  Broadly speaking a rock is an essential part of the crust of the earth, and includes loose material, like sand, mud, or volcanic ashes, as well as compact and solid masses, like sandstone and granite. Rocks are aggregates of minerals, either several minerals grouped together, as are mica, quartz and feldspar to make granite, or large quantities of a single mineral, like quartz grains to make sandstone. The rocks are most conveniently classified according to their mode of origin, into three main groups, igneous, sedimentary, and metamorphic. The igneous rocks are those which have solidified from a molten magma, like lavas, granites, etc. The sedimentary rocks are those which represent accumulations of fragments or grains, derived from various sources, usually the weathering of other rocks, and deposited by such agents as water, wind and organisms. Metamorphic rocks are those which were originally either igneous or sedimentary, but have been altered by the actions of heat, pressure and water, so that the primary character has been changed, often to such an extent as to be obscured. Rocks once formed in any of the above ways are being constantly altered in character by the Igneous RocksIgneous rocks are those which have formed from material that has been melted, which involves temperatures around 1300° C.; or, if there is water in the original material, temperatures as low as 800° C. will suffice. Considering the increase of temperature to be a degree for every 50 feet downward, this involves the rocks having been at depths of 5 to 10 miles below the surface. While at such depths the temperature must be high enough to melt rocks, the great pressure of the overlying rocks seems to keep them solid; for we know that the center of the earth is solid, as is shown by a variety of observations, such as the rate at which earthquake waves are transmitted through the earth, the lack of tidal effects, etc. However, there is every reason to believe that if the pressure is removed from the rocks which are five to ten miles below the surface, there is heat enough at those depths to melt them. When the crust of the earth is folded, as when mountain ranges are formed, the areas under the arches or upward folds are relieved of pressure. Then those rocks, which are under the arches and are relieved, become molten. The molten magma may well up and fill the space beneath the arch where it would cool again very slowly; or, if there is fissuring during the folding, some of the molten material may be forced out through the fissures and pour out over the surface as lava. Another area in which pressures may be locally relieved is in the region of faulting, where the crust of the earth is broken The molten material is termed the magma, and when it reaches the surface, great quantities of water vapor and other gases escape: or these gases may even escape from magmas which do not reach the surface, rising through fissures. As these hot vapors pass through the fissures, they are cooled, and may deposit part or all of their dissolved compounds in the fissure, making veins. Lava is the magma minus the vapors. Magmas vary greatly from place to place, indicating that they are formed locally and do not come from any general interior reservoir, as has sometimes been suggested. When the molten magmas escape to the surface, they are termed extrusive, and as they spread out in a layer this is termed a sheet. This rise and overflow may be quiet, and from time to time one outpouring may follow another making sheet after sheet. Or after one outpouring, the pressure below may cease for a time and allow the lava to solidify and make a cap or cover over the opening. Before more lava can rise, this cover must be removed. This usually happens in an explosive manner, the lava below, with the increasing pressure exerted by its expanding gases, finally exerting enough pressure, so that the cover is broken, or shattered and thrown in thousands of fragments into the air, In case the molten magmas under the relieved areas do not reach the surface they are termed intrusive. Such magmas may remain in the space under a mountain fold, or be forced in fissures part way to the surface. When the magma is forced into more or less vertical cracks and there solidifies, and these are exposed by erosion, they are termed dikes. Sometimes the magmas have risen part way to the surface and then pushed their way between two horizontal layers of rock and there hardened, in which case they are termed sills, when uncovered. The Palisades along the Hudson River are the exposed edge of a sill. Again the molten magmas may well up and spread between two horizontal layers, but come faster than they can spread horizontally, and then the magma takes the form of a half sphere, and the overlying layers of rock are domed up over it. Such a mass is termed a laccolith. In all these cases the mass of igneous rock is only discovered when the overlying rocks have been eroded off. The great mass of molten magma under the arches of mountain ranges simply cools slowly into a granitic type of rock. These masses are exposed when the thousands of feet of overlying rock are eroded off. When these masses are exposed, if of but a few miles in extent, they are called stocks, but, if of many miles in length and breadth, they are batholiths, In all the above cases the exterior of the molten mass cools first, and forms a shell around the rest. The shell determines the size of the mass. As the cooling continues into the interior, it also solidifies, and as all rocks shrink on cooling, cracks develop, separating the mass into smaller pieces. There is usually no regularity about these cracks and the mass is divided into blocks from six inches to three feet in diameter. However, in some cases, especially in sills and laccoliths where the cooling is slower, the shrinkage may be marked by a regular system of cracks which bound the rock into more or less regular hexagonal columns. The Palisades and the Devil’s Tower in Wyoming (See Plate 52) show this structure. The Devil’s Tower is the remnant of a laccolith, all except the central core of which has been eroded away. All of the above terms have nothing to do with composition, but refer entirely to the manner of occurrence. While the igneous rocks are classified according to their composition, the rate at which they cooled has much to do with their texture, and certain names apply to the texture. For instance when the molten lava cools very rapidly, there is no time for the formation of crystals, and the resulting rock is glassy or non-crystalline. If the cooling is slow as in large bodies, crystals have time to form and grow to considerable size as in granites. Between these all grades may occur; and one classification of igneous rocks expresses their rate of cooling, in such terms as the following. Glassy—lavas which have cooled so quickly that they are without distinct crystallization, such as obsidian, pitchstone, etc. Dense or felsitic—lavas which have cooled less rapidly, so that crystals have formed, but in which the crystals are too small to be identified by the unaided eye, such as felsite or basalt. Porphyritic—magmas from which, in solidifying, one mineral has crystallized out first and the crystals have grown to considerable size, while the rest have remained small. Granitoid—magmas which have solidified slowly, so that all the minerals have crystallized completely, and the component crystals are large enough to be recognized readily, as in granite. Fragmental—a term applied to the fragments which have resulted from explosive eruptions of igneous rocks. These fragments may be loose or consolidated. Volcanic ashes are typical. Porous—a term applied to the lava near the upper surface, which is filled with gas cavities, such as pumice. Amygdoloidal—is the term applied to porous lavas, when the cavities have been filled by other minerals, such as calcite or some of the zeolites. In determining a rock, first decide whether it is igneous, sedimentary or metamorphic. The igneous character is recognized by its being either glassy, or composed of masses of crystals irregularly arranged, there being neither layering nor bedding. CLASSIFICATION OF IGNEOUS ROCKS
When it is located as igneous, turn to the key on page 177 and decide as to which type of texture is present. If glassy, the color, luster and type of construction will place it. If the rock is crystalline, first decide whether feldspar is present, and if present, what type: then determine the dark mineral, and lastly whether quartz or olivine is present. In dense rocks the presence of quartz may be determined by trying the hardness, for none of the other constituents of igneous rocks have so great hardness. For example, if it is found that a rock is composed of orthoclase hornblende and quartz, and the texture is granitoid, it is granite: or if the rock is plagioclase feldspar and pyroxene of any sort, it is gabbro, etc.
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| Inorganic origin: | ||
| 1. | Coarse fragmentary material resulting from weathering | talus |
| 2. | The same fragmentary material cemented | breccia |
| 3. | Unsorted material resulting from rock weathering | soil |
| 4. | Coarse fragments rounded by the action of water and wind | gravel |
| 5. | The same material cemented | conglomerate |
| 6. | Finer material deposited by water or wind | sand |
| 7. | The same material cemented | sandstone |
| 8. | The finest material, mostly kaolin, deposited by water | clay |
| 9. | The finest material, deposited by wind | loess |
| 10. | The same material cemented | shale |
| 11. | Fine particles of lime, pure or impure | marl |
| 12. | The same material cemented | limestone |
| 13. | Unassorted material left by the glacial ice | till |
| 14. | The same material cemented | tillite |
| Organic Origin: | ||
| 15. | Limes made from shells, etc. | coquina, chalk, coral rock, etc. |
| 16. | Silica from the shells of plants, etc. | diatomaceous earth, etc. |
| 17. | Carbon from plants | peat, lignite, coal, etc. |
| 18. | Hydrocarbons from animals | petroleum, asphalt, amber, etc. |
| 19. | Phosphates from animals | guano, phosphate rock, etc. |
Talus
Where weathering is very active, especially on or below steep mountain slopes, a mass of loose, angular fragments accumulates. This material is termed talus, a term which refers only to the physical character of the material, and not at all to its composition. If weathering continues these fragments will be
Breccia
Pl. 58
Pl. 58
The term breccia is used to cover all those rocks which are composed of angular fragments, of any composition, and above sand in size, when they are cemented into a solid mass, by any sort of cementing agent. Here the term is used in its broad sense, as compared with the way it was used under igneous rocks.
Breccias may result from the cementing of talus, but more often the breaking up of the material into angular fragments was due to other causes, such as crushing along a fault plane, or in the movements involved in mountain making. In such cases the breccia is of limited extent, but may occur repeatedly in the same neighborhood. Limestone, which has been crushed and then recemented, often makes a rock which takes a good polish and is used in several localities as an ornamental stone in place of marble, in fact often goes in trade circles under the name of “marble.” The breccia figured on Plate 58 is such a limestone.
Over most of the earth’s surface there is a covering of rock waste, the product of weathering, some of which is
Soils range from the finest, composed mostly of clay, to coarse ones, composed of sand, gravel or even boulders. Clay, the finest grained soil, is composed of particles only about ¹/1000th of a millimeter in diameter, of which it would take 720,000 billion particles to make a gram’s weight. Ordinary soils however have about 2 to 5 million particles to the gram.
The average specific gravity of soil with the usual amount of humus in it is from 2.55 to 2.75. In this case however the specific gravity is of less importance than is the volume weight. A cubic foot of water weighs 62½ pounds, that of soil from 75 to 80 pounds, the extremes being 30 lb. for peaty soil and 110 lb. for calcareous sand. The terms “heavy” and “light,” used in agriculture do not refer to the volume weight, for clay which is actually relatively light (70-75 lb. per cubic foot) is classed as a “heavy” soil; while sand, of much greater actual weight, is
Soil is usually divided into an upper darker-colored layer, termed loam, and into a lower, lighter-colored layer, termed subsoil. The presence of humus, resulting from the decomposition of plant and animal remains is the factor which darkens the color and distinguishes the loam; so that loam is a complex of inorganic rock particles plus more or less humus, colloid compounds, bacteria, living plants and animals. The subsoil is mainly rock particles. The distinctions between these two layers break down in arid soils, and often also in swampy regions.
It is this layer of soil on which the water of every rain and flood works, picking part of it up and carrying it along, step by step, to the sea. Though the amount moved on any one day is small, the sum of all the soil transported is enormous, a large river carrying annual incredible amounts. For instance the Mississippi annually deposits in the Gulf of Mexico 476,900,000 metric tons (2204 lb. to the metric ton), of which about a third is in solution. At this rate it takes about 7000 to 9000 years to remove a foot from over the whole drainage basin. This is considerably slower than is the case of some other rivers. While on the one hand soil is being continuously carried away from the surface, on the other hand it is being constantly renewed from below, by the weathering action of water, air and temperature.
Gravel
Gravel is a mass of loose fragments of rock, which have been rounded by water and deposited with little or no sorting, so that larger and smaller pebbles and sand all occur together. It is the deposit laid down by comparatively fast water in inland lakes or along the storm-beaten shores of the sea. Where a swift stream enters quiet water, as where it empties into a lake, there it quickly drops its coarse material as gravel, usually thus building a delta. Gravel also occurs in stream beds, where for any reason the rate of flow is checked. During the recent glacial period, the ice sheet brought down great masses of unsorted material, which was deposited as till, or in moraines. Much of this was then picked up by the running water and moved longer or shorter distances, so that, all over the glaciated country of the northern and eastern United States, there are unusually large numbers of gravel deposits. Gravels are all water laid, and usually show more or less clearly the bedded or stratified structure.
The size of the component pebbles of gravel ranges from great boulders to fine sand, and the finer gravels grade into the coarser sands, the line between gravel and sand being drawn at about the size of a pea, the coarser being gravel, the finer sand.
Gravel is widely used as ballast for railroads and in making highways, because of its tendence to pack well, while the hard pebbles resist wear. It is also widely used in concrete work, bonding in well with the cement, and making it go from three to five times as far.
Conglomerate
Pl. 58
Pl. 58
Conglomerates are composed of rounded pebbles and sand of varying sizes, cemented together into a solid rock. The pebbles may run up to boulders in size, but they have all been more or less rounded by water, and transported some distance. The pebbles may all be of the same composition, or may represent a variety of rocks. When the pebbles are all, or most all, of one sort, the resulting conglomerate is termed a quartz-conglomerate, a limestone-conglomerate, a gneiss-conglomerate, etc. So too the cementing material varies in kind, silica, calcite and iron oxide being the commonest. The color will depend on both the component pebbles and the cement, sometimes one dominating, sometimes the other. There are some of the quartz- and limestone-conglomerates which can be cut and polished to make very handsome stone.
Conglomerates represent consolidated gravels, and always indicate an aqueous origin, quite often the delta of an ancient stream, or the invasion of the sea over the land; so they have become of importance to geologists in interpreting past events.
Sand
Sand is a mass of small rock particles, from the size of a pea down to ¹/500 of an inch in diameter. The material may be any sort of rock, or a mixture of two or more kinds. Sand may be the result of the disintegration of older rocks at the point where it is now found, in which case the grains have the shapes they had in the original rock; but more
Those sands, which lie where they were formed are called residual, and such sand is usually composed of a mixture of angular grains, some harder and others softer, such as quartz, feldspar, mica and hornblende, all mixed together. Where the sand has been transported, only the more resistant minerals have remained, such as quartz, magnetite, cassiderite, etc.; with which there are at times rarer minerals, such as gold, platinum, garnets or topaz. Such sands are known as gold-bearing, topaz-bearing, etc.
The sands from different localities differ greatly. The streams gather the rock particles, and sort them according to the size, which the water flowing at any given rate can carry. When the water is slowed down, it drops all the particles above the size which the new rate of speed can handle. The grains of sand from the bed of a stream are usually more or less angular. The further they are carried, the more they are knocked together and rounded; so that after being carried to the sea, and then thrown up on the beaches, they have been well rounded, especially the larger grains. As the air is less viscid than the water, sand which is transported by the wind, is even more rounded; so that desert sands show the most complete rounding, indeed are even polished; and this is true even of the smaller grains. It is the wind-blown, or desert sands, which flow so evenly in an hourglass. Between the angular residual sands and the
Sands made of particles of lime, calcareous sands, are less resistant to wear than are those of quartz. In regions where the water is “soft” (free from lime), they do not last long, as they are dissolved; but in a limestone region where the water is “hard” (saturated with lime), the grains are not so quickly dissolved and may accumulate into beds of great thickness, as in Florida. Along some shores of the ocean, there occur “green sands,” which are ordinary quartz sands mixed with the dark green mineral glauconite, which is a potassium iron silicate, forming on the ocean bottom as a result of the action of decaying animal matter on iron-bearing clays and potassium-bearing silicates, like feldspar. This is particularly characteristic of some of the sands along the coast of New Jersey.
In places, especially in the beds of rivers, there occur “quicksands.” This is a deposit of fine sand, mixed with a considerable amount of clay, and saturated with water; so that it will not support the weight of a man or an animal. Much that goes under the name of quicksand is a fluid mud, covered with a thin layer of sand.
Sand is used for a wide variety of commercial purposes, and under these conditions gets various trade names; for instance “glass sand” is a pure, colorless to white, quartz sand, which is used as one of the components in making glass. It must
Sandstone
When sand of any sort is cemented so as to make a solid rock, it is termed sandstone, which varies widely according to the size, color and composition of the grains, and also with the sort and amount of the cement. When the size of the grains is larger than that of a pea, sandstone grades into conglomerate; when smaller than ¹/500th of an inch, especially if mixed with clay, it grades into shale. There are all grades of firmness, due to the amount and kind of cement, ranging from those which have little or no cement, but are compact as a result of the pressure of the overlying rocks, to those in which the cement has filled all the pore spaces. In general there is a considerable amount of space between the grains of sand; so that a sandstone will absorb large amounts of water, up to 25% of its bulk.
Sandstones are usually bedded rocks and are relatively easy to quarry, and most of them are not so firmly cemented, but that they can be readily worked or cut into shape by the stone cutter; and so, certain sandstones are very popular for building stone or for trimming on buildings, where they are not too much exposed to the weather.
Sandstone gets a variety of names according to the cement.
Siliceous sandstone is cemented with silica and usually very hard.
Calcareous sandstone is cemented with lime and usually rather soft.
Ferruginous sandstone is cemented with one of the iron oxides.
Argillaceous sandstone is held together with clay impurities, and is usually both soft and of undesirable color.
According to their composition there is also a number of varieties.
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,
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
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
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
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
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,
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,
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 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
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.
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
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
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
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
Bituminous Coal
soft 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
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
Anthracite
hard coal
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
Amber
Pl. 61
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
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
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
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
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
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
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
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
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
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
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
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
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
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 granite, syenite, etc. | gneiss | |
| fine igneous rocks such as trachite, rhyolite, etc. | schist |
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
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
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 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
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
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
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 or argillite, 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
Pl. 66
Pl. 66
Phyllite is a thinly cleavable, finely micaceous rock of uniform composition, which is intermediate between slate and mica schist. In this case the
Marble
Pl. 66
Pl. 66
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 metamorphic
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
Soapstone
Soapstone
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.
It is bluish-gray to green in color, often soft enough to cut with a knife, and has a greasy feel. It is very resistant to heat and acids; for which reasons it has proved very useful commercially in making hearthstones, laundry tubs, and fire backs; and, when powdered, in making certain lubricants. The Indians, in the days before Columbus, took advantage of the ease with which it is cut, to make from it large pots for holding liquids, which are today among the greatest treasures in collections of Indian relics. They also carved pipe-bowls and various ornaments and amulets from soapstone.
It is found in Vermont, Massachusetts, New York, New Jersey, Pennsylvania, Maryland, Virginia, North Carolina, Georgia and California.
Serpentine
Pl. 67
Pl. 67
Pure serpentine is the hydrated silicate of magnesium, as described among the minerals on page 138. Serpentine rock is serpentine with more or less impurities, such as pyroxene, amphibole, olivine, magnetite, chromite, calcite, magnesite, etc. It often also contains mica and such garnets as pyrope, as accessory minerals. Serpentine, like steatite, always occurs in and with metamorphic rocks, and was originally a metamorphic rock, but has since been changed by the hydration of its silicates, when it came into the zone in which ground water is present. In the first instance it was some sort of shale, clay and dolomite, which was metamorphosed to an amphibole or pyroxene schist. When this was exposed to the action of
Serpentine rock is used as an ornamental stone for interior decoration, because it takes a high polish and has pleasing colors, various shades of green. It is however decidedly soft and will stand very little exposure to weather, and it is also filled with seams which make it difficult to get out large slabs.
Serpentine rock occurs fairly commonly in the metamorphic belt of New England and the Piedmont Plateau, and in some of the western states, especially California, Oregon, and Washington.
Ophiolite
Ophicalcite
Ophicalcite
This name is given to marbles which are streaked and spotted with serpentine. They are a mixture of green serpentine and a white or nearly white
Ophicalcite occurs in and with metamorphic rocks, and represents an impure limestone which has been metamorphised, the lime becoming marble, and the impurities becoming such silicates as pyroxene, amphibole, or olivine. This metamorphic rock has then come into the zone of ground-water and the silicate minerals have been changed by hydration to serpentine. Ophicalcite is then a metamorphic rock, in which secondary chemical changes have since taken place. It may have a wide range of accessory minerals present, such as magnetite, chromite, pyrope among the garnets, olivine, etc. Verde antique is a trade name for one of the ophiolites.
While not abundant, ophicalcite is in good demand as an ornamental stone for interior work; for it takes a high polish, and is beautiful; but, on the other hand, it will not stand exposure to the weather for the calcite is soluble, and there are numerous seams and cracks in it making it difficult to obtain large slabs.
It occurs in Quebec, Canada, in the Green Mountains of Vermont, and in the Adirondack Mountains.
