Part 7

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.

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.

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.