Part 7

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.

[Illustration: =HOW THE HISTORY OF THE EARTH IS EMBEDDED IN THE ROCKS=

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. | +-------------+------------------+------------------+-------------------+