed to this monster. The hero Perseus rescued her and married her.
Pegasus, the winged horse, sprang from the blood of the frightful Gorgon, Medusa, whom Perseus had slain not long before he rescued Andromeda from the sea-monster. According to the most ancient account, Pegasus became the horse of Jupiter, for whom he carried the thunder and lightning; but he afterward came to be considered the horse of Aurora, and finally of the Muses. Modern poets rarely speak of him except as connected with the Muses.
The Dragon, according to some of the poets, was the one that guarded the golden apples of the Hesperides; according to others, the monster sacred to Mars which Cadmus killed in Bœotia.
The Lyre is said to be the one which Apollo gave to Orpheus. After the death of Orpheus, Jupiter placed it among the stars at the intercession of Apollo and the Muses.
The Crown was the bridal gift of Bacchus to Ariadne, transferred to the heavens after her death.
Aquila is probably the eagle into which Merops was changed. It was placed among the stars by Juno. Some, however, make it the Eagle of Jupiter.
Cygnus or Cycnus, according to Ovid, was a relative of Phaëthon. While lamenting the unhappy fate of his kinsman on the banks of the Eridanus, he was changed by Apollo into a swan, and placed among the stars.
Sagittarius was said by the Greeks to be the Centaur Cheiron, the instructor of Peleus, Achilles[37]and Diomed. It is pretty certain, however, that all the zodiacal constellations are of Egyptian origin, and represent twelve Egyptian deities who presided over the months of the year. Thus Aries was Jupiter Ammon; Taurus, the bull Apis; Gemini, the inseparable gods Horus and Harpocrates; and so on. The Greeks adopted the figures, and invented stories of their own to explain them.
Scorpio, in the Egyptian zodiac, represented the monster Typhon. Originally this constellation extended also over the space now filled by Libra.
Ophiuchus represents Æsculpius, the god of medicine. Serpents were sacred to him, probably because they were a symbol of prudence and renovation, and were believed to have the power of discovering herbs of wondrous powers.
Aquarius, in Greek fable, was Ganymede, the Phrygian boy who became the cup-bearer of the gods in place of Hebe.
Taurus, as has been stated above, was the Egyptian Apis. The Greeks made it the bull which carried off Europa. The Pleiades are usually called the daughters of Atlas, whence their name Atlantides. Milton speaks of them as “the seven Atlantic Sisters.”
According to one legend the seventh was Sterope, who became invisible because she had loved a mortal; according to another, her name was Electra, and she left her place that she might not witness the downfall of Troy, which was founded by her son, Dardanus.
The Hyades, according to one of several stories, were sisters of the Pleiades. The name probably means “the Rainy,” since their rising announced wet weather.
Cetus is said by most writers to be the sea-monster from which Perseus rescued Andromeda.
Orion was a famous giant and hunter, who loved the daughter of Oinopion, King of Chios. As her father was slow to consent to her marriage, Orion attempted to carry off the maiden; whereupon Oinopion, with the help of Bacchus, put out his eyes. But the hero, in obedience to an oracle, exposed his eye-balls to the rays of the rising sun, and thus regained his sight. The accounts of his subsequent life, and of his death, are various and conflicting. According to some, Aurora loved him and carried him off; but, as the gods were angry at this, Diana killed him with an arrow. Others say that Diana loved him, and that Apollo, indignant at his sister’s affection for the hero, once pointed out a distant object on the surface of the sea, and challenged her to hit it. It was the head of Orion swimming, and the unerring shot of the goddess pierced it with a fatal wound. Another fable asserts that Orion boasted that he would conquer every animal; but the earth sent forth a scorpion which destroyed him.
Canis Major and Minor are the dogs of Orion, and are pursuing the Hare.
The Twins, Castor and Pollux, the sons of Jupiter and Leda, are the theme of many a fable. They were especially worshipped as the protectors of those who sailed the seas, for Neptune had rewarded their brotherly love by giving them power over winds and waves, that they might assist the shipwrecked.
Leo, according to the Greek story, was the famous Nemean lion slain by Hercules. Jupiter placed it in the heavens in honor of the exploit.
The Hydra also commemorates one of the twelve labors of Hercules—the destruction of the hundred-headed monster of the Lernæan lake.
Virgo represents Astræa, the goddess of innocence and purity, or, as some say, of justice. She was the last of the gods to withdraw from earth at the close of “the golden age.”
Libra, or the Balance, is the emblem of justice, and is usually associated with the fable of Astræa.
Argo Navis is the famous ship in which Jason and his companions sailed to find the Golden Fleece.
This slight sketch of the leading fables connected with the constellations will serve to show how completely the Greeks “nationalized the heavens.”
Astronomy(as-tron´om-i). The science which treats of the heavenly bodies, explaining the motions, times and causes of the motions, distances, magnitudes, gravities, light, etc., of the sun, moon, and stars, the nature and causes of the eclipses of the sun and moon, the conjunction and apposition of the planets, and any other of their mutual aspects, with the times when they did or will happen.
Aberration(ab-er-ā´shun). A small apparent motion of the fixed stars, occasioned by the progressive motion of light and the earth’s annual motion in its orbit. By this they sometimes appear twenty seconds distant from their true situation.
Amplitude(am´pli-tud). An arc of the horizon intercepted between the true east and west points and the center of the sun, or a star at its rising or setting.
Anomaly(an-om´al-i). The angular distance of a planet from its perihelion, as seen from the sun; either true, mean, or eccentric.
Aphelion(af-ēl´yun). That point of a planet’s orbit which is most distant from the sun.
Apogee(ap´o-jē). That point in the orbit of the moon which is at the greatest distance from the earth.
Apparition(ap-par-ish´un). The first appearance of a star or other luminary after having been obscured.
Ap´pulse. The approach of a planet towards a conjunction with the sun or any of the fixed stars.
Apsis(ap´sis). The two points of a planet’s orbit in which it is at its greatest and least distance from the sun.
Aquarius(a-kwā´ri-us). The eleventh sign of the zodiac, which the sun enters about the 21st of January.
Asteroids(as´ter-oids). The small planets that circulate between the orbits of Mars and Jupiter.
Ax´is(ax´is). The imaginary line passing through the center and poles of the earth, on which it performs its diurnal revolutions from west to east.
Azimuth(az´im-uth). An arc of the horizon intercepted between the meridian of the place and the vertical circle passing through the center of a celestial object.
Can´cer. The fourth sign of the zodiac, being that of the summer solstice, which the sun enters about the 21st of June.
Capricorn(kap´ri-korn). The tenth sign of the zodiac, which the sun enters about the 21st of December, at the winter solstice.
Colure(kol´ur). Two great circles, supposed to intersect each other at right angles in the poles of the world, one of them passing through the solstitial and the other through the equinoctial points of the ecliptic, viz., Cancer and Capricorn, Aries and Libra, dividing the ecliptic into four equal parts.
Coma(kō´ma). A dense, nebulous covering, which surround the nucleus or body of a comet.
Com´et. A member of the solar system, commonly consisting of three parts: the nucleus, the envelope or coma, and the tail; but one or more of these parts is frequently wanting.
Conjunc´tion. The meeting of two heavenly bodies in the same point or place in the heavens.
Constella´tion. A number of stars which appear as if situated near each other in the heavens, and are considered as forming a particular division.
Cynosure(sin´o-shōōrorsī´). A name of the constellation Ursa Minor, or the Lesser Bear, which contains, in the tail, the pole star by which mariners are guided.
Declination(dek-lin-a´shun). Distance of any object from the celestial equator, either northward or southward.
Disk. The face or visible projection of a celestial body, usually predicated of the sun, moon, or planets; but the stars have also apparent disks.
Eclipse´. An obscuration or interception of the light of the sun, moon, or other luminous body.
Eclip´tic. The great circle of the heavens which the sun appears to describe in his annual revolution.
Equa´tor. The great circle of the sphere, equally distant from the two poles of the world, or having the same poles as the world.
Equinox(ē´kwi-noks). The precise time when the sun enters one of the equinoctial points, making the day and night of equal length.
Faculae(fa´ku-lē). Certain spots sometimes seen on the sun’s disk, which appear brighter than the rest of his surface.
Fixed Stars. Those which retain the same or very nearly the same position with respect to each other.
Gal´axy. The Milky-Way.
Gemini(jem´i-nī). The third sign or constellation in the zodiac, which the sun enters about the 21st of May.
Geocentric(jē-o-sen´trik)Par´allax. The apparent change of a body’s place that would arise from a change of the spectator’s station from the surface to the center of the earth.
Ha´lo. A luminous circle, usually prismatically colored round the sun or moon, and supposed to be caused by the refraction of light through crystals of ice in the atmosphere.
Heliocentric(hē-li-o-sen´trik)Par´allax. The arc of the great circle of the celestial sphere, drawn from the heliocentric to the geocentric place of a body.
Heliometer(hē-li-om´e-ter). An instrument for measuring with exactness the apparent diameter of the sun, moon, planets, etc.
Hori´zon. A circle touching the earth at the place of the spectator, and bounded by the line in which the earth and skies seem to meet.
Le´o(Lat., the Lion). The fifth sign of the zodiac which the sun enters about the 22d of July.
Libra(lī´bra), the Balance. The seventh sign of the zodiac, which the sun enters at the autumnal equinox, in September.
Luna´tion. The period of a revolution of the moon round the earth, or the time from one new moon to the next.
Maculae(mak´u-lē). Dark spots on the surfaces of sun and moon, and on some of the planets.
Moon. A secondary planet or satellite of the earth, whose light, borrowed from the sun, serves to dispel the darkness of night.
Nadir(nā´dir). The point of the heavens or lower hemisphere directly opposite the zenith.
Neb´ulae(neb´u-lē). Misty appearances among the stars, usually, but not always, resolved by telescope into myriads of small stars.
Nodes(nōdes). The two points in which the orbit of a planet intersects the ecliptic.
Nuta´tion. A vibratory motion of the earth’s axis, arising from periodical fluctuations in the obliquity of the ecliptic.
Occulta´tion. The hiding of a heavenly body from our sight by the intervention of some other of the heavenly bodies.
Or´bit. The path described by a heavenly body in its periodical revolution.
Par´allax. The change of place in a heavenly body in consequence of being viewed from different points.
Penum´bra. A partial shadow or obscurity on the margin of the perfect shadow in an eclipse, or between the perfect shadow, where the light is entirely intercepted, and the full light.
Perigee(per´i-jē). That point in the orbit of the sun or moon in which it is at the least distance from the earth.
Perihelion(per-i-hē´li-on). That part of the orbit of a planet or comet in which it is at its least distance from the sun.
Plan´et. The name given to a few bright and conspicuous stars which are constantly changing their apparent situations in the celestial sphere.
Precession(pre-sesh´un)of the Equinoxes. A continual shifting of the equinoctial points from east to west.
Radius Vector. An imaginary line joining the center of the sun and the center of a body revolving about it.
Retrocession(rē-tro-sesh´un)of the Equinoxes. The going backward of the equinoctial points.
Sagittarius(saj-i-tā´ri-us). One of the twelve signs of the zodiac, which the sun enters about November 22.
Sat´ellite. A small planet revolving round another planet.
Scor´pio. The eighth sign of the zodiac, which the sun enters about October 23.
Selenography(sel-en-og´raf-i). The description of the surface of the moon.
Sign. The twelfth part of the ecliptic.
Solstice(sol´stis). The time when the sun, in its annual revolution, arrives at that point in the ecliptic farthest north or south of the equator, or reaches its greatest northern or southern declination.
Star. An apparently small, luminous body in the heavens, that shines in the night, or when its light is not obscured by clouds or lost in the brighter effulgence of the sun.
Sun. The central body of our system, about which all the planets and comets revolve, and by which their motions are regulated and controlled.
Taurus(taw´rus). The second sign of the zodiac, which the sun enters about the 20th of April.
Virgo(ver´go). The sixth sign of the zodiac, which the sun enters in August.
Ze´nith. The point in the heavens directly overhead.
THE EARTH AS A PLANET
ITS STRUCTURE:Interior,Crust,Rocks,Fossils,Heat
GEOLOGICAL VIEW OF GROWTH OF THE EARTH
SURFACE OF THE EARTH:Land Forms:Continents,Islands,Mountains,Plains;Water Forms:Springs,Rivers,Lakes,Oceans
CELEBRATED MOUNTAIN PEAKS AND RANGES
ATMOSPHERE, CLIMATE AND WEATHER
NATURAL WONDERS AND FORCES:Volcanoes,Earthquakes,Geysers,Caverns,Waterfalls,Whirlpools,Tides,Deserts,Ocean Depths,Clouds,Seasons,Glaciers,Icebergs,Snow,Rain,Hail,Dew,Coral Islands and Reefs
DICTIONARY OF MINERAL PRODUCTS
TABLES FOR THE IDENTIFICATION OF MINERALS
GEMS AND PRECIOUS STONES
PRONOUNCING DICTIONARY OF SCIENTIFIC TERMS ABOUT THE EARTH
NUMEROUS ILLUSTRATIONS, CHARTS AND MAPS
Large illustration(465 kB)
BOOK OF THE EARTH
Science tells us that the Earth was once a shining star, a globe of liquid fire. As it cooled down, a crust formed over its surface, composed chiefly of rocks and metals. This crust was rent by the force of the gases shut up within, and thus the mountains, valleys, gorges, and volcanoes were formed. The Earth, indeed, is still upheaving and subsiding, but so slowly that we rarely feel it. Through these agencies the distribution of land and water on the surface of the earth has undergone great changes. The shape of the Earth is that of a sphere somewhat flattened at the poles, and it has a diameter of about 8,000 miles. The solid crust is called thelithosphere—which is surrounded by an envelope of air—theatmosphere—and in part by an envelope of water—thehydrosphere.
HOW THE EARTH WOULD APPEAR IF CUT THROUGH THE CENTERBeneath the rocky crust of the earth, thirty-five miles in thickness, there is a broad belt of heavier material to a depth of nine hundred miles. Within this shell lies the great metallic core.
HOW THE EARTH WOULD APPEAR IF CUT THROUGH THE CENTER
Beneath the rocky crust of the earth, thirty-five miles in thickness, there is a broad belt of heavier material to a depth of nine hundred miles. Within this shell lies the great metallic core.
Our first glimpse of the earth as a planet shows it as a nebulous star, still intensely hot, and with no solid nucleus, rotating on its own axis, and at the same time revolving around the sun in a nearly circular orbit.
At first it seems hardly possible that the earth could have been a star. But, if we go down beneath the surface of the earth, we find that at a depth of forty or fifty feet there is very slight variation in temperature. When we go yet deeper, as in mines, we find that the earth grows hotter as we descend. The temperature increases on an average about one degree Fahrenheit for every sixty-four feet descent. But this amount is variable according to the locality, geologicalformation, and dip of strata. In the Calumet and Hecla Mine, observations show an increase of one degree in about every one hundred and twenty-five feet. At Paris, the water from a depth of 1794 feet has a temperature of eighty-two degrees; at Salzwerth, in Germany, from a depth of 2144 feet, a temperature of ninety-one degrees. Natural hot springs, rising from unknown depths, are sometimes scalding hot. One in Arkansas has a temperature of one hundred and eighty degrees.
At a depth of twenty miles, with this continual increase of temperature, the ground must be fully red-hot; and not very much farther down the heat must be sufficient to melt every known substance. The solid earth, then, is merely a thin crust, covering a sea of liquid fire below. The streams of lava poured forth from volcanoes are a proof of the existence of this molten mass beneath our feet.
If we examine the solid crust of the earth we shall not long be at a loss in regard to the origin of this internal heat. We are all familiar with the burning of coal. Now coal is mainly a substance calledcarbon, and when it burns it unites withoxygen, one of the gases in the air. Many rarer substances, such as silicon, and the metals magnesium, calcium, and sodium, are even more inflammable than carbon, and in burning give rise to solid products. Now the rocks in the earth are found to be made up almost wholly of these very inflammable substances combined with oxygen. The solid portions of the earth, then, are nothing but the ashes and cinders of a great conflagration. Even the waters are made up of hydrogen, one of the most inflammable substances, united with this same oxygen, and, strange as it may seem, they too, are the products of combustion. When, therefore, the materials of which the earth is formed were burning, our planet must have been a fiery star, and the great heat must have reduced all the products of the conflagration to a liquid state.
When the fire went out for lack of fuel the mass began to cool at the surface, and a solid crust was finally formed, which with the lapse of time became thicker and thicker. This crust shut in the steam and gases generated in the fiery ocean underneath; and these, acting upon the crust with enormous pressure, heaved it into ridges. At times the strain caused the crust to crack, and forced the melted mass up through it, and in this way hills and mountains were formed. The thicker the crust the greater the strain it would bear before it gave way, and the greater the amount of molten matter driven out through the rent. The highest mountains, then, are the last that were uplifted. In some cases the openings thus made in the crust were never completely closed, and thus volcanoes were formed. These act like safety-valves, and prevent the forces within from accumulating sufficiently to cause fresh rents. But notwithstanding the relief thus given to the pent-up forces, they still manifest themselves in earthquakes.
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.
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 showthat its density is about five and one-half times that of pure water. Its mass is, therefore, approximately six thousand trillion tons.
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 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 great bulk of the earth consists of thelithosphere, 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.
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 namerock.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.
GranitePorphyryBasaltHornblendeCOMPOSITION AND TEXTURE OF STONE AS REVEALED BY THE MICROSCOPE
GranitePorphyry
BasaltHornblende
COMPOSITION AND TEXTURE OF STONE AS REVEALED BY THE MICROSCOPE
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 ofsandstone, 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 ofcalcium 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 ofquartz, 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 ofthreedifferent 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 isquartz. 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 calledfelspar. 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 ismica. 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.
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 adefinite chemical composition and constant physical propertieswhich define them asminerals.
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.
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 aresilica,limeandaluminum. 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.
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:
The ten elements given above form 99.24 of the earth’s solid crust.
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 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 instancesthis 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.