The Geological Story Briefly Told: An Introduction to Geology for the ...

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Contents

Gkologi 11

Part L ROCKS OB WHAT THE EARTH 18 MADE OF . . . 13-88

I. MINERALS 16

Part I. Rocks, Or What The Earth Is Made Of

Rocks consist of minerals; the ores and gems they contain are some of the minerals; and throughout the earth, to its center, the solid rocks have a crystalline structure.

The most abundant element in minerals and in the earth's constitution is Oxygen. It makes part of the atmosphere, is one of the two constituents of water, and is an essential part of all rock-making minerals. Owing to its strong attractions for other elements, iron rusts, — that is, becomes ' oxidized; and most iron-bearing minerals in the rocks oxidize, wherever exposed to air and moisture. This oxidation produces the decay of the rocks, and promotes the conversion of them into gravel, sand, and soil.

Of next importance in the constitution of rocks is Silicon (named from the Latin for jlinf). All the rocks excepting limestones contain silicon in combination with oxygen, and also usually other elements.

14 CONSTITUTION OF BOOKS.

Next comes Carbon (named from the Latin for coal). It is the chief ingredient of coal and one of the constituents of limestone and of mineral oil and gas. It is a chief constituent also of all animal and vegetable tissues. In its pure crystallized state it is diamond, the hardest of all minerals, and also graphite or plumbago (the material of lead pencils), which is among the softest of minerals.

Aluminum (so named from the Latin for day), combined with oxygen and other elements, is a chief constituent of clay, and also of very many of the rocks and hard minerals.

Calcium (from the Latin for lime) is a chief constituent of limestone. Magnesium, potassium, sodium, are other common elements of rocks.

Sulphur occurs pure in nature, especially in volcanic regions. It is a constituent of many ores, and also of gypsum among rock-making materials. Arsenic is an ingredient of many ores, but not of any rock.

Hydrogen (named from the Greek for water-producer) is the element that is combined with oxygen to form water. It is an essential constituent also of animal and vegetable tissues. It forms, with carbon, mineral oil and gas.

The crystalline structure of the earth's minerals and rocks is universal. A mineral shows it often in having symmetrically arranged facets over its surface, which are generally brilliant in luster. Some of these forms are ■ shown in the figure of quartz on page 17, of garnet, hornblende, and augite on page 22, and of calcite on page 24.

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ELEMENTS. It}

The crystalline structure is commonly shown also in the presence of planes of easy fracture or cleavage in one or more directions ; and when in two or three directions, in causing many crystals to break into fragments of symmetrical form when struck with a hammer. The planes of fracture of a broken bar of iron are surfaces of angular points because of this cleavage, each grain being an imperfect crystal with cleavage planes; and so is the plane of fracture of white marble, the grains in the surface being angular and lustrous. So it is in granite, and through all the earth's deepest foundations wherever the rocks are not melted.

The crystalline structure is assumed on consolidation, whether this takes place from a state of fusion, or of solution, or of vapor. The vapor or moisture of the air is changed to crystals of water, or to flakes, which are aggregations of crystals, with every snowstorm; and the water of ponds crystallizes in becoming ice. Even the sand of the seashore is crystalline; for every grain is a fragment of a crystal.

If conditions are favorable for very bIow, quiet consolidation, single crystals of large size may form ; but if too rapid, the solid snow becomes an aggregation of crystalline grains. This is the condition of granite and of all other igneous rocks except the glassy ones ; for glass, like animal and vegetable tissues, is an exception to the general rule, in being non-crystalline. Even glass becomes crystallized if melted again and slowly cooled; but it is then turned into stone.

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16 CONSTITUTION OF BOCKS.

It 18 thus manifest that the principle which gives to the gem its beauty of form and brilliancy pervades the whole system of material existence. The elements of beauty are everywhere.

I. MINERALS.

Minerals are distinguished by their chemical composition. But, in general, the kinds may be determined by an examination of their color, hardness, luster, Bpecific gravity, and fusibility.

When distinctly crystallized, minerals may be distinguished also by their crystalline characters; for besides symmetry in the general arrangement of the faces, there is, in the same mineral species, a like angle between corresponding faces, and also between like cleavage directions.

1. Consisting of Silica.

Quartz. — Quartz is the most common of the materials of rocks. It is well fitted for this first place; for (1) it is one of the hardest of minerals, the point of a knife-blade or edge of a file making no impression on it; (2) it does not melt in the hottest fire; and (3) it is not dissolved by water, or corroded by either of the common acids. Its durability is its great quality. With a piece of quartz it is easy to write one's name on glass. Another quality of it, distinguishing it from many minerals it resembles, is that it breaks as easily in one direction as another; that is, its crystals have no cleavage.

QUARTZ. 17

It is of various colors and kinds. Flint and hornstone are dark-colored, massive quartz. The smooth-surfaced stones of a pebble bank, whether white, brown, yellow, or black, if uniform (not speckled) in color, are almost all quartz. Mountains thousands of feet high are sometimes made of quartz rocks. The sands of a seashore are mostly quartz, because the grinding of particle against particle which goes on under the heavy dash or swift flow of the waters wears out all other materials, and leaves only the hard quartz particles behind.

Quartz is often found in crystals. The figure annexed shows the form of one of them. It is a regular six-sided prism (iii), with a six-sided pyramid at each end; and it is often as transparent as glass. Frequently the crystals are attached by one end in great numbers to a surface of rock, so that this surface is brilliant with little pyramids of quartz set crowdedly over it, or with pyramids raised on prisins. The inclination of the face of the prism to the adjoining face of the pyramid is always the same (141° 47"), wherever the quartz crystal may come from. These glassy crystals are wholly natural productions, having their forms perfect and luster brilliant when first taken from the rocks.

While some quartz crystals are clear and colorless, others have a purple color, and these are the amethyst of jewelry. Others have a light-yellow color, looking like topaz, and are called false topaz; and others a clear smoky-brown color, and these are the cairngorm stone of Scotland.

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Fig. L

18 CONSTITUTION OP BOCKS.

Agate ia quartz in which the color is arranged xa. thin bands or layers of different shades of color, as white, smoky-brown, red, etc.

Quartz, called in chemistry silica, is a compound of silicon and oxygen, in the ratio of 1 atom of the former to 2 of the latter, and the chemical formula is therefore SiOj.

Quartz, while so enduring, if pulverized and heated, after mixing it with soda, potash, lime, magnesia, or oxide of iron, fuses easily and forms glass. Ordinary glass is made by melting together quartz sand and soda. Again, hot waters containing soda or potash in solution will dissolve out the silica of silica-bearing minerals, and on cooling will deposit it again. The waters of hot springs often contain silica, which, they have taken, along with soda or potash, from some rock with which they have been in contact Through deposits from such solutions (1) agates have been made; (2) fissures in rocks have been filled with quartz, and the fractures thus mended; and (3) the sands of sand beds and gravel of gravel beds have often been cemented into the hardest of rocks.

Opal is also silica, but it differs from quartz in being softer, of less specific gravity, and never crystallized, and it is dissolved much more readily by hot alkaline waters; the precious opal has a beautiful play of colors arising from internal reflections. The silica of Diatoms and of the deposit made by geysers (called geyserite) is in the state of opal.

2. Silicates.

Silica, while existing in rocks abundantly as quartz, also makes, on an average, a third of all their other minerals, limestones excepted; that is, it exists combined with other substances, making various common minerals. These- minerals containing silica are called silicates.

Some of the substances that are combined with silica in the common silicates are the following:

1. Alumina. — Consists of aluminum and oxygen in the ratio 2 : 3, and has therefore the formula AW

In the pure state it constitutes the mineral corundum (a blue variety of which is the blue gem, sapphire, and a red variety, the ruby). It is infusible, like quartz, and harder than all other minerals excepting the diamond. Because so hard, it is ground up to make emery, for grinding and polishing hard stoneB and metals.

2. Lime. — A constituent of all limestone, as well as of many silicates. Quicklime, which is used for making mortar, is more or less pure lime. Consists of one atom of calcium and one of oxygen, and has therefore the symbol CaO.

3. Magnesia. — A constituent of Epsom salt. Consists of magnesium and oxygen in the proportion 1 : 1, and has the symbol MgO.

i. Potash. — An ingredient of wood ashes, hence the name. Consists of potassium (or Jcalium) and oxygen, in the proportion 2:1} its symbol is K,0.

20 CONSTITUTION OP ROCKS.

6. Soda. — An ingredient of the ashes of seaweeds. Consists of sodium (or natrium) and oxygen in the proportion 2:1; its symbol is NajO. Sodium and chlorine constitute common salt, the symbol being KaCl.

6. Iron oxides. — Consist of iron {ferrum, in Latin) and oxygen; one oxide has the symbol FeO, and another Fe a Oj.

Some of the more common silicates are the following:

1. Feldspar. — A feldspar contains, besides silica, the elements of alumina, and of potash, soda, or lime. It is fusible, and is therefore a constituent of most volcanic or igneous rocks.

Feldspar has usually a white or flesh-red color, and sometimes might be mistaken for quartz. But (1) it is not quite so hard as quartz, though too hard to be scratched easily with a knife ; and, besides, (2) it melts when highly heated ; (3) it cleaves in one direction with a bright, even surface, brilliant in the sunshine, and also in another direction but less easily, at right angles or nearly so to the former. While quartz has no cleavage, feldspar has cleavage in two directions transverse to each other.

Common feldspar (called orthoctaae in mineralogy) is a potash feldspar, containing the elements of potash along with those of alumina and silica; another form is a soda feldspar, and is called albite, from its usual white color; others are soda-andAime feldspars, and one of these, called labradorite, is a constituent of trap, basalt, and other igneous rocks ; and another is a lime feldspar.

L

SILICATES. 21

2. Mica. — Mica (often wrongly called isinglass) splits very easily into leaves, thinner than the thinnest paper, which are tough and elastic, and frequently transparent, It does not melt easily, but fuses on the thin edges with high heat. It is the transparent material commonly used in the doors of stoves. Some mica is white, or gray ; it is oftener brownish, and very frequently black. Like feldspar, it contains the elements of silica and alumina, with potash. The common light-colored kind is free from magnesia and is called muscovile; the black kind contains magnesia and iron and is called biottie.

3. Hornblende. — Black hornblende, when occurring in rocks, often looks much like mica, showing lustrous cleavage surfaces ; but it is a brittle mineral, and hence cannot, like mica, be split into thin, flexible leaves or scales with the point of a knife. It makes very tough rocks, hence the first part of the name, horn; the rocks are heavy and sometimes look like an ore of iron, hence the second part, blende, a German word meaning blind or deceitful. It is a silicate, that is, it contains silica, but with it there are iron oxide, magnesia, and lime without potash. Hornblende also occurs of green, brown, and white colors. A. green radiated kind is called actinolite; a white kind, tremolite; a finely fibrous kind having the fiber flexible, asbesttis.

4. Augite or Pyroxene. — Augite is black or dark-green pyroxene, having the same composition as hornblende, and differing only in the shape of its crystals. It is named

22 CONSTITUTION OP ROCKS.

from a Greek word signifying luster, because its crystals are often bright, though not more so than those of hornblende. Two of the crystals of hornblende are represented in Figs. 2, 3, and one of those of augite in. Fig. 4. The angle of the prism of augite (or that between / and J in Fig. 4) is about 87° ; while the angle of the prism of hornblende (between J and/ in Fig. 2) is 1244°; it is mainly owing to this difference that hornblende and augite have distinct names.

Tigs, a, 8, Hornblende ; 4, Augite ; 0, Garnet In mica echini.

6. Garnet. — Usually in dark-red crystals, but often also black, and occurring imbedded in mica schist and other rocks, as represented in Fig. 5. The crystals, here represented, have 12 rhombic faces, or are dodecahedrons. Another common form has 24 faces, and is called a trapezohedron. The most common varieties of garnet contain silica, alumina, iron oxide, and lime. When transparent it is used as a gem.

6. Talc. — A very soft mineral, feeling greasy in the fingers, consisting of silica, magnesia, and water. A granular kind is soapatone.

CARBONATES. 23

7. Serpentine. — A soft mineral or rock, usually greenish, of the same constituents as talc, but in different proportions.

8. Chlorite. — A soft dark-green mineral, having the elements of tale, along with alumina, and often some iron oxide. It is sometimes mica-like in structure.

Other common silicates are staurolite, whose crystals are often crosses; cyanite, usually in bluish bladed forms; epidote, commonly of a yellowish-green color; tourmaline, often in 3- to 9-sided prisms of a black color, the luster pitch-like on a surface of fracture; chrysolite or olivine, a, greenish glass-like silicate of magnesia and iron, common in some igneous rocks, especially basalt.

3. Carbonates.

Carbon has been described as one of the constituents of limestone, mineral coal, charcoal, mineral gas, and mineral oil. One atom of carbon combined with 2 of oxygen forms carbon dioxide, often called carbonic acid; its symbol is COg. Carbonic acid is the gas that escapes from ordinary effervescent waters, such as soda water. It is present in the atmosphere, constituting 3 volumes in every 10,000. Compounds of carbonic acid are called carbonates.

1. Calclte. — -Calcite is carbonate of lime, also called calcium carbonate; its formula is CO a + CaO (or CaC0 3 ). It occurs in crystals that break easily in three directions, affording forms with rhombic faces, like Fig. 6 ; the angles between the faces are 105° 5' and 74° 55'. A very common form is called dog-tooth

2t CONSTITUTION OP BOCKS.

spar; the shape is shown in Fig. 7. Another kind is a 6-sided prism with a low pyramid at either end (Fig. 8). Calcite is easily scratched with the point of a knife. Most limestone is more or less pure ealcite. When calcite or limestone is burnt, carbonic acid escapes as a gas, and the lime ia left. This lime is the so-called quicklime, for making mortar. When a grain of calcite is put into dilute hydrochloric acid, carbonic acid gas is given off freely, producing a brisk effervescence, and the calcite, if it is pure, becomes wholly dissolved. By means of (1) its effervescence with acid, (2) its low degree of hardness, (3) its infusibility in the hottest fire and its burning to quicklime Instead of fusing, calcite or limestone is easily distinguished from feldspar and other minerals. The cleavages in crystals of calcite also separate it from feldspar; for the number of directions is three, and the angle between them is about 105° instead of about 90°. Limestone is mostly massive calcite, more or less impure, but partly dolomite.

2. Hagnesian Limestone, or Dolomite. — Limestone sometimes contains magnesia in place of half of the lime, and it is then called in mineralogy, dolomite, after Dolomieu, a French geologist of the last century. Dolomite, or magnesian limestone, does not effervesce freely unless the acid is heated, and in this respect it differs from calcite. In aspect, calcite and dolomite, are closely alike.

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SULPHATES AND ORES.

4. Sulphates.

Sulphur, when combined with oxygen in the proportion 1 : 3, forms, along with water, the strong acid, sulphuric acid, called often oil of vitriol.

Gypsum. — Gypsum is a sulphate of lime (or calcium sulphate) containing water. It is one of the ingredients deposited by sea water on evaporation. Occasionally it forms large rock masses. The mineral crystallizes in rhombic and arrow-head forms, and has an easy cleavage in one direction. It is soft, pearly in luster, and white to gray and black in color. When fine granular, and fitted for making vases or for other ornamental use, it is called alabaster.

Ho other sulphate occurs in rock masses. Barite, or barium sulphate, also called heavy spar, is a frequent associate of ores in veins. Copperas, or green vitriol, is an iron sulphate, — a common result of the decomposition of iron ores containing sulphur.

6. Ores.

Ores include those metal-bearing minerals that are of economical value. The metal in many ores is in combination with sulphur or oxygen, but in some with arsenic, antimony, and other elements, or with carbonic acid, sulphuric acid, phosphoric acid, and other acids.

The following are a few of the common ores; — ■

1. Pyrite. — Pyrite has nearly the color and luster of

26 CONSTITUTION OF ROCKS.

brass. It is bo hard that it will strike fire with steel

(whence its name, from the Greek for fire), and in this it differs from a yellow ore of copper, called chakopyrite or Fia. 9. copper pyrites, which it much resembles. It

is very often in cubes, like Fig. 9. It consists of aulf bur and iron, nearly 48 parts by weight in 100 being iron; the chemical symbol is FeS E . Pj«*te. Both of the constituents have a strong affinity

for oxygen; and consequently pyrite often changes to vitriol or an iron sulphate, or to the oxide of iron called liTnonite. It is of no use as an ore of iron, because of the difficulty of separating the sulphur; but it is often employed for the making of vitriol. It is the most generally distributed of all metallic minerals, occurring in particles through most rocks, crystalline as well as imcrystalline. Owing to the tendency to alteration just mentioned, it has caused the destruction or disintegration of rocks over the earth's surface to a greater extent than any other agency.

2. Magnetite, or Magnetic Iron Ore. — An iron-black oxide of iron, having a black powder. It is attractable by the magnet. It often occurs in black octahedrons. It is common in northern New York, in Orange County, New York, in Sussex County, New Jersey, and in many other regions, where it constitutes beds of great thickness, and is worked for making iron. It consists of oxygen and iron in the proportion of 4 atoms of the former to 3 of the latter (Fe s O,) ; and contains 72 parts of iron in 100.

ORES. 27

3. Hematite, or Specular Iron Ore. — An oxide of iron, of iron-black color when in crystals, but often bright red when massive, the powder being red. Red ocher is earthy hematite. It is not strongly attracted by a magnet. Like magnetite, it occurs in great beds in northern New York, in the Marquette region, near Lake Superior, in Michigan, and in many other places. It consists of oxygen and iron in the proportion of 3 atoms of the former to 2 of the latter (Fe,0») and contains, when pure, 70 parts by weight of iron in 100.

Nearly all rocks of a reddish or red color owe their color to this oxide of iron.

Hematite and magnetite occur, with small exceptions, in beds instead of veins. When the beds are vertical or nearly so, they look like veins.

4. Limonite. — A brown, brownish-yellow, or black ore of iron, affording a brownisfnyeUow powder; it is sometimes called brown hematite. YeUow ocher is impure or earthy limonite. It differs in composition from hematite only in containing water. When heated, the water is driven off, and it becomes red, and is then hematite in composition. It contains, when pure, about 60 per cent of iron. It is a result of the decomposition of other iron ores, and forms great beds in some regions, as near Salisbury in Connecticut, and in Richmond, Massachusetts. It is often found in bogs, and is then called bog iron. ore. Limonite is often disseminated through clays, giving them a yellowish or brownish color ; and such clays turn red when heated, because they

28 CONSTITUTION OP ROCKS.

lose the water which makes limonite differ from hematite. For this reason, bricks are usually red. Clay for making white pottery must contain no iron.

5. Siderite, or Spathic Iron Ore. — A gray to brown iron carbonate, without metallic luster, consisting of oxide of iron and carbonic acid. When pure, about 48 parts in 100 are iron. It occurs crystallized, and also in impure massive nodular forms. The iron ore of many coal regions is this massive nodular variety. It is a heavy mineral (the specific gravity 3.5 or above), and by this quality it may be distinguished from other grayish or brownish stones. In heated hydrochloric acid it effervesces, owing to the escape of carbonic acid. This ore, like limonite, is sometimes present Sparingly in clays.

6. Chalcopyrite, or Yellow Copper Ore, — A brass-colored mineral consisting of sulphur, iron, and copper, about a third of which is copper. It .is scratched easily with a knife, and affords a dark-green powder, differing thus from pyrite, the mineral it most resembles. It is sometimes mistaken for gold; but, unlike gold, it is brittle. It occurs for the most part in veins with other ores.

7. Galenite, or Lead Ore. — A lead-gray ore, brittle and easily pulverized, and affording a lead-gray powder, consisting of sulphur and lead, the lead making over eighty-six per cent. It often cleaves into cubic or rectangular forms. It is the common kind of lead ore. It often contains a little silver, and then is worked as a silver ore; It occurs

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LIMESTONE AND SANDSTONE. 29

in cavities in limestones, in northern Illinois, . Wisconsin, and Missouri, in Leadville, Colorado, and in Derbyshire, England. It is often found also in veins. At Leadville, and at other places in the Rocky Mountains, the deposits in limestone are directly connected with veins, and are part of the results of vein-making.

8. Malachite. — Green copper carbonate, often accompanying other copper ores. Azurite is a blue copper carbonate, much less common.

IL KINDS OF ROCKS.

The following are the characters of some of the common kinds of rocks : —

1. Limestone ; Magnesian Limestone. — These rocks are partly described on pages 23 and 24. They are of dull shades of color, from white through gray, yellow, red, and brown to black, and of all degrees of texture, from the compactness of flint to a coarse granular texture. The test by acids, by heat, and by use of the point of a knife in trial of the hardness, are the means of distinguishing limestones from other rocks. Chalk is limestone. Ordinary marble is limestone, and sometimes the magnesian kind.

The different kinds of limestone are called calcareous rocks, from the Latin calx, meaning lime.

2. Sandstone. — Sandstone is a rock made of sand. The sand may be quartz, like the sand of most seashores; or it

30 KINDB OP BOCKS.

may be powdered granite, or other powdered rock. Sand, when gathered into beds and consolidated, makes sandstone. Sandstones are the moat common of rocks. They have various dull colors, from white through gray, yellow, and brown to brownish-red and red.

3. Conglomerate. — A conglomerate or pudding stone is a consolidated gravel bed, — gravel being sand mixed with pebbles or small stones. The stones are sometimes large, even a foot in diameter. They are often of quartz, sometimes of other hard rocks, and occasionally of limestone.

4. Shale. — Shale is a fine mud or clay consolidated into a rock having a slaty fracture, but less evenly slaty and less firm than true slate. The colors vary, like the colors of mud or clay, from gray and yellowish shades through red and brown to black. Black is a common color, because the plants and animals that live and die in the mud or over it contain carbon, the chief element of coal, and contribute portions of carbonaceous substances to the mud. Such black shales, when burnt, usually become white or nearly so, because the vegetable or animal material is then burnt out. For the same reason black limestones afford white quicklime.

The loose earthy material of the world, in and out of the water, is mostly either sand, gravel, mud, or clay ; and thus it has been through all ages. Sand is finely pulverized rock. Mud is the same, for the most part, but finer; and it may contain rock that is decomposed as well as pulverized. Olay is a fine kind of mud; it is mainly either pulverized feldspar

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CRYSTALLINE HOCKS. 81

along with quartz in fine grains, or else decomposed feldspar with more or less quartz. It comes from the pulverizing of granite, gneiss, and other rocks containing feldspar, or from their decomposition. Clay often contains iron; and when burnt to make brick it becomes red. Gravel is mixed sand and pebbles.

The consolidation of sand makes sandstones; of pebble beds, conglomerates; of fine mud or clay, shale.

5. Argillaceous Sandstone. — When sands are clayey, the beds make, when consolidated, a clayey, that is, argillaceous, sandstone (argilla, in Latin, meaning day). Such sandstones usually break into thin slabs, in which case they are said to be laminated sandstones ; and, if of sufficient hardness, they make good nagging stones for sidewalks. The common flagging stone used in New York and adjoining states is an argillaceous sandstone.

6. Slate. — Slate, or argillyte, differs from shale in breaking much more evenly, and being much firmer. The slates used for roofing are examples.

7. Granite. — Granite is one of the crystalline rocks, its ingredients being, not worn grains like those of a sandstone or conglomerate, but crystalline grains, all having been rendered crystalline together by a process in which heat was concerned (pp. 39, 42). It consists of grains of three minerals, quartz, feldspar, mica, mixed promiscuously together. The quartz grains are usually grayish or smoky in color (commonly of a darker tint than the feldspar), and have no cleavjogle —

82 KINDS OP BOCKS.

age. The grains of feldspar have cleavage, and therefore show smooth, sparkling surfaces when a fragment of granite is exposed to the sun, and their color is usually white or flesh-red. The mica is much softer than the feldspar, and with a point of a knife blade its grains may be divided into thin, flexible scales ; its colors are white, brownish, or black.

8. Gneiss. — Gneiss has the same constituents as granite; but these constituents are arranged more or less in planes, and, owing to the mica, the rock splits into thick layers, and on a cross fracture appears banded. On account of its splitting into layers gneiss is aaid to be a schistose rock (this term being derived from a Greek word meaning to divide, .and pronounced as if spelt shistose). This schistose structure is the only one distinguishing it from granite. It is somewhat like the laminated structure.

9. Mica Schist. — Mica schist has the same constituents as granite and gneiss, but the quartz and mica are by far the most abundant, especially the mica; and on account of the large proportion of mica, mica schist divides into thin layers. It glistens in the sunshine, owing to the scales of mica. Sometimes the scales of mica are too small to be distinct, and contain some water in combination, and then it is called kydromica schist.

The rocks, granite and gneiss, and gneiss and mica schist, pass into one another through indefinite shadings. There are granites that are slightly gneiss-like, and others of all grades to true gneiss; and there are all grades from gneiss

CRYSTALLINE ROCKS. 8ft