This represents the extreme among coarsely crystalline igneous rocks, a whole mass made up of one mineral, and that some one of the pyroxene group. If the mineral can be exactly determined, the rock may be still more definitely named. For instance if it is all augite, then the rock would be called augitite. Like the preceding rocks, pyroxenite is an intrusive rock, usually found in dikes, which are connected with gabbro, and it represents the segregation of one mineral out of the gabbro, and its solidification at one point. Hornblende, magnetite and pyrrhotite may be present as accessory minerals. This is not a common rock, but it illustrates the fact that all possible combinations do occur, if the circumstances have warranted it. It is found near Baltimore, Md., Webster, N. C., and in Montana.
This is a combination of orthoclase feldspar, quartz, and either hornblende, mica or augite in which the crystals are of such small size that they can not be identified with the naked eye. In composition it corresponds to granite, but it is much finer in texture. It differs from trachite by having quartz while the latter has none. This can usually be determined by trying the hardness as none of the other minerals are as hard as 7. It is much harder to distinguish it from dacite which differs only in having plagioclase feldspar in place of the orthoclase, and only the microscope will enable one to make this distinction. Where the distinctioncannot be made these light-colored lavas are often calledfelsite.
Rhyolite is usually an extrusive lava, occurring in sheets, but sometimes it is intrusive, occurring in sills, dikes, and laccoliths. In all these cases the lava has solidified so rapidly, that the crystals are tiny, and only the general effect of a crystalline structure is distinguishable. Rhyolites may occur with porphyritic structure, in which case the presence of the larger feldspar crystals will help to distinguish whether they are orthoclase or not, making the determination easier. The color of rhyolites is green, red or gray, always a decided light shade.
Rhyolites are abundant in the western states, as in the Black Hills, the Yellowstone Park, Colorado, Nevada, California, etc.
The combination of orthoclase feldspar with mica, hornblende or augite is termed trachite, if the texture is dense. It is usually an extrusive lava of light color (green, red or gray), and corresponds in composition to syenite. It can be distinguished from rhyolite by having no quartz, and so nothing to show a hardness above 5.5; but it is difficult to distinguish it from andesite, which differs only in having plagioclase feldspar in place of orthoclase. It sometimes occurs with a porphyritic structure, in which case the feldspar crystals are usually large enough to be distinguished.
Trachites are not abundant in America, but some are found in the Black Hills of South Dakota, in Custer Co., Colo., and in Montana.
The union of plagioclase feldspar, quartz, and either hornblende or mica is termed dacite, if the texture is dense. It is an extrusive lava, occurring mostly in sheets and dikes. It corresponds in composition to quartz-diorite. As the texture is dense it is difficult to distinguish dacite from rhyolite, for both have quartz and differ only in the character of the feldspar, so it is quite common to use the term felsite which does not distinguish between the two, and only states that the rock is dense, light-colored and extrusive. When, as often occurs, the texture is porphyritic, and the feldspars are the large crystals, then exact determination is fairly easy.
Dacites are rather common, occurring on McClelland Peak, Nev., in the Eureka district, Nev., on Lassen’s Peak, Calif., Sepulchre Mt. in the Yellowstone Park, etc.
The union of plagioclase feldspar with mica, hornblende or augite, makes andesite if the texture is dense. The lack of quartz, and so no mineral which has a hardness of over 5.5, makes it possible to distinguish andesite from dacite or rhyolite, but it is hard to distinguish this rock from trachite, which differs only on having orthoclase feldspar in place of plagioclase. When the texture is porphyritic and the feldspars are the large crystals, then it is easy to make the distinction. Andesite gets its name from being the characteristic lava of the Andes Mountains, and is the commonest of all the extruded, light-colored lavas, being the lava ofhundreds of flows throughout the western United States.
The union of plagioclase feldspar and biotite is the commonest type. Plagioclase with hornblende or augite is less common, and, when they do occur, they are usually distinguished ashornblende-andesiteoraugite-andesite. Magnetite, apatite and zircon may be present as accessory minerals.
The lavas of Mt. Hood, Shasta, Rainier and others of the volcanic peaks of the Cascade Range, those at Eureka and Comstock in Nevada, in the Yellowstone National Park, and the porphyries of many peaks in Colorado, like the Henry Mts., etc., which are exposed laccolithic intrusions, are all andesites, as are many more.
The combination of plagioclase feldspar with olivine and augite (or any other pyroxene) makes a heavy, dark-colored, black to dark-brown rock which, if its texture is dense or porphyritic, is termed basalt. This usually has more or less magnetite in it as an accessory mineral, indeed the magnetite may be so abundant as to be a component part of the rock. This magnetite makes trouble for anyone trying to use a compass on or about basalt rocks. These are extrusive or intrusive rocks and correspond in composition to gabbro.
Basalts are among the commonest of igneous rocks, and are popularly designated “trap,” much used as a road ballast on account of its toughness, which is largely due to its dense texture. Thecoast of New England is seamed with dikes of basalt, and through the Adirondack and White Mountains there are a host of these dikes. The crests of such mountains, as the Holyoke Range, the Tom Range, the Talcott Mts., East and West Rocks at New Haven, etc., are all basalt sheets. The Palisades, First Wachung and Second Wachung Mountains of New Jersey are sills of basalt. The Lake Superior region is crisscrossed with basalt dikes. That greatest of all lava fields the Columbia Plateau, covering over 200,000 square miles on the Snake and Columbia Rivers in Oregon, Washington and Idaho, is all basalt. So it goes all down through Nevada, New Mexico and California.
This is a term which properly refers to texture alone, indicating a lava, which has cooled in such a manner that one mineral has crystallized out of the magma first and developed to a larger size, while the mass of the material formed tiny crystals in which the larger ones are embedded. The large crystals are technically known asphenocrysts. The surrounding mass of tiny crystals is termed thematrix. This porphyritic structure is especially characteristic of lavas which have been extruded in large masses, and of intruded lavas in such places as sills and laccoliths.
The term porphyry today has the above precise meaning. It is a much abused word, and has had all sorts of meanings. In the past it was first used to refer to lavas in general, then it cameto be applied to lavas which had been erupted before Tertiary times, that is to all ancient lava sheets. This idea soon proved incorrect, lavas being of the same composition whether ancient or recent. In the West the word is often colloquially used today to designate almost every kind of igneous rock occurring in sheets or dikes, if in any way connected with ore deposits.
When the composition of a rock with porphyritic textures can be determined, the name due to the composition is coupled with that due to texture, making such terms astrachite-porphyry,basalt-porphyry, etc.
Tuff, a term not to be confused with tufa onpage 215, is the name used to designate the finer fragmental ejecta of volcanic eruptions, which are also often referred to as “volcanic ash,” but the word, ash, conveys the false impression that the rock is a remnant of something burned, and is therefore not a good term. When first ejected, tuff is loose material, but it is usually soon cemented to make a more or less firm mass of rock, for which the term, tuff, is still retained. In some cases, while still loose, it is carried by streams to a distance and deposited in more or less sorted and layered beds: and the finer tuff is often carried by the winds and laid down, at a considerable distance from its source, in so called “ash beds.” In both these cases, sedimentary characteristics have been added to the tuff, and layering which is characteristic of sedimentary deposits, is present. These transported tuff beds are really sedimentary,but as there is little change in the material, they are referred to here and not again. These tuff beds are not at all uncommon in the sedimentary deposits of Tertiary age in the Rocky Mountain region. The coarser material of volcanic eruptions usually goes under the head of breccia.
This term is used to describe the coarse fragmental ejecta of volcanic eruptions. It is also used, in the section under sedimentary rocks, in a broad sense to include all angular unworn fragmental material, whether of igneous or sedimentary origin. For this reason, when dealing with igneous rocks, it is usual to designate the fragments according to their composition, making such terms astrachite-breccia,rhyolite-breccia, etc.
While still loose (and also even when cemented into beds of rock), it is customary to designate the smaller fragments, from the size of a grain of wheat up to an inch or two in diameter, aslapilli; the larger fragments, from two inches up to a foot or so in diameter, asbombs; and the largest masses, often tons in weight, asvolcanic blocks.
Lavas, which have cooled so quickly that crystals have not had time to form, have a glassy appearance, and are termed obsidian. If the color is dark, due to the presence of large amounts of those elements which make dark minerals, this lava is termedbasalt-obsidian. Obsidian ischaracterized by its glassy texture, a hardness around 6, and by breaking with a conchoidal fracture, so called because the surface is marked by a series of concentric ridges, something like the lines of growth on a shell. Obsidians vary greatly in color, but are usually red or green to black, and translucent on thin edges. While glassy, all the obsidians contain embryonic crystals, which appear like dust particles floating in the glassy matrix, or there may even be a few larger crystals present, which are often arranged in flow lines. Most all large masses of obsidian have streaks or layers of stony material in them where crystallization has set in, in a limited way.
Near the upper surface, obsidians usually have gas cavities scattered through them, and these may be small and few, or large and numerous. Indeed the cavities may be so numerous as to dominate and give the rock a frothy appearance. In this case, if the cavities are small and more or less uniform, the rock is calledpumice; if they are larger it isscoria. If, as often happens when the lava is ancient and has been buried beneath other rocks, the cavities have been filled with some secondary mineral, then the lava is called anamygdoloid.
Obsidian is found in many localities, especially where there are recent volcanoes, the most famous places being the obsidian cliffs in the Yellowstone Park, those near Mono Lake in California, and many other localities in the Rocky Mountains, the Sierra Nevadas, and the Cascade Mountains.
This is very like obsidian in appearance, but differs in that the glassy material contains from five to ten per cent of water in its composition, the most obvious effect of which is to make the luster resinous, instead of vitreous, as is characteristic of obsidian. The colors are commonly red, green or brown. Pitchstone is associated with recent volcanoes, and some fine specimens have come from Silver Cliffs, Colo., and various parts of New Mexico and Nevada.
Perlite is a glassy lava, containing two to four per cent of water, which, on cooling, has cracked into numerous rounded masses, with a concentric structure, reminding one of the layers of an onion.
While lava is cooling, there is a constant escape of gases, mostly steam, and as these rise through the molten mass they make cavities, near the upper surface, that portion on top often becoming frothy. If this solidifies quickly so that the gas cavities are preserved it is scoria. When the gas cavities are small and uniformly distributed, the rock is called pumice, and often used as a scouring agent. When the cavities are large and irregular the term scoria is generally used. Molten lavas may form various structures, according to the conditions under which they cool, dripping through cracks or from the roof of caves, which often form where the molten lava escapes from a hardened shell, and making stalactites, stalagmites,etc. The very thin lava of the Hawaiian volcanoes may even be blown by the wind into fine threads, known as “Pele’s hair.”
The presence of the gas cavities is so characteristic of the upper surface of lavas which have been extruded; that, where one is dealing with older lavas, now buried beneath other rocks, this fact helps to determine whether the mass is a sheet, rather than a sill; for, in the case of the sill, the lava was forced between layers of sedimentary rocks, and the burden of the overlying rocks did not permit the escape of steam and therefore the upper surface of sills does not have the scoriaceous structure.
When the upper surface of a lava is filled with steam holes, and this lava has been buried beneath other rocks, the seeping waters slowly bring such minerals as quartz, calcite and zeolites and fill the cavities. Such a rock is known as an amygdoloid. It is often confused with porphyry; but, if examined closely, it will be seen that the outlines of the gas cavities are rounded, while the outlines of a crystal, like a phenocryst, are always angular. This will be clear if the amygdoloid onPlate 56is compared with the porphyry onPlate 55.
To this class belong all those rocks which have been laid down by water or wind, or are the results of organic depositions. They includeloose material like sand or day, and also the same materials, when cemented into more or less solid rocks, like sandstone or shale. So long as the material has not been altered from what it was when laid down, the rock is termed sedimentary.
In general the material of which these rocks are composed comes from the weathering and disintegration of other rocks. This does not apply to the organic deposits, for each type of which there is a peculiar mode of formation. To illustrate the typical formation of sedimentary rocks, we may look at the fate of a granite when exposed. At once the surface is attacked by changes of temperature, frost and rain. The various minerals of the granite expand and contract with every change of temperature, but each component mineral has a different coefficient of expansion under heat, so that minute cracks are quickly formed between the minerals. Water gets into these cracks and begins to dissolve the minerals. Feldspar is the most easily attacked, part of it being dissolved and carried away, a small part changing to quartz, and by far the largest part changing to kaolin. The dark mineral is also attacked and partly dissolved, and partly changed to kaolin and iron oxides. The quartz resists solution almost completely. Of these products the kaolin and iron oxides are carried far away and deposited in still water. The quartz and perhaps some of the dark mineral are heavier and carried more slowly, being deposited as sand. This happens to granite everywhere, but in the regions where there isfrost the action is greatly hastened; for water gets into the cracks and expands every time it freezes and thus widens the cracks rapidly, which greatly facilitates the entrance and movement of water in the rock. In a similar way any original rock will be disintegrated, and the residue, after the soluble part has been carried away, becomes sand or clay or mud.
Particles of quartz, kaolin, and lime, separately, or mixed, loose or more or less cemented, with accompanying impurities, make up the great bulk of the sedimentary rocks. They are usually arranged in layers, of varying thickness, as they were laid down by water or the wind. In the same way layered accumulations which are either products of plants or animals, or parts of the plants or animals, are considered sedimentary, as for instance, coal, chalk, petroleum, etc.
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 befurther broken up into one of the finer grained rocks, which the water can carry away and deposit elsewhere. There is little or no layering in talus. If the talus is not carried away but is cemented where it was formed, the resulting mass is termed breccia, but this is not very commonly the case.
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 onPlate 58is such a limestone.
Over most of the earth’s surface there is a covering of rock waste, the product of weathering, some of which isunassorted, and some of it sorted by water or wind. This is all termed soil. It is an ever-moving cover resulting from the decomposition of the underlying rocks, to which have been added in places layers of rock waste brought from afar by the streams. Some soils are rock waste which had been carried clear to the ocean and deposited on the floor of the sea, and is now above sea level, because the floor of the sea has been elevated. Inasmuch as the underlying rocks vary in composition, and as there are areas of transported material, it is clear that the composition of soils must vary from place to place, both as to composition and texture.
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, isclassed as a “light” soil. These terms as used in agriculture refer to the ease with which the soils are worked, and to their penetrability by plant roots.
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 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.
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 aquartz-conglomerate, alimestone-conglomerate, agneiss-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 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 moreoften the sand grains have been transported greater or lesser distances, and in the process have been more or less rounded.
Those sands, which lie where they were formed are calledresidual, 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 asgold-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 thepolished desert sands, there are of course all grades. Glacial sands are angular or “sharp” almost to the degree characteristic of residual sands; and lake-shore sands are between river sands and sea sands in the degree of rounding.
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 mustbe free from impurities, as these color the glass, and much of the sand used for this purpose is quartz, crushed to a fine sand-like condition. “Moulding sand” is a rather fine-grained quartz sand, with a small but very definite admixture of clay, and this is used to make the moulds for castings in foundries. “Polishing sand” is one composed of angular fragments of quartz, usually from stream beds or glacial deposits, or even crushed quartz, and is used for cutting and polishing marble, for sandpaper, and for polishing wood and softer stones. There are many other special uses, like building, ballast, filters, furnaces, etc., in which quartz sand is used, being screened if necessary to get the right sizes.
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.In moist climates where it freezes, this makes many sandstones unsuitable for use as building stones, as they are likely to spale, or chip off, as is seen in the “brown stone” so much used in New York City.
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 sandstoneis cemented with silica and usually very hard.
Calcareous sandstoneis cemented with lime and usually rather soft.
Ferruginous sandstoneis cemented with one of the iron oxides.
Argillaceous sandstoneis 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.
Arkoseis a sandstone composed of quartz and feldspar grains, usually derived from the disintegration of granite and not transported far.
Graywackeis 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.
Gritis a term applied to a coarse sandstone,composed of angular quartz fragments, and used to a considerable extent for millstones.
Flagstoneis 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.
Freestoneis a thick-bedded sandstone, not over hard, so called, because it can be worked freely and equally well in all directions.
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 sluggishstreams. In fresh water the settling of the clay is a very slow process, requiring days, or when very fine, weeks, before the water wholly clears. In salt water, however, the clay sort of coagulates, the particles gathering together in tiny balls, which settle rapidly, so that the water is soon clear.
According to their mode of origin clays are classified as residual, sedimentary, marine, swamp, lake, flood-plain, eolian, etc. But when their uses are considered a very different classification is made, based mostly on their composition, and we speak of China clays or kaolins, fire or refractory clays, paving-brick clays, sewer-pipe, stone-ware, brick, gumbo and slip clays.
Thekaolinorchina claysare 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 claysare 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 claysmay 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 atbest a second grade fire clay. In coal mining sections it is customary to term those beds of clay either above or below the coal, “fire clay”; but this is an unfortunate designation, for though some of them are true fire clays, the most of them are not.
Stone-ware claysare 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 claysmust be plastic, and carry a considerable amount of fluxing material, as the surface of the pipe is expected to vitrify in the burning.
Brick claysare 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 claysrange 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 claysare those with a high percentage of fluxing material; so that, when baked at moderatetemperatures, the surface fuses into a glassy brown or green glaze.
Adobeis an impure calcareous clay, widely used in the western United States for making sun-dried bricks.
Gumbois 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.
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.
When pure or impure clays, or loess, are consolidated, they are all grouped under the name shale. It usually possesses a layered or stratified structure,which makes it possible to split it into thin layers. Of all the sedimentary rocks shale is the commonest, and it may occur in all the places where clay could occur, but the most widely distributed shale is that which made the sea bottom of former times and is more or less calcareous, like the piece onPlate 59, in which bits of shells are still visible. Shale has the same wide variation in composition as has clay, the various types being designated according to the impurity which is present, as:
argillaceous shale, made mostly of clay,
arenaceous shale, shale with more or less sand as an impurity,
calcareous shale, or one with more or less lime as an impurity,
ferruginous shale, or one with iron compounds as impurities,
bituminous shale, or one colored black by the presence of organic matter, remains of either plants or animals.
While of no value as building material, shale may be ground or crushed, and used as a substitute for any corresponding clay, and thus many manufacturers use shale in making fire-clay products, bricks, tile, etc.
Where limestones or shells of any sort have been pulverized, and mixed with more or less impurities, especially clay, the resulting unconsolidated mass is known as marl. It is usually associated with marine formations, and is the finer débris which has settled on the ocean bottom well out from shore,that is out beyond the sandy and mud deposits. Finding it therefore usually indicates a sea bottom recently elevated. It is very characteristic of the southern coastal states, from Maryland all along to Texas.
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 fromthe solution of its lime by rains, but has stood up very well in dry climates.
The varieties of limestone are mostly distinguished according to their mode of origin, some of them being as follows.
Bog Limeis 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 asForamenifera 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’smost famous lectures is the one on chalk, found in hisEssays and Lay Sermons.
Coral Rockis 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 Limestoneis 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 Limestoneis a very fine-grained, compact limestone with clay impurities, the finest of the grain making it usable for making the stone plates used in lithographic printing.On slabs of this limestone figures are drawn in reverse with a special crayon. Then the slab is treated with acid, those parts which are not protected by the drawing being etched away, while the points protected by the drawing remain in low relief. From this slab figures can then be printed.
Travertineis 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 orcalc sinter. Such masses are common around Caledonia, N. Y., Mammoth Hot Springs in the Yellowstone Park, etc.
Onyx marbleis 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 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.