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.
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.
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 alayer of brown to black mud, known aspeat. 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 aslignite. If this lignite is buried still deeper, with consequently more pressure and more time, it changes into the still denser blackbituminous 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 toanthracite, 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 accompanymetamorphism. Then all the associated hydrogen, oxygen and moisture are driven off, and only the carbon remains, which is then known asgraphite. All steps between the stages especially designated occur. The following represent steps only in the series of changes.
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 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 wouldnot 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.
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 ascoke, 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 theby-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.
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 conversionto jelly-like masses by bacteria in the fresh-water marshes of those ancient days.
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 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 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 supplycomes 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 700A.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 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 beingelektronfrom which our word electricity comes. All these finds were of Baltic amber which was doubtless gathered and traded by those earlymen. 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.
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 anasphalt, 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 haveoil-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 withoil 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 isimpossible to find petroleum in igneous or metamorphic rocks.
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 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 outby 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.
Where petroleum has escaped through pores in the rocks, or by way of fissures, and has come to the surface of theearth, 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.
GilsoniteorUintaite, a black bitumen with a brilliant luster and a conchoidal fracture, a hardness of 2 to 2½, and a specific gravity of 1.06.
Maltais 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 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 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.
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 andhave 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.
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 isessentially 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 outlinesof 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 designatedschistosityif irregular andslaty cleavageif 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. Inaddition 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 63is 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 anotherthis 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.
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 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 64shows one gneiss which is in a less advanced stage, the pebbles being simply flattened and the matrixpartly 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:
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 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 sometimesground 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 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 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 thegreater 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.
The color ranges from gray through red, green and purple to black. The grays and black are due to the presence of more or less carbonaceous material in the original rock, the carbon compounds having changed to graphite. The reds and purple are due to the presence of iron oxides, and the green to the presence of chlorite.
While the particles of slate are so small as to be indistinguishable to the unaided eye, the use of thin sections under the microscope shows that slate is composed mostly of quartz and mica, with a wide range of accessory minerals, like chlorite, feldspar, magnetite, hematite, pyrite, calcite, graphite, etc.
According to their chief constituents slates may be distinguished as argillaceous-slate orargillite, bituminous-slate, calcareous-slate, siliceous-slate, etc.
Slate will be found here and there in the metamorphic areas of New England, the Piedmont Plateau, the Lake Superior region, and in many places in the west.
Phyllite is a thinly cleavable, finely micaceous rock of uniform composition, which is intermediate between slate and mica schist. In this case theflakes of mica are large enough to be distinguishable to the eye, but most of the rest of the material can only be identified with the aid of a microscope. It is mostly quartz and sericite. Phyllite represents a degree of metamorphism greater than for slate, but less than for schist; and it may grade into either of these other rocks. Garnets, pyrite, etc., may be present as accessory minerals. The color ranges from nearly white to black, and it is likely to occur in the same places as do slates.
This is a broad term, and includes all those rocks composed essentially of calcium carbonate (limestones) or its mixture with magnesium carbonate (dolomite), which are crystalline, or of granular structure, as a result of metamorphism. It takes less heat to metamorphose a limestone, and for this reason the marbles have a more crystalline structure than most metamorphic rocks; and they do not have the tendency to split or cleave which is so characteristic of most metamorphic rocks. It is only when there is a large amount of mica present that the typical schistosity appears. Commercially the term marble is used to include true marble and also those limestones which will take a high polish; but in this book, and geologically speaking, no rock is a marble unless it has crystalline structure.
Marbles range widely in color according to their impurities. Pure marble is white. Carbonaceous material in the antecedent limestone is changed to graphite in the metamorphicprocess, and makes the marble black, but appears usually in streaks or spots, rather than in any uniform color. An all black “marble” is usually a limestone. The presence of iron colors the marble red or pink. Chlorite makes it green, etc.
Various accessory minerals are common in marbles, such as mica, pyroxene, amphibole, grossularite among the garnets, magnetite, spinel, pyrite, etc., through a long list.
Because it cuts readily in all directions and takes a high polish, marble is widely used as a building stone. In the moist climate of the United States it suffers in being soluble in rain water when used on the outside of a building: but for interior decoration it furnishes some of the finest effects.
The largest marble quarries are developed in Vermont, Massachusetts, New York, Pennsylvania, Georgia, Alabama, Colorado, California, and Washington.
Steatite is a rock composed essentially of talc, which is associated with more or less impurities, such as mica, tremolite, enstatite, quartz, magnetite, etc. It is found in and with metamorphic rocks, and is a rock which has been modified by hydration from a metamorphic predecessor. It was probably first a tremolite or enstatite schist, in which, after the metamorphic rock came into the zone where ground water exists, the tremolite or enstatite was altered to talc, the impurities remaining much as they were in the first place.