Chapter IV: The Rocks (2)

Arkose is a sandstone composed of quartz and feldspar grains, usually derived from the disintegration of granite and not transported far.

Graywacke is a sandstone composed of quartz, feldspar, and some other mineral, like hornblende-augite, etc., also derived from the disintegration of granites and not transported far.

Grit is a term applied to a coarse sandstone, composed of angular quartz fragments, and used to a considerable extent for millstones.

Flagstone is a thin bedded sandstone, often with mica, which splits easily and uniformly along the bedding planes; so that it can be quarried in large slabs. It was widely used for sidewalks before the advent of concrete.

Freestone is a thick-bedded sandstone, not over hard, so called, because it can be worked freely and equally well in all directions.

Clay

Clay is a term used to describe a mass of fine particles, the most prominent property of which is plasticity when wet. Clays range from masses of pure kaolin to masses of kaolin and related minerals mixed with as much as 60% of impurities, which may be sand, lime, iron oxides, etc. The particles of a fine clay range around ¹/₁₀₀₀ of a millimeter in diameter, while the impurities may be, and usually are, of larger size, up to the size of sand grains.

All clays are of secondary origin, the result of weathering, especially of feldspars, though clays may also result from the weathering of serpentines, gabbros, etc. In some cases after the weathering of feldspar or limestones, the clay may remain just where it was formed, as a residual deposit; but, being so fine-grained, it is usually transported by rain water or by the wind and deposited somewhere else as a sedimentary bed. The quiet waters of a lake are favorable places for such deposits, and many clay beds represent former lake bottoms. Impure clays are often laid down on the flood plains of sluggish streams. In fresh water the settling of the clay is a very slow process, requiring days, or when very fine, weeks, before the water wholly clears. In salt water, however, the clay sort of coagulates, the particles gathering together in tiny balls, which settle rapidly, so that the water is soon clear.

According to their mode of origin clays are classified as residual, sedimentary, marine, swamp, lake, flood-plain, eolian, etc. But when their uses are considered a very different classification is made, based mostly on their composition, and we speak of China clays or kaolins, fire or refractory clays, paving-brick clays, sewer-pipe, stone-ware, brick, gumbo and slip clays.

The kaolin or china clays are residual clays, usually resulting from the decomposition of pegmatite dikes. They must be white when burned, free from iron oxides, and fairly plastic. A good deal of china clay is made by crushing feldspar.

Ball clays are sedimentary clays which remain white when burned, are usually very plastic, and free from iron oxides. They are mostly used in the making of various sorts of china.

Fire clays may or may not have iron oxides in them, but they must be free or nearly free from fluxing materials, such as lime, magnesia and the alkalies (sodium and potassium compounds). They may be more or less plastic, the essential quality being their ability to withstand high temperatures without fusing. Silica (as sand) tends to diminish the refractory quality; so that a clay otherwise suitable, if it has sand in it, becomes at best a second grade fire clay. In coal mining sections it is customary to term those beds of clay either above or below the coal, “fire clay”; but this is an unfortunate designation, for though some of them are true fire clays, the most of them are not.

Stone-ware clays are those with considerable sand and up to five per cent of fluxing materials. They must be plastic enough to be readily worked, and then burn to a dense body at comparatively low temperatures.

Sewer-pipe clays must be plastic, and carry a considerable amount of fluxing material, as the surface of the pipe is expected to vitrify in the burning.

Brick clays are low grade clays and vary greatly in composition. The main requisites are that they mould easily and bake hard at relatively low temperatures with as little warping and cracking as possible. As most clays shrink both in the air drying and in the baking, sand is added when the clay is being mixed. The color is mostly due to the presence of iron impurities. If there are iron oxides and little or no lime, the brick bakes to a red color, but if there is an excess of lime over the iron oxides, it bakes to a cream or buff color, which on vitrifying turns green.

Paving-brick clays range from surface clays, to semirefractory clays, shale being often used. The essential component is enough fluxing material, so that the bricks shall begin to vitrify, or fuse, at not too high temperatures.

Slip clays are those with a high percentage of fluxing material; so that, when baked at moderate temperatures, the surface fuses into a glassy brown or green glaze.

Adobe is an impure calcareous clay, widely used in the western United States for making sun-dried bricks.

Gumbo is a term applied to fine-grained plastic clays which shrink too much in the burning to be useful in manufactures. They can be burned to make an excellent ballast for railroads and highways. They are especially abundant in the Middle Western States.

Loess

This is the name given to a fine grained homogeneous clay-like material, which is a mixture of clay, fine angular fragments of sand, flakes of mica and more or less calcareous matter. It is usually without stratification, and cleaves vertically, so that, when eroded, it forms steep cliffs. Loess covers great areas in the Mississippi Valley, in the Rhine Valley, and in North Central China. By some it is thought to be an accumulation of dust in those regions where the prevailing winds were of diminished velocity and where the grass or other vegetation has served to catch and hold the material; by others it is thought of as a river and lake deposit; and by still others it is thought to be due to the combination of the two modes, wind and flood. The writer inclines to the first view expressed.

Shale Pl. 59

When pure or impure clays, or loess, are consolidated, they are all grouped under the name shale. It usually possesses a layered or stratified structure, which makes it possible to split it into thin layers. Of all the sedimentary rocks shale is the commonest, and it may occur in all the places where clay could occur, but the most widely distributed shale is that which made the sea bottom of former times and is more or less calcareous, like the piece on Plate 59, in which bits of shells are still visible. Shale has the same wide variation in composition as has clay, the various types being designated according to the impurity which is present, as:

_argillaceous shale_, made mostly of clay,

_arenaceous shale_, shale with more or less sand as an impurity,

_calcareous shale_, or one with more or less lime as an impurity,

_ferruginous shale_, or one with iron compounds as impurities,

_bituminous shale_, or one colored black by the presence of organic matter, remains of either plants or animals.

While of no value as building material, shale may be ground or crushed, and used as a substitute for any corresponding clay, and thus many manufacturers use shale in making fire-clay products, bricks, tile, etc.

Marl

Where limestones or shells of any sort have been pulverized, and mixed with more or less impurities, especially clay, the resulting unconsolidated mass is known as marl. It is usually associated with marine formations, and is the finer débris which has settled on the ocean bottom well out from shore, that is out beyond the sandy and mud deposits. Finding it therefore usually indicates a sea bottom recently elevated. It is very characteristic of the southern coastal states, from Maryland all along to Texas.

Limestone

Any mass of marl, or aggregate of calcareous shells, corals, etc., which has become consolidated is known as limestone. It may, and usually does, have a wide range of impurities, chief of which are clay, sand, iron oxides, and bituminous matter, like plant or animal remains. Pure limestone is white, but due to impurities it ranges through grays, greens, browns, to black, and even red, but this last is rarer. It is easily identified by the presence of calcium carbonate, which effervesces in hydrochloric acid. It most often represents deposits in fairly deep water on ocean bottoms of the past, but there is also a wide range of limestones which were formed in fresh water.

Limestone is often burned at temperatures just above 900° C, at which point carbon dioxide goes off as a gas, and leaves calcium oxide, or lime. When this is mixed with water it makes calcium hydroxide, or slaked lime, which is mixed with sand to give it body, and is used as mortar. When exposed to the air, the slaked lime gives up water, and takes back from the air carbon dioxide, and again becomes calcium carbonate with its original hardness. Limestone is also used as one of the elements in all cements. It is also considerably used as a building stone, which, however, suffers in moist climates from the solution of its lime by rains, but has stood up very well in dry climates.

The varieties of limestone are mostly distinguished according to their mode of origin, some of them being as follows.

Bog Lime is a white calcareous powdery deposit on the bottom of ponds in limestone regions, a deposit precipitated from solution by the action of the plants inhabiting the ponds.

Coquina (Plate 59) is the rock formed by the rather loose consolidation of shells and shell fragments. It is particularly characteristic of tropical regions, and is very abundant near St. Augustine, Fla., in which region it was, and still is, cut into blocks and used for building stone. In that mild climate it has stood very well.

Chalk (Plate 60) is a soft fine-grained limestone, formed in the ocean by the accumulation of myriads of the tiny shells of Foramenifera, which are single celled animals, living either a floating life near the surface of the sea, or a creeping life on the bottom. Chalk is composed mostly of the shells of floating Foramenifera, which when the animals died, settled to the bottom and there accumulated, mostly at depths of 600 feet or more. When the mass of unconsolidated shells is dredged up from depths of 50 to 2000 fathoms, it is known as _Foramenifera ooze_. Chalk beds are then indications of an uplifted sea bottom. When consolidated, if pure or nearly so, it makes a white chalk, and the beds may be of considerable thickness, as is the case of the famous cliffs near Dover on either side of the English Channel. One of Huxley’s most famous lectures is the one on chalk, found in his _Essays and Lay Sermons_.

Coral Rock is made by the cementation of fragments of corals. The binding material, as in most stones, is lime; and this sort of rock is associated with coral reefs of either the past or the present. One of the best illustrations of this being the “Dolomite Mountains” in Tyrol. Coral rock, like coquina, has been cut into blocks and used as building stone, as in Bermuda.

Encrinal Limestone (Plate 60) is a rock made by the cementation of fragments of the skeleton of crinoids. These animals belong to the group, echinoderms, and are now extinct except for a few so called “sea-lilies.” They were animals with a central mouth surrounded by long, jointed, flexible arms in multiples of five, and below this a small body inclosed in calcareous plates, all at the top of a long jointed stem. They lived in the sea and in the earlier geological times must have been very abundant; for their remains are so common in places as to make whole layers of limestone.

Hydraulic Limestone is a fine-grained, compact, yellowish limestone with from 13 to 17% of sand, and some clay; which, when it is burned at a temperature a little higher than that used in burning lime, makes a product, that, while not as strong as Portland cement, still like it sets under water.

Lithographic Limestone is a very fine-grained, compact limestone with clay impurities, the finest of the grain making it usable for making the stone plates used in lithographic printing. On slabs of this limestone figures are drawn in reverse with a special crayon. Then the slab is treated with acid, those parts which are not protected by the drawing being etched away, while the points protected by the drawing remain in low relief. From this slab figures can then be printed.

Travertine is a general name, applied to calcareous deposits from fresh water lakes or streams, and has been precipitated either as a result of cooling or evaporation. Some travertines are porous, while others are dense; some are white, while others are colored, often beautifully, by impurities in the water.

Porous deposits of travertine, when made on grass or other like substances, are known as tufa or _calc sinter_. Such masses are common around Caledonia, N. Y., Mammoth Hot Springs in the Yellowstone Park, etc.

Onyx marble is a dense travertine, usually formed as a result of the deposition of lime from the water of springs. It is often banded, due to the presence of impurities in the water at one time, and their absence at other times.

Till

Till is an unconsolidated mass of boulders, pebbles, sand and fine clay, the unsorted material left behind by glaciers when they melted. The boulders and pebbles, while they show some wear, are not rounded like those that have been transported by streams, but have a more or less angular shape; and some of them are polished or striated on one side, where, while frozen in the ice, they were rubbed along the bottom.

One of the most recent geological events in America was the extension of the ice sheet, now covering Greenland, down over north and northeastern North America, until it extended as far south as northern New Jersey, the Ohio River and the Missouri River, and as far west as the Rocky Mountains, but not over the Great Basin, the Cascade Ranges or Alaska. This great mass of ice, thousands of feet thick, moved from two centers, one either side of Hudson Bay, scraping up the loose soil, and grinding off the exposed surfaces of the underlying rock. All this material it carried southward, until the melting along its lower margin equaled the rate at which it advanced. When the melting was faster than the advance the glacial sheet retreated. At the southern limit of the advance this débris was dropped, either making long ridges (moraines) or while the ice was retreating, thicker or thinner sheets. This deposited débris is till.

The soil, and especially the subsoil, in all the regions formerly covered by the ice sheet, is made up very largely of this till; which, where it is undisturbed is often called “hardpan.” When till is mixed with humus it becomes loam. This mixture of material, varying all the way from the fine powdered products of the ice grinding to the great boulder it picked up and carried south, is characteristic of this or any other glaciated country. When this section of country was settled, the boulders and stone were a hindrance to cultivation, and were picked up and piled into stone walls, which are one of the first features to strike the eye.

Tillite

When till is consolidated into solid rock, it is known as tillite. In several cases it has been found buried far beneath the more recent sedimentary rocks; testifying that there were other glacial periods beside the last one which furnished the till.

The Coal Series

Disregarding minor constituents, the plants are largely made up of cellulose, which is a combination of carbon, hydrogen, and oxygen, (C₆H₁₀O₅). If this is heated in the air, where there is plenty of oxygen, it disintegrates, or burns, making carbon dioxide and water; but if the heating is done where the oxygen is excluded, as in a kiln, the hydrogen and oxygen will be driven off and the carbon will remain behind as charcoal. In Nature similar reactions go on, but more slowly. Vegetable matter, exposed to the air, disintegrates into carbon dioxide and water, and there is no solid residue. However, if the vegetable matter is under water, which excludes the air more or less completely including the oxygen in it, then disintegration still takes place, but the products formed are water, (H₂O) marsh gas (CH₄), and some carbon dioxide (CO₂), but a considerable part of the carbon remains behind and accumulates.

Thus in bogs, swamps and ponds, where dead vegetation, especially that growing in the water, piles up, the oxidation is incomplete; so that there gradually accumulates on the bottom a layer of brown to black mud, known as _peat_. More plant remains are constantly being added, and the layer may increase to several feet in thickness. The decomposition is incomplete and some oxygen and hydrogen remain, but the carbon is in a constantly increasing ratio and in proportion far above that in cellulose. In the cold northern climates sphagnum moss is the most efficient peat producing plant, but in temperate and tropical climates the moss is replaced by the leaves, twigs, trunks, etc., of trees, bushes, and vines.

If these peat beds are buried beneath a layer or layers of sediment, especially clay, the peat is sealed up and oxidation stops almost entirely. With the pressure of the superincumbent beds, the peat becomes more and more compact, and changes to a dark-brown or black color. It is then known as _lignite_. If this lignite is buried still deeper, with consequently more pressure and more time, it changes into the still denser black _bituminous coal_. This is as far as it will go unless some new agent is added to the forces already working.

The next step in the series of changes forming coal is associated with mountain making. In case the layers of rock containing beds of coal are folded, and that presupposes at least a moderate increase in heat, the bituminous coal is altered to _anthracite_, which is still denser, and so hard that it breaks with a conchoidal fracture. Alteration may be carried a step still farther, in case the rocks between which lie beds of coal are effected by such high temperatures as accompany metamorphism. Then all the associated hydrogen, oxygen and moisture are driven off, and only the carbon remains, which is then known as _graphite_. All steps between the stages especially designated occur. The following represent steps only in the series of changes.

Peat

Peat is a mass of unconsolidated vegetable matter, which has accumulated under water, and in which the original plant remains are still, at least in part, discernible. It contains a large amount of water, so that before it can be used as a fuel, it is cut out in blocks, which are piled up and left for a time to dry before using. It burns with a long flame and considerable smoke. This country is so well supplied with other fuels, that so far peat has been but little used.

Lignite _brown coal_

Lignite is more compact than peat, and is found buried to some depth under layers of clay or sandstone. It is dark brown to black in color, and still retains pretty clear traces of the plants from which it was derived. It also usually contains a considerable amount of moisture, and when this is dried out, it tends to crumble badly, so that it is undesirable to handle it much, or to ship it far, before using. It has a fair fuel value and is fairly widely used; but it is very desirable that some method be found, by which lignite could be treated to obtain its by-products, and at the same time make it more compact, so it would not crumble with the handling incident to using it in furnaces. There are extensive lignite deposits in this country in North and South Dakota, Montana, Wyoming, Colorado, New Mexico, Texas, Louisiana, and Mississippi.

Bituminous Coal _soft coal_

This type of coal is compact, black in color, and breaks readily, but does not crumble as badly as lignite. It contains considerable water, and still has some hydrogen and oxygen compounds in it. Bituminous coal is the product of plant remains which have been preserved for long periods, (millions of years), sealed from the air by the overlying beds of rock. The pressure has made it compact, and nearly all traces of the original plants have disappeared.

Bituminous coal is our most abundant fuel, occurring the world over in seams from less than an inch in thickness to some over fifteen feet thick. The United States is peculiarly fortunate in the abundant and easily accessible deposits of this type of coal, in Pennsylvania, West Virginia, Ohio, Kentucky, Tennessee, Indiana, Illinois, Michigan, Iowa, Missouri, Kansas, Nebraska, Texas, Utah, and Colorado.

The volatile constituents, hydrogen and oxygen compounds, of bituminous coal may be driven off by heating the coal in closed ovens, and the residual mass is known as _coke_, almost pure carbon. This is distillation, and the ovens in which this is done, without trying to save the volatile products, are called bee-hive ovens, while the more modern ovens which save the by-products are called by-products ovens. A ton of bituminous coal treated in the typical by-products oven, will yield on the average 1410 lb. of coke, 7.1 gallons of tar, 18.9 pounds of ammonia sulphate, etc., 2.4 gallons of light oils, 10440 cubic feet of illuminating gas, about half of this last being used to furnish the heat for the distillation. The coal-tar dye industry is built on the tar thus produced. Toluol, benzol, etc., come from the light oils; and half the gas produced is available for household illumination, etc. Coke is demanded, as it is a superior fuel for melting iron ores, iron and steel, and is made regardless of whether the by-products are used. The coke thus produced is hard, clean, and vesicular; but for some reason as yet unknown, by no means all bituminous coal will produce a coke which has this porous structure. These latter are known as “non-coking,” and are of little use to the steel industry.

Cannel Coal

This is a compact variety of non-coking bituminous coal, with a dull luster and a conchoidal fracture. It contains the largest proportion of volatile hydrocarbon compounds of any variety of coal; so that when the supply of petroleum and natural gas gives out, this will be one of the important sources of obtaining substitutes. Cannel coals occur in Ohio, Indiana, and eastern Kentucky. This cannel coal owes its peculiar fatty nature to the material from which it is derived, it being supposed to have resulted from the accumulation of the spores of lycopod trees, and their conversion to jelly-like masses by bacteria in the fresh-water marshes of those ancient days.

Anthracite _hard coal_

Anthracite coal is hard, black, has a luster, and breaks with a conchoidal fracture. It contains but a low percentage of volatile matter, and so burns with a short flame, and less smoke, than is the case with the other coals. It is always associated with folded rocks, and appears to have been formed as a result of the combined pressure and the higher temperatures, which accompanied mountain making. Still the temperature was not high enough to metamorphose the adjacent rocks. Most of our anthracite comes from northeastern Pennsylvania.

Carbonite

Carbonite is natural coke. It occurs in coal seams which have been cut by dikes or intrusions of igneous rocks, the coal having been thus coked by natural processes. It is not vesicular like artificial coke, for which reason it is not useful as a fuel. Some carbonite is found in the Cerillos coal field of New Mexico, in Colorado, and Virginia.

Jet

Jet is a dense variety of lignite, a fossil wood of black color, which takes a high polish and cuts easily into various ornamental shapes. It has been used for ornaments since early ancient times, beads of jet being found in the early bronze period in England, the supply probably coming from the Yorkshire coast, whence the principal supply comes even to the present day. In Switzerland and Belgium it was used still earlier, even as far back as the Palæolithic age. Jet seems then to have had a talismanic value, and to have been worn to protect the owner. About 700 A.D. crosses and rosaries began to be made of jet, the custom starting at Whitby Abbey, the material being obtained nearby, so that it came to be known as “Whitby jet,” and in the eighteenth century became very popular. In recent times it has been used mostly as jewelry suitable for mourning.

Amber Pl. 61

Amber is a gum which oozed from coniferous trees and was petrified. It is associated with lignite beds of middle Tertiary age. It is usually pale-yellow in color, but at times has a reddish or brownish tinge, and is more or less transparent. It occurs in rounded irregular lumps, up to ten pounds in weight, though most pieces are smaller; and is mostly picked up along certain coasts where it is washed ashore by the waves. Since the earliest records amber has been cast up on the shores of the Baltic, and it was used by peoples as early as in the stone age for ornaments and amulets. It has been found among the remains of the cave dwellers of Switzerland, in Assyrian and Egyptian ruins of prehistoric age, and in Mycenæ in the prehistoric graves of the Greeks, the first recorded reference to it being in Homer, and the Greek name for amber being _elektron_ from which our word electricity comes. All these finds were of Baltic amber which was doubtless gathered and traded by those early men. Even down to the present many men make their living, riding along the shore at low tide and hunting for the amber washed ashore by the waves. As early as 1860 the German geologists concluded that the source of the amber must be lignite beds outcropping beneath the sea level, and started mining for the amber with fair success, so that today two types of Baltic amber are distinguished, “sea stone” which is washed ashore, and “mine stone” taken from the mines. Beside the Baltic locality, it is found along the shores of the Adriatic, Sicily, France, China, and occasionally of North America.

Some pieces of amber are found with insects inclosed and preserved almost as perfectly as if collected yesterday. They were apparently entangled in the gum while still viscid and completely embedded, before fossilization.

The Petroleum Series

Certain sedimentary rocks contain larger or smaller quantities of natural gas, petroleum, mineral tar and asphalt. These are compounds of carbon and hydrogen, or hydrocarbons, and range from gases to solids, each being a mixture of two or more hydrocarbon compounds. The crude petroleum may have either a paraffin base or an asphalt base: in the former case, when the gas, gasoline, kerosene, etc., have been removed by distillation, the solid residue will be paraffin, as in most of the Pennsylvania crude oils; while in the latter case, the solid residue will be an asphalt, as in most of the California and Texas crude oils. In the case of the paraffin series all the compounds belong to the paraffin group, while the asphalt is due to the presence, in addition to the paraffin group, of some of the benzine series of hydrocarbons.

Petroleum is found in sands and shales, which were originally deposited on ancient sea bottoms, the shales generally being the real source of the petroleum. The oil was once the fatty portion of animal bodies (perhaps to some extent of plant bodies), and was separated during decomposition as a result of bacterial activity. Oil thus produced is in tiny droplets, which have a great affinity for clay. After being freed by the bacteria, the oil droplets in muddy water attach themselves to particles of clay, and as the clay settles the oil is carried down with it, the two eventually making a bituminous shale. In clear water, or in water which is in motion, the oil droplets rise to the surface and eventually distill into the air.

The oil, or petroleum, may stay diffused through the shales, in which case we have _oil-bearing shales_, with sometimes as much as 20% of oil. Were there but ¹/₁₀₀₀ of a per cent of oil in a layer of shale 1500 feet thick, this would amount to 750,000 barrels per square mile which is equal to a rich production from wells. When the oil in shale amounts to three per cent or more, it is commercially usable. There are large stretches of petroleum-bearing rocks in New York, Pennsylvania, Ohio, Indiana, and all the way out to the Pacific coast, some of them with oil so abundant, that a blow of the hammer will cause them to smell of petroleum.

In case these oil-bearing shales have been heavily overburdened and compressed, the petroleum may have been more or less completely pressed out of them. Then the droplets uniting have formed a liquid, which has moved out from the shale, and gone wherever it could find open spaces. Sandstones have frequently offered their pore space, and as it filled, have been thus saturated with petroleum. If the sandstones were open to the air, or if fissures extended from them to the surface, the oil has escaped to the surface and evaporated into the air. But in those cases where the sandstone (or other permeable rocks) was covered by an impervious layer, like a dense shale or clay, the oil was confined below the covering layer of rock. Crude oil is lighter than water; so that when natural gas, petroleum and water were all present in the rocks, the gas lies on top, the petroleum next, and the water underneath. With this in mind it is easy to see, that in slightly folded or undulating layers of rock, the gas and petroleum would be caught under upraised folds and domes. This is the basis of prospecting for oil.

If petroleum-bearing layers are depressed far enough beneath the surface to be affected by the high temperatures of the earth’s interior, or have been near volcanic activity, of course the petroleum has been distilled by natural processes, and at most only the residues, like paraffin or asphalt, have remained. For this reason it is impossible to find petroleum in igneous or metamorphic rocks.

Natural gas

Natural gas is the lightest portion of crude oil, and consists mostly of marsh gas (“fire damp,” CH₄) together with other light hydrocarbons, like ethane (C₂H₆), ethylene (C₂H₄), and some carbon dioxide and monoxide. It is colorless, odorless, and burns with a luminous flame. Mixed with air it is explosive. It is found in sedimentary rocks, mostly sandstones, either with or without petroleum. Usually it is under considerable pressure, and escapes with great force wherever a hole permits. In time the gas all escapes through the hole or well, and then the well “runs out.” If petroleum is present under the natural gas, the hole may become an “oil well,” from which petroleum may be pumped, until it in turn is exhausted. The end of an oil supply is usually indicated by the appearance of water in the well. Natural gas is mostly associated with oil districts, as in Pennsylvania, Ohio, Illinois, Texas, California, etc.

Petroleum Crude Oil Pl. 61

Petroleum is a mixture of paraffin compounds all the way from the gases, through gasoline, kerosene, lubricating oils, and vasoline to paraffin. In some of the crude oils there is also an admixture of compounds from the benzine series, in which case, when all the volatile compounds have been distilled off, an asphalt remains. The different components of petroleum may be separated out by heating the crude oil in closed tanks, and drawing off the various substances at the proper temperatures.

Petroleum occurs in sedimentary rocks of marine origin, usually rocks which also contain the shells of some of the animals, the soft parts of which made the oil. To have been preserved the millions of years since the petroleum was first formed, the oil-bearing layers must have been covered by some impervious layer of rock, beneath the domes and anticlines of which the oil has lain ever since. When such a dome or anticlinal fold is perforated by a well, the released oil flows to the surface with a greater or less rush, according to the pressure. Wells may keep flowing for 20 years, sometimes more, sometimes much less. Those which flow with the greatest pressure usually are relatively short lived, at times lasting only a year or two. When this easily obtained oil is exhausted, there is an even greater supply to be obtained by the distillation of the bituminous shales. Petroleum never occurs in igneous or metamorphic rocks, but is found in either sandstones or shales, in places favorable for accumulation, all across that great stretch of ancient sea bottoms, extending from the Appalachian Mountains to the Rocky Mountains, and in the Great Basin between the Rocky Mountains and the Sierra Nevada Range, and also to the west of the Sierras.

Bitumen

Where petroleum has escaped through pores in the rocks, or by way of fissures, and has come to the surface of the earth, the lighter components, thus exposed to the air, have vaporized and escaped, leaving behind a more or less solid residue, which is known as bitumen. If the escape was through a fissure, the bitumen may have accumulated in the fissure until it was filled, making vein bitumen. Or the escape may have been so rapid that the petroleum formed a pool or lake from the surface of which evaporation took place. In time such a pool will give off the gases and volatile compounds, only a residue remaining to make a pitch lake, like the one at Rancho Le Brea near Los Angeles, or an asphalt lake like the one on the island of Trinidad. On account of their varying hardness and composition, some of these bitumens have received special names; as:

Albertite, a black bitumen with a brilliant luster on broken surfaces, a hardness between 1 and 2, and a specific gravity a shade over 1.

Grahamite, a black bitumen, which is brittle, but has a dull luster, a hardness of 2, and a specific gravity of 1.15.

Gilsonite or Uintaite, a black bitumen with a brilliant luster and a conchoidal fracture, a hardness of 2 to 2½, and a specific gravity of 1.06.

Malta is a semi-liquid viscid natural bitumen, which has a considerable distribution in California.

The above varieties of bitumen look a good deal like coal, but are easily distinguished by their lightness (weight about half that of coal), and the fact that with only moderate heat they melt, and become a thick liquid like tar.

Guano

Guano is the accumulation of the excrement of birds (or of other animals like bats) on areas so dry that, though soluble, it is not leached and washed away. It may also contain some of the bones and mummified carcasses of the birds which died on the spot. The greatest of these deposits are on several small islands, just off the west coast of Peru, and now “farmed” by the Peruvian government. In this country there are no true guano beds, except a few accumulations of bat guano in certain caves of Kentucky and Texas, but these are not large enough to become of commercial importance.

Phosphate Rock

Phosphate rock is one composed chiefly of calcium phosphate along with various impurities, such as clay and lime. It occurs in beds, irregular masses, or as concretionary nodules in limestone or sand.

The bedded varieties are in the older sedimentary rocks, in which the phosphate runs from a small percentage up to as high as 85%. Ultimately the phosphate came from either animal excrement, or from bacterial decomposition of animal carcasses and bones. In all the beds it seems to be true that in the first instance the phosphate was laid down as a disseminated deposit in marine beds, usually limestones. Later by the action of water leaching through the rocks, the phosphate was dissolved, and then redeposited elsewhere in a more concentrated form. This may be either in the underlying sandstones, but is more often in limestones, replacing the original lime.

In these secondary deposits, if the phosphate has been laid down in cavities, the resulting phosphate will be in nodular masses. In the case of the Florida and Carolina deposits, these nodules have been freed from their matrix and washed along the river beds, remaining as pebbles in the river sands. The bed deposits are mostly in Kentucky and Idaho. The commercial use for such phosphate rocks is of course the making of fertilizers.

Diatomaceous Earth Pl. 62

Diatoms are tiny single-celled plants living in uncounted millions in the fresh and salt water. Each diatom builds around itself two shells which fit into each other like the cover and box of a pill-box, and each shell is marvelously ornamented. The shells are composed of silica of the opal type. In size the diatoms range from ¹/₅₀₀₀ of an inch in diameter up to the size of a pin head, and they live in such numbers that ordinary surface waters have hundreds of them to the quart, and where they are flourishing up to 250,000 in a quart. When the plants die, or in order to reproduce abandon the shells, these shells fall to the bottom of the pond or the sea, and there accumulate, often making a layer from a few inches thick up to hundreds of feet in extreme cases. If unconsolidated, this mass of tiny shells is known as diatomaceous earth; but if they are consolidated it is called tripolite, so named because the first of them used commercially came from Tripoli.

As the shells are tiny and uniform in size and have a hardness of 6, the diatomaceous earth is used to make a great variety of polishes, scouring soaps, tooth paste, as a filler in certain kinds of paper, in making waterglass, as an absorbent for nitroglycerine, and as packing in insulating compounds, where asbestos would otherwise be used.

Deposits of freshwater diatoms are found all over the United States, usually in thin layers of limited extent, especially in Massachusetts, New York, Michigan, etc. The marine deposits of diatoms are on a much larger scale, there being beds of diatoms in Anne Arundel, Calvert and Charles Counties, Md., up to 25 or 30 feet in thickness. In Santa Barbara County, Cal., there is one bed 2400 feet thick and another 4700 feet thick, beside many other smaller ones. The enormous former wealth of life indicated by these great deposits may be suggested, when it is remembered that it takes about 120,000,000 to make an ounce in weight. They reproduce on an average about once in five days, so that from a single diatom the offspring possible under favorable conditions would amount to over 16,000,000 in four months or over 60 tons in a year. Of such an order is the potential increase of animals or plants, no matter how small, if the rate of reproduction is high.

Metamorphic Rocks

Either a sedimentary or an igneous rock, which has been altered by the combined activities of heat, pressure and chemical action, becomes a metamorphic rock. The process is essentially one, during which the layers of rock come under the influence of such temperatures as are associated with the formation of granite or lavas. Such material as is actually melted becomes igneous rock, but adjacent to the masses actually melted are other rocks which do not melt but, according to the temperature, are more or less changed, and these are the metamorphic rocks. At a distance from the molten masses the changes are minor, but close to the molten magmas extensive changes take place. Though not actually melted the rock near the heat center may be softened, usually is, in which case pebbles and grains or even crystals become soft and plastic, and, as a result of the great pressure, are flattened, giving the rock, when it cools again, a striated appearance. At these high temperatures the water in the rock and also some other substances vaporize, and the hot steam and vapor are active agents in making a great many chemical changes. In some cases material like clay is changed into micas, or chlorite, etc.; in other cases the elements of a mineral will be segregated and large crystals will appear scattered through the metamorphic rock, such as garnets, staurolites, etc.

If one studies a layer of rock both near and far from the molten mass, all grades of change will appear. For example, at a distance a conglomerate maybe unaltered; somewhat nearer the molten mass, the heat and steam may have softened (but not melted) the pebbles and then the pressure has flattened them as though they were dough; and nearest the molten mass, the outlines of the pebbles are lost, only a layered effect remaining, and many of the materials have changed into new minerals, like mica, garnets, etc., but still the layered effect is preserved.

One of the effects of heat and pressure is to flatten the component particles of the rock, so that it tends to split in a direction at right angles to the direction of the pressure, just as particles of flour are softened and flattened under the pressure of the roller; and then when the crust is baked it splits or cleaves at right angles to the direction in which the pressure was exerted by the roller. This tendency to split is not to be confused with either the layering, characteristic of sedimentary rocks, nor the cleavage characteristic of minerals. It has nothing to do with the way the particles were originally deposited, nor with their cleavage; but is due to the pressure, and resembles the pie crust splitting, being irregular and flaky. This is designated _schistosity_ if irregular and _slaty cleavage_ if regular. Schistosity refers to the flaky manner of splitting into thin scales as in mica schists. Slaty cleavage is more regular, this being due to the fact that the material of which slate is made is small particles of clay of uniform size.

The metamorphic rocks are generally more or less folded, as they are always associated with mountain making. These major folds are of large size, from a hundred feet across to several miles from one side to the other. Such folds may also occur in sedimentary rocks or even in igneous rocks and simply express the great lines of yielding, or movement of the crust of the earth. In addition to this there is minor folding or contorting which is characteristic of metamorphic rocks only. When the rocks were heated by their nearness to the molten igneous magmas, they must expand, but being overburdened by thick layers of other rocks, there is no opportunity for yielding vertically, so the layers crumple, making minor folds from a fraction of an inch to a few feet across. Such crumpling, which is so very conspicuous especially where there are bands of quartzite in the rock, is entirely characteristic of metamorphic rocks. It is seen on hosts of the rocks about New York City, all over New England, and in any other metamorphic region. Plate 63 is a photograph of such a crumpled rock which has been smoothed by the glacial ice.

The metamorphic rocks are the most difficult of all the rocks to determine and understand, because the amount of change through which they have gone is greatest, but for this same reason they offer the most interest, for the agents which caused the changes are of the most dramatic type of any that occur in Nature. From one place to another a single layer of metamorphic rock changes according to the greater or less heat to which it was subjected, making a series of related rocks of the same composition but with varied amount of alteration. For this reason in naming metamorphic rocks, a type is named, and from that there will be gradations in one or more directions, both according to composition, and according to amount of heat involved. If it is possible to follow a given layer of metamorphic rock from one place to another this is of great interest; for by this means, many variations in the type will be found, both those resulting from a different amount of heat, and those due to the local changes in the composition of the original rock.

One further consideration has to be kept in mind. When a rock is metamorphosed the high temperatures either drive off all water, or the water may be used up in the making of some of the complex minerals. When such a metamorphic rock later comes near the surface and is exposed to the presence of ground water, and that leaching down from the surface into the rocks, several of the minerals formed at high temperatures will take up this water and make new minerals such as serpentine, chlorite, etc. They are always associated with metamorphic rocks, and have been metamorphic rocks, but since then have become hydrated, forming minerals not at all characteristic of high temperature.

The following shows the relation of the sedimentary and igneous rocks to their metamorphic equivalents.

_Loose sediment_ _Consolidated sediment_ _Metamorphic
equivalent_

gravel conglomerate gneiss
sand (quartz) sandstone quartzite
mud (sand and clay) shale schist
clay shale slate or phyllite
marl limestone marble
peat bituminous coal anthracite to
graphite
coarse igneous rocks such as gneiss
granite, syenite, etc.
fine igneous rocks such as schist
trachite, rhyolite, etc.

In working out the past history of any given region, much of it is done on the basis of this series of equivalents. The finding of limestone, for instance, indicates that the given area was at one time under the sea to a considerable depth, that is from 100 to 1000 feet, but not ocean-bottom depths which run in tens of thousands of feet. Marble indicates the same thing, and so one can go on through all these types of rock.

Gneiss Pl. 64

Gneiss is an old word used by the Saxon miners, and is often very loosely used. Here it is used in its structural sense, and a gneiss may be defined as: a banded metamorphic rock, derived either from a sedimentary or an igneous rock, and is composed of feldspar, quartz, and mica or hornblende, and is coarse enough, so that the constituent minerals can be determined by the eye. It corresponds to a granite, or some sedimentary rock like gravel or conglomerate.

Due to the action of pressure, all the gneisses are banded, and the original constituent particles or crystals are distorted. The lines of banding may be long or short, straight, curved or contorted. When the banding is not conspicuous, the gneiss tends toward a granite. When the banding is thin and the structure appears flaky, the gneiss tends toward a schist. The color varies according to the constituent minerals, from nearly white, through red, gray, brown, or green to nearly black. Plate 64 shows one gneiss which is in a less advanced stage, the pebbles being simply flattened and the matrix partly altered to micaceous minerals, and a second gneiss which is so far advanced that the original constituents are all altered to other minerals and only the banded structure remains. This latter type would have required but little more heat to have completed the melting and changed this to a granite.

Gneisses are very compact and have little or no pore space in them. They are hard and strong and resist weathering well, so that they are widely used as building stone: but they are not as good as granite for this purpose, as they split more readily in one direction and can not therefore be worked so uniformly as can granite.

There are many varieties of gneiss, based either on their origin, composition, or their structure, as follows:

Granite-gneiss is one derived by metamorphism from granite.
Syenite-gneiss is one derived by metamorphism from syenite.
Diorite-gneiss is one derived by metamorphism from diorite.
Gabbro-gneiss is one derived by metamorphism from gabbro.
Biotite-gneiss is one composed of quartz, feldspar and biotite.
Muscovite-gneiss is one composed of quartz, feldspar and muscovite.
Hornblende-gneiss is one composed of quartz, feldspar and hornblende.
Banded-gneiss is one in which the banded structure shows clearly.
Foliated-gneiss is one in which there is thin irregular layering.
Augen-gneiss is one which has concretionary lumps scattered through
it.

Gneisses have a wide distribution over all New England, most of Canada, the Piedmont Plateau, the Lake Superior region, the Rocky Mountains, the Sierra Nevada and the Cascade Ranges.

Quartzite

Quartzite is metamorphosed sand or sandstone, and frequently grades into one or the other. It is a hard compact crystalline rock, which breaks with a splintery or conchoidal fracture. It is distinguished from sandstone by the almost complete lack of pore spaces, its greater hardness and by its crystalline structure. In practice it may be distinguished by the fact that a sandstone in breaking separates between the grains of sand, while a quartzite breaks through the grains.

Some quartzites are almost pure quartz, but others contain impurities of clay, lime or iron, which were in the original sandstone. These alter in the metamorphism to such accessory minerals as feldspar, mica, cyanite, magnetite, hematite, calcite, graphite, etc. The color of quartzite when pure is white, but may be altered to red, yellow, or green by the presence of these accessory minerals.

On account of the difficulty of working the quartzites, they are not much used in building, though they are very durable. When crushed they often make excellent road ballast, or filling for concrete work. The pure varieties are sometimes ground and used in the manufacture of glass.

According to the accessory mineral, the following varieties may be distinguished; chloritic-quartzite, micaceous-quartzite, feldspathic-quartzite, etc.

Quartzites are common in the New England, the Piedmont Plateau, and Lake Superior metamorphic regions, and also in many western localities.

Schist Pl. 65

Schist is a loosely used term, but is used here in its structural sense. It includes those metamorphic rocks which are foliated or composed of thin scaly layers, all more or less alike. The principle minerals are recognizable with the naked eye. In general schists lack feldspar, but there are some special cases in which it may be present. Quartz is an abundant component of schists; and with it there will be one or more minerals of the following groups: mica, chlorite, talc, amphibole or pyroxene. Frequently there are also accessory minerals present, like garnet, staurolite, tourmaline, pyrite, magnetite, etc.

All schists have the schistose structure, and split in one direction with a more or less smooth, though often irregular, surface. At right angles to this surface they break with greater or less difficulty and with a frayed edge. As they get coarser, the schists may grade into gneisses, losing their scaly structure: while on the other side, as the constituent minerals become finer and so small as to be difficult of recognition, schists may grade into slates.

The varieties of schist are based on the mineral associated with the quartz; as mica-schist, chlorite-schist, hornblende-schist, talc-schist, etc.

The color also is due to the constituent minerals other than quartz and ranges widely, mica-schists being white to brown or nearly black, chlorite-schists some shade of green, hornblende-schists from dark green to black, talc-schists white, pale-green, yellowish or gray, etc.

Schists are found all over the same regions as gneisses and quartzites, _i.e._, New England (especially good exposures of schist being seen about New York City), the Lake Superior region, Rocky Mountains, etc. Beside these regions where it occurs native, there are boulders of schist all over the glaciated areas of eastern and northern United States.

Slate

Slate is a metamorphic rock which will split into thin or thick sheets, and is composed of grains so fine as to be indistinguishable to the unaided eye. The cleavage is the result of pressure during metamorphism, and has nothing to do with the bedding or stratification of the sedimentary rock from which it was derived. The original bedding planes may appear as streaks, often more or less plicated, and running at any angle with the cleavage. If these bedding streaks are abundant or very marked, they may make a slate unsuitable for commercial uses. The slaty cleavage may be very perfect and smooth so that the rock splits into fine sheets, in which case it is often used for roofing slate; but by far the greater part of the slates have a cleavage which is not smooth or perfect enough so that they can be so used. Slates are the metamorphic equivalents of shales and muds, and represent the effect of great pressure but with less heat than is associated with schists or phyllite, and consequently with less alteration of the original mineral grains.