Chapter VII: The Origin and Descent of Rocks (2)

=Iron Ore-beds.=—In a somewhat different way iron ore-deposits are formed by the precipitation of iron oxide or iron carbonate from solutions of ferrous compounds. The ferrous compounds in solution were leached from iron-bearing rocks by percolating waters. The most familiar case is that of iron-bearing springs. On exposure to the air, the iron compounds in solution undergo change, and ferric oxide is thrown down, usually forming _limonite_ (Fe₂O₃,3H₂O), but sometimes _hematite_ (Fe₂O₃). This change is common in marshes and gives origin to “bog-ore.” Similar deposits take place in certain shallow lakes, and hence are known as “lake ore.” Iron ore sometimes also forms at the bottom of a peaty bed or in muddy soil. In connection with the great coal formations, beds of iron carbonate (_siderite_) occur. Organic matter seems to play a great part both in the original solution and the later deposition of these ores. From certain soils and clay-beds on which the ancient coal-producing forests grew, the iron has been almost completely removed, either by the action of the roots, or more probably by organic acids arising from their decay and from the decaying vegetation on the surface. On flowing into shallow bodies of water or into marshes, the waters containing such dissolved iron compounds usually throw down their iron content either as a carbonate (siderite), or as a hydrous ferric oxide (limonite). The siderite is formed where decaying vegetation is present to furnish abundant carbon dioxide and to partially protect the iron solution from oxidation, and the limonite where free oxidation takes place. Sand, silt, clay, or calcium carbonate often accumulates with the iron precipitate, and the result is an impure deposit which becomes an _ironstone_. Such deposits often become segregated into nodules, as will be explained later. It is thought that diatoms sometimes aid in the deposit of iron ore in shallow waters.

=Silicious deposits.=—In the decomposition of igneous rocks, a certain portion of the silica, as well as of the bases, is dissolved and carried away in solution. Certain organisms extract this from solution for their skeletons, just as others extract calcium carbonate. The accumulation of these silicious skeletons often forms silicious rocks. The _diatom_, _radiolarian_, and other oozes (Fig. 353) of the deep sea are the great examples. Sometimes layers of infusorial earth, _tripolite_, arise from the shells of diatoms and other aquatic organisms secreting silica. The waters in which such earths accumulate are rather shallow, and either fresh or salt. The most familiar examples of indurated rocks formed in this general method are the _flints_ and _cherts_ (impure flints) that occur in limestone and chalk, chiefly as nodules, but sometimes in distinct beds.

=Organic rocks.=—While most limestones, chalks, flints, cherts, and the silicious and calcareous oozes are formed through the agency of organisms, they are not themselves strictly organic. There is, however, a small but important group of rocks formed directly from organic matter. In favorable situations the woody parts of plants, falling into water, are so far preserved from decay that they accumulate in beds, and by slow changes pass into _peat_, _lignite_, _bituminous coal_, _anthracite_, and _graphite_. The first of these is composed essentially of carbohydrates and hydrocarbons much as plants are, while the last two are mainly carbon, and the intermediate members represent stages of passage from the first to the last. They are all derived from the strictly organic part of the plants, and spring essentially from the atmosphere and hydrosphere. They are only indirectly associated with the evolutions of the inorganic series.

INTERNAL ALTERATIONS OF ROCKS.

Besides the extreme alterations of rocks at the surface of the earth by which they pass into solution and into residual mantle-rock, and at length by transportation and re-sedimentation become stratified rocks, as just described, those rocks which are not at the surface are subject to changes that give rise to several varieties of _altered rocks_. These changes are taking place constantly under ordinary conditions, though usually very slowly. Under great pressure and heat the changes are relatively rapid and intense, and lead to results not reached under other conditions. These more profound changes are termed _metamorphism_, and will be considered later. It is, however, important to recognize the great fact that the outer part of the earth, for perhaps 20,000 or 30,000 feet, is more or less fractured and permeated by water containing in solution various substances dissolved from the atmosphere, the soil, and the rocks through which it has already passed, and that this permeating and circulating water is now, and for long ages has been, working changes in the rocks, partly by dissolving matter out of them, partly by depositing matter in them, and partly by furnishing a medium through which new combinations of their constituents may take place. This outer fractured portion of the lithosphere has been called the _zone of fracture_.[201]

=Oxidation and deoxidation.=—At and above the surface of the underground water, where the rocks are easily reached by atmospheric waters carrying much free oxygen, and by the air itself, _oxidation_ prevails. Through oxidation the ferrous oxides are changed to ferric oxides, a change which is usually manifested by a transition from a gray, green, or blue color, to buff, brown, yellow, or red. The partial progress of such oxidation is often shown in a fractured block or bowlder whose exterior shows the latter colors, while the interior shows the former. The sulphides, of which common pyrites (FeS₂) is the most familiar, are oxidized into sulphates, and then sometimes pass on into the higher oxides and other compounds. Thus copperas (FeSO₄) arises from pyrites (FeS₂) by direct oxidation of both Fe and S. The sulphuric acid of this compound, uniting with some base stronger than the ferrous oxide, gives rise to further oxidation and results in hematite (Fe₂O₃) and limonite (Fe₂O₃,3H₂O). In general, the mineral constituents of the rocks in this upper zone take on their maximum states of oxidation. This oxidation affects more or less profoundly the character of the rock as a whole. Deeper in the earth oxidation is less prevalent, and the action is sometimes reversed and deoxidation takes place. So also wherever organic matter is undergoing decomposition deoxidation is likely to occur.

=Solution and deposition.=—Solution preponderates in the upper part of the zone of fracture, but deposition is prevalent in its deeper parts. The calcium carbonate and silica dissolved near the surface are often deposited below as calcite and quartz. The sulphates and other sulphur compounds that are formed and dissolved near the surface are apt to be changed into sulphides lower down by deoxidation. The soluble oxides and other compounds formed near the surface are often likewise precipitated below. This is particularly true where the descending waters encounter decomposing organic matter, and where they mingle with waters that have followed other routes and have become charged with different solutes. On coming together, reaction between the constituents takes place, resulting sometimes in new solutions and sometimes in precipitation.

If these lower deposits of calcite, quartz, sulphides, etc., are made in the pores of the rock, they change its texture and composition. If they are made in fissures they constitute _veins_, and if a sufficient percentage of the vein matter consists of valuable metallic compounds, they constitute _ores_.

As the waters descend they suffer greater and greater pressure and some increase of temperature, and these changes modify their power to hold substances in solution. In general, the waters increase in solvent power, but the effect is different for different mineral substances, and hence as a rule the waters are taking up some substances and laying down others as they proceed. After penetrating to greater or less depths, the waters may come again to the surface, either because they are pushed up by the higher head of the waters behind, or because they become warmer and thus lighter, and are forced up by the heavier cold waters above, or else they pass up by diffusion through the descending waters. In any case, the deep, warm waters, usually rather highly charged with material dissolved in their previous courses, are apt to deposit some of their burden as they ascend to horizons of lower pressures and temperatures. They are particularly liable to make deposits where they commingle with other waters differently charged with solutes. Thus internal changes in the body of the rocks are, and for ages have been, taking place. In the upper part of the depositing zone, calcite is the dominant mineral deposited, while in the lower, quartz is more common; but much depends on local conditions and other influences, and no rigid rule holds good.

=Hydration and dehydration.=—Water sometimes unites directly with some of the constituents of a rock and produces hydrated minerals, i.e., minerals that have water as an element of their constitution, not simply water absorbed into their pores. A large class of minerals known as zeolites, because they swell up and undergo life-like contortions when their basic water is driven off by heat, are examples of hydrous products. A more familiar example is limonite (Fe₂O₃,3H₂O), of which yellow ocher is a variety, which on heating sufficiently gives off its water and becomes hematite (Fe₂O₃) or red ocher. The turning of yellow clay to red brick on burning is a familiar example of dehydration. The general tendency in the upper zones penetrated by water is toward hydration. In the lower zones, where the pressure is great, Van Hise holds that there is a tendency toward dehydration, if the rocks have been previously hydrated. This may be the case if rocks have once been near the surface and later deeply buried by the accumulation of sediments on them. If the principle holds, rocks subjected to intense lateral pressure may be dehydrated.

=Carbonation and decarbonation.=—The igneous rocks are largely silicates. The carbonic acid of the surface-waters and of the air acting upon them, converts them, in part, into carbonates. In this way has arisen most of the original supply of calcium and magnesium carbonates. Original carbonates formed in this way are precipitated and redissolved again and again. The carbonates in river-waters are much more largely solutions of previously solid carbonates than original carbonates formed from the silicates. The potassium and sodium of the silicates also form carbonates, but by preference they unite with the sulphur and chlorine, and hence appear more largely as sulphates and chlorides.

Carbonation is usually accompanied by oxidation and hydration. These several processes break up the complex and relatively unstable silicates into simpler and more stable silicates, carbonates, and oxides. This is illustrated by the following formulas illustrative of the changes undergone by augite and labradorite, two common rock-forming minerals.

The composition of augite may be represented by the formula

{ CaO.(Mg,Fe)O.2SiO₂
{ (Mg,Fe)O.(Al,Fe)₂O₃.SiO₂.

Assuming Mg and Fe to be equal in amount in the first half of the above formula, and Mg and Fe to be equal in the first part of the second half, and Al and Fe to be equal in the last part of the second half, doubling the whole and allowing it to be acted on by CO₂ and H₂O, we have

2CaO.2MgO.2FeO.Al₂O₃.Fe₂O₃.6SiO₂ + 6CO₂ + 2H₂O
= 2CaCO₃ + 2MgCO₃ + 2H₂O.Al₂O₃.2SiO₂ + 2FeCO₃ + Fe₂O₃ + 4SiO₂.

The hydrous silicate of the last part of the equation is kaolin.

The composition of labradorite is represented by the formula

{ CaO.Al₂O₃.2SiO₂
{ Na₂O.Al₂O₃.6SiO₂.

Assuming the two molecules represented by this formula to be equally abundant, and allowing the whole to be acted on by H₂O and CO₂, we have

CaO.Na₂O.2Al₂O₃.8SiO₂ + 4H₂O + 2CO₂
= CaCO₃ + Na₂CO₃ + 2(2H₂O.Al₂O₃.2SiO₂) + 4SiO₂.

When waters charged with carbonates descend into the earth they are likely to precipitate a portion of their burden, forming calcite and other crystalline carbonates, and hence these are among the most common minerals found in veins and rock cavities. Carbonates are also deposited when carbonate-charged waters come to the surface and evaporate or lose a part of their carbon dioxide.

Decarbonation also takes place, but it is, at least at the surface, a much less common process, and its conditions are less well understood. Sufficiently high heat will drive off the carbon dioxide, as in the artificial process of burning lime, but this is rarely observed in nature. Even lava intrusions do not usually reduce limestone to caustic lime at any appreciable distance from the contact. It is believed, however, that in the deeper zones, where high pressure and heat prevail, carbonates are changed into silicates, thus in a way reversing the process that prevails at the surface, and setting free again a portion of the carbon dioxide that had become locked up in the formation of the carbonates. To this action some of the carbon dioxide of deep-seated thermal springs is assigned.

The carbonation of the silicates takes place at the expense of the carbon dioxide of the atmosphere and hydrosphere, and hence in proportion as the igneous rocks are changed into carbonates, the atmosphere and hydrosphere are depleted of carbon dioxide, new supplies being neglected. As plants are dependent on carbon dioxide for their principal food, and as animals are dependent on plants for their food, directly or indirectly, the process of carbonation has a profound bearing on the life-history of the earth, and will often invite attention in the historical chapters. It is sufficient here to note that carbonation is one of the chief processes in the alteration of igneous rocks and furnishes, directly and indirectly, a larger percentage of the mineral substances dissolved in the waters that flow from the land, than any other single process.

=Molecular rearrangements.=—Besides these and similar changes that involve additions and subtractions through the agency of percolating water, the molecules of some of the rock constituents rearrange themselves, or the elements enter into new chemical relations; thus, pyroxene may pass into hornblende by a change of the crystalline arrangement of the molecules. The change may sometimes be caught in progress, the outer part of the crystal being hornblende (which when thus formed is called _uralite_), while the heart of the crystal remains pyroxene. So aragonite may pass into calcite.

By changes of the foregoing kinds, many crystalline rocks are much altered. Some become _chloritic_ from the development of the soft, green hydrated mineral, chlorite, derived from the pyroxene, amphibole, biotite, and perhaps other silicates of the original rock. Others become _talcose_ from the development of talc, a very soft, unctuous, hydrous magnesian silicate developed from the magnesian minerals of the original rock. _Soapstone_ or _steatite_ is a rock composed essentially of such secondary material. _Serpentine_ is a rock made up of a similar secondary mineral (serpentine) apparently derived from chrysolite (olivine) and other magnesian minerals. _Epidote_, a complex lime-iron-alumina silicate, often recognizable by its peculiar pistachio-green color, is derived from other silicates, and is rather common in many varieties of crystalline rocks. _Melaphyre_ is a name applied rather loosely and variously to certain altered basic rocks of the basalt family. _Diabase_ is essentially an altered dolerite. Nearly all the very ancient basaltic rocks show notable degrees of alteration, even though they appear to have escaped unusual dynamic conditions since their original formation, and hence their alteration seems to have resulted chiefly from the operation of unobtrusive agencies, chief among which is the circulation of water.

_The Salient Features of Rock Descent._

The foregoing processes by which primitive or igneous rocks are disintegrated and their constituents converted into fragmental material may be said to constitute _the descent of rocks_ in its fuller sense. Viewed chemically, the great features of the process are (1) the breaking down of the complex silicates, and (2) the gathering of the resultant simpler silicates (mainly aluminum silicates) into the silt and clay beds, (3) the assembling of a large part of the free acidic element (the quartz) into the sand and gravel beds, and (4) the concentration of a large part of the earthy basic element (the calcium, magnesium, and iron oxides) into the calcareous, magnesian, and iron deposits, while (5) a large part of the alkaline basic remainder (the sodium, and potassium oxides) is dissolved and held in the sea-water. Physically, the great features are (1) the disaggregation of the antecedent rock, and (2) the separation from one another of products which are physically unlike, that is, the coarser from the finer, and the heavier from the lighter, and (3) the aggregation of these diverse materials in more or less distinct beds. It is to be noted that while the rearrangement of the sediments is made on the basis of their physical characters, it results in chemical differentiation as well, for the products of rock decay, which are physically diverse, are often chemically diverse as well. The physical assortment and the stratification are to be looked upon as a step in the direction of a simpler grouping of the material. On the whole, the process is descensional in character.

THE REASCENSIONAL PROCESS.

Running hand in hand with this descensional process, there has always been a reascensional process by which the coherence, the crystallization, and in some measure the complex composition of the rocks are restored. This is partially due to external mechanical agencies, but chiefly to internal chemical and molecular forces.

Two general phases of this reconstructional work are recognized. The first, simplest and most universal, is that by which the incoherent materials produced by the descensional processes, i.e., the muds, sands, and clastic materials generally, are hardened into firm, coherent shales, sandstones, and limestones, and incidentally more or less changed in composition and molecular arrangement. The second is that by which more profound changes of induration and of composition are wrought, bringing the rock back to a state resembling its original crystalline character. This is known as _metamorphism_. Often, however, it is but an extension and intensification of the more common processes of the first class. Metamorphism is essentially reconstruction.

=Induration under ordinary pressures and temperatures.=—All kinds of loose fragmental material, whether soils, earths, clays, sands, gravels, volcanic ashes, cinders, or other forms of clastic or pyroclastic material, may become hardened into firm rock either by _pressure_, or by _cementation_, or by both. Pressure and cementation commonly act together and aid each other. The ordinary pressures arise from the weight of the overlying material, and these of course increase with depth. Extraordinary pressures arise from the shrinkage of the earth and perhaps from other sources. The fragments of the clastic material, on being pressed together for long periods, weld more or less at the points of contact. If they are irregular, angular, or elongate, they come to interlock more or less like the fragments of macadam, and this coöperates with the welding. The process is greatly aided by water-bearing solutions of lime, silica, etc. which are deposited at the points where the fragments press upon each other. It is here that the capillary spaces are most minute and deposition is most liable to take place. Sometimes a film of mineral matter is laid down over the surfaces of the fragments and serves to bind them together. This process goes on wherever the ground-waters are in a depositing condition, just as the opposite process of disintegration takes place wherever the waters are in a solvent state. At and near the surface of the land, the waters are usually in the latter condition and disintegration is in progress, as already noted, but this is not always so. At times and places, the water from within the rock-mass may come to the surface and evaporate, and in so doing leave all its dissolved material on the surface, or within the outer pores of the mass, as cementing material. The exterior thus becomes firmly bound together, “case-hardened,” as it is termed. This may be seen in the drying of a lump of mud, the exterior of which often becomes quite firm. It is seen in quarry-rock, especially sandstone, which is sometimes soft and easily worked when taken wet from the earth, but which hardens as the water—the “sap” of the quarrymen—dries out and deposits its solutes in the capillary spaces of the grains of the surface. It is obvious that it is the very last of the “sap” which contains the most concentrated solutes, and that this last remnant is held in the minute capillary spaces where the grains touch each other, and hence the last stage of drying leaves the cement at the points where it is most effective. In natural exposures of sandstone, the pores of the outer shell sometimes become almost completely filled in this way with silicious deposits, and the sandstone is changed into a quartzite.

In the sea, and in the deep water underground, the common habit of the water is to deposit more than to dissolve, though it is doing more or less of both. As a rule, therefore, loose material in these situations becomes bound more or less firmly into rock, and hence what were originally loose sand beds become _sandstones_; what were soft muds become _shales_ or _limestone_, according to composition; what was gravel becomes _conglomerate_; what was chipstone becomes _breccia_; what were volcanic ashes, cinders, and lapilli become _tuffs_; and what were masses of volcanic blocks and coarse fragments become _agglomerates_.

+Fig.+ 354.—Quartz crystal enlarged by secondary growth. The shaded
outline represents the outline of the sand grain; the solid lines,
the outline after secondary growth. Magnified 67 diameters. (Van
Hise.)

+Fig.+ 355.—Sandstone and quartzite texture. The shaded outlines
represent the surfaces of the sand grains before growth, the
intervening white portions, the added quartz, and the black portions,
unfilled spaces. Open spaces characterize sandstone. When the spaces
are filled with quartz, the rock becomes quartzite. Magnified 35
diameters. (Van Hise.)]

The cementing process works at times in specially interesting ways. In quartz sandstones, the grains are worn fragments of quartz crystals, formed originally in quartz-bearing rock. The crystalline force in these remnants controls the arrangement of the new molecules of silica deposited about them. The result is that the new deposits tend to build up the original forms of the crystals from which the sand grains were derived (Fig. 354). Sometimes a film of iron oxide has formed about the grain of sand before the addition of the new silica. This, or some difference of color, may clearly distinguish the original grain from subsequent additions. Sometimes the adjacent grains of sandstone are rebuilt in this way until the interstices are completely filled. When this has been accomplished, the sandstone becomes a quartzite (Fig. 355). Most quartzites indeed appear to have been formed in this way, but mainly under special conditions that promote the deposition of silica. Grains of other minerals, such as feldspar, are subject to similar secondary enlargement (Fig. 356).

Sometimes the new material is deposited in the form of concentric shells about the particles of sediment, building them up into little spheres. Rock formed of such spherules is known as _oolite_, from the resemblance of the grains to the roe of fish (Fig. 357). Sometimes the nuclei of the concretions are grains of quartz sand, and the added concentric layers are of calcium carbonate. In this case the structure is quite obvious; but perhaps more frequently the nuclei are minute and difficult to identify, and the concentric shells make up the main mass of the grains. Certain formations, as the oolitic limestone of Indiana and elsewhere, and the Upper and Lower Oolites of England, are characterized by this structure. In most cases these accretions probably grew in depositing waters that gently rolled the grains while layers were being added. They thus do not fall under the head of cementation after the beds were formed; but concentric additions to the grains appear sometimes to have taken place after they were formed into beds.

=Cavity filling.=—When cavities of some size occur in rocks and the percolating waters are in a depositing state, the interiors of the cavities are sometimes lined with concentric layers of deposit. Here, instead of building _out_ from a nucleus, the waters build _in_ from the walls of the cavity. The _agate structure_ (Fig. 358) is a case of this kind, in which the successive layers are commonly silica in the form of chalcedony and differ from each other in color and texture. Often before the cavity is entirely filled, the deposit changes from chalcedony, to crystals of quartz, which grow with their bases on the walls and their pyramidal points toward the center of the cavity. _Geodes_ are examples of a similar process in which the cavity is but partially filled with crystals which have their bases set on the walls of the cavity and their points directed inwards (Fig. 359). The crystals of geodes are most commonly quartz or calcite, but they may be any other mineral that the waters are capable of depositing. Very large cavities lined in this way are known to miners as _vuggs_, and these grade on into caves lined with crystals and with _stalactite_ and _stalagmite_. These are the largest expression of the solidifying process by means of internal deposition.

=Fissure-filling; veins.=—Cracks, crevices, and fissures filled by deposition in a similar way give rise to _veins_ (Fig. 360). Here the filling grows from the walls toward the center, and hence often has a banded appearance. By this filling of cracks and crevices, the circulating water heals the breaks in the rocks. Frequently a crushed zone is thus restored to a solid state. When the fissures are deep and wide and traverse different formations, conditions are afforded for very complex deposits, and for the concentration of rare and valuable material originally dispersed through a great mass of rock. Ore deposition in such veins is usually treated as a theme by itself, but it is really but a declared expression of the work which the percolating waters are doing throughout all the rocks which they penetrate. Most of the fine crystals that grace mineralogical collections were formed in cavities and fissures by deposition from circulating mineralized waters.

=Solution as well as deposition.=—A further phase of the process needs attention. The percolating waters are constantly taking up matter as well as throwing it down, and so, while they are cementing fragments together and healing fractures, they are also removing material, and a rock may be growing porous and cavernous at the same time that its fragments are being united. Cavities may be formed at one stage and filled at another; matter may be taken up at one point and put down at another, and so an internal reconstruction is in slow progress.

=Concretions.=—A notable phase of this internal reconstruction is the assembling together of like kinds of matter. For instance, silica that was probably deposited in the form of the silicious shells and spicules of plants and animals, and was disseminated through the sediments as originally formed, is aggregated into nodules of chert or flint (Fig. 361); similarly, concretions of ferrous carbonate or calcium carbonate grow in sands, silts, or muds; clusters of crystals of pyrite (FeS₂), of sphalerite (ZnS), and galenite (PbS) are formed in clayey layers, pressing the clay back as they grow; and in many other cases, kind comes to kind. Some concretions probably form during the accumulation of the beds in which they lie.

=Replacements and pseudomorphs.=—So also there are replacements, sometimes resulting in imitative or false forms. Frequently the calcium carbonate of corals, molluscan shells, etc., is replaced by silica, and this substitution is brought about so gradually, particle by particle, that the minutest details of structure are sometimes fully preserved. This is often of great service in their study, since the limestone in which they are imbedded may often be dissolved away, while the silicified fossil is unaffected. So woody matter is sometimes replaced by silica, forming silicified wood. Similarly, the molecules of one crystal are sometimes replaced by different material, as the molecules of calcite by zinc carbonate, giving a pseudomorph of zinc carbonate after calcite.

=Incipient crystallization.=—A more general change is incipient crystallization. Some common limestones and dolomites are now largely made up of small crystals, though the mass was originally a calcareous mud or ooze. Incipient crystals are formed in shales and other sediments. This process, like the preceding, is a kind of incipient metamorphism or reconstruction, but it is a pervasive process, taking place under ordinary conditions of heat and pressure, and through the agency of circulating ground-waters.

By these and similar processes the fragmental deposits are solidified into firm rock and undergo internal changes which more or less reorganize the matter of which they are composed. The process is a very slow one usually. Some of the sands and muds of very early geologic ages are yet imperfectly solidified; e.g., much of the St. Peter’s sandstone, a very ancient formation, is yet so incoherent as to break down into sand in being dug out, and is used for mortar sand much more than for building stone. Some of the Hudson River shales of scarcely less age are more nearly clay than hard rock. But these are examples of excessive slowness and slightness of change. In general, all but the most recent deposits show notable progress in reconstruction.

_Reconstruction under Exceptional Conditions._

Two special conditions greatly influence changes in rocks, viz., pressure and heat. Their action gives rise to three general cases, but these blend indefinitely: (1) exceptional pressure without great heat, (2) great heat without exceptional pressure, and (3) great heat and great pressure conjoined. Exceptional pressure may arise from the weight of overlying rocks, or from lateral thrust due to the shrinkage of the globe, and occasionally from other causes. Exceptional heat may arise from pressure, from the intrusion of hot lavas, and occasionally from other sources. In the case of intruded lavas there may or may not be exceptional pressure. Thrust usually gives heat as well as pressure, but if lateral thrust acts on rocks near the surface, they may be mashed into new forms without becoming very exceptionally heated, though some rise of temperature is inevitable.

.)

(1) =Slaty structure.=—When rocks made up of clastic particles are compressed in a given direction and are relatively free to expand at right angles to the direction of pressure, the particles that are already elongated tend to take positions with their longer axes at right angles to the direction of pressure, and all particles, whether elongate or not, are more or less flattened in a plane transverse to the direction of pressure. This may be readily seen where the particles are large (Fig. 362). As a result of the orientation and flattening of their particles, rocks so affected split more readily between the elongate and flattened particles than across them. In other words, the rocks cleave along planes normal to the direction of compression, and break with difficulty and with rough fracture across the planes of cleavage. The condition thus induced is known as slaty structure (Fig. 363), and is best illustrated by roofing-slate, which was originally a mud, later a shale, and finally assumed the slaty condition under strong compression. Sometimes the original bedding may still be seen running across the induced cleavage planes (Fig. 364). As the original mud beds were horizontal or nearly so, and as the thrust is usually horizontal or nearly so, the induced cleavage commonly crosses the bedding planes at a high angle (Fig. 364); but after the beds are tilted or bent, the lines of pressure take new directions relative to the bedding planes, and the angles between the original bedding and the slaty cleavages usually become smaller, and may even disappear in exceptional cases. Limestones, sandstones, and conglomerates are not so easily compressed as mudstones, and they usually take on only an imperfect cleavage normal to the direction of pressure. Often they merely show some little compacting, while the shaly strata between them are converted into slate. Obviously the direction of slaty cleavage may be used to determine the direction of the compressing force, and is thus serviceable in dynamic studies.

=Foliation, schistosity.=—A more intense application of pressure in a given direction is capable of breaking down and deforming the most resistant rock. This must necessarily be attended with the evolution of much heat, and thermal effects are mingled with pressure effects, but the thermal effects may be neglected for the moment. The first stage of the mechanical effect of the compression may be to crush the rock more or less. It thus becomes granular or fragmental, and is really a peculiar species of clastic rock (_autoclastic_). At a further stage, the fragmented material may be pressed into layers or leaves, much as in the development of slaty cleavage, but as a result of the nature of the material, the cleavage is less perfect. This is often attended by more or less shearing of the material upon itself, and thus a rude fissility and foliation is developed. The result, including the attendant metamorphism about to be described, is a _foliated_ or _schistose structure_ (Figs. 365 and 366). Even the most massive rocks may be reduced to the foliated form by this process; thus, a granite may be mashed into a _gneiss_—which is a granite in composition, but has a foliated structure—or a basalt may be converted into a _schist_, a common term for foliated crystalline rocks. Porphyritic rock rendered schistose by pressure is shown in Fig. 366. When massive rocks like granite or basalt are thus crushed down into the foliated form, the process is in a sense degradational. It is a kind of _katamorphism_ or downward change. It is often difficult to differentiate the schists thus derived by degrading massive rocks, from those developed by ascensional processes from clastic formations (_anamorphism_). The action of heat is important in the evolution of schists of both classes, but the effects of heat may best be taken up where it acts measurably alone.

=Metamorphism by heat.=—When a mass of lava is poured out upon the surface, it bakes the mantle-rock which it overruns, in greater or less degree, depending on the mass and temperature. The nature of the effect is much the same as in the process of brick-making, a dehydration of the material, a hardening of the loose matter by the partial welding of the particles, and sometimes the partial fusion of the surface and the development of new compounds, usually glassy, but sometimes partially crystalline. In both the natural and the artificial process, the time element is short, the pressure trivial, and the water action limited. If the heat were to become sufficiently intense, the result would be fusion, i.e., a lava which would solidify into a glass. In such a case, the rock cycle would be carried back to the initial molten state and a new cycle instituted, but this does not usually take place when lava merely overflows the surface.

If lavas, instead of rising to the surface, wedge in between layers of rock and form _sills_, or interstratified sheets, the surface above as well as that below is baked, and as the excess of heat of the lava can only escape through the neighboring rock, the effects for a given mass of lava are more considerable, and as the time element and the water action (and sometimes the pressure) are usually greater than in the case of extruded lavas, the effects tend rather toward chemical and crystalline change than to simple baking. This tendency increases with increase in the mass of the lava and in its temperature. Sometimes enormous masses of very hot lava are thrust in between or among the strata that lie beneath the surface, and bring to bear upon them intense heat for a long period. So also, when a vent or fissure is the passageway for lavas that continue to come to the surface for long periods, as in the case of persistent volcanoes, the rocks which form the walls of the vent or fissure are heated for a long time, and this gives rise to metamorphism through heat, without very unusual pressure, but usually with the free aid of water. In these cases the chief effect is chemical recombination and crystallization. In the limestones and sandstones it is simple; in the shales more complex. In pure limestones and dolomites little chemical change takes place, but the molecules are rearranged into larger and more perfect crystals, and _marble_ is the result. The coarseness of the crystals is, in a general way, a measure of the length of time during which the heat acts, and of its intensity, but much depends on the freedom of the attendant water circulation. Crystals an inch or two across are sometimes formed in the contact zone, where the attendant water action is important. If impurities, as silica, alumina, iron, etc., are present, various minerals, such as _tremolite_ and _actinolite_, may be formed in the marble. In pure quartzose sandstones, the effect is to cause the building up of the quartz grains until the interspaces are essentially filled and the whole becomes a massive _quartzite_. Here, as in the marbles, impurities form adventitious crystals, a very common one being _hematite_, formed from the segregation of the ferric oxide of the sandstone.

In the shales, the material to be acted upon is more complex, for, while the main mass is an aluminum silicate, there is usually much free quartz, not a little potash and iron, and more or less of lime, magnesia, soda, and other ingredients, for the muds from which the shales arose contained not only the fully decomposed matter of the original crystalline rocks, but the fine matter worn from them by wind and water without decomposition. When this mixed matter is acted upon by high heat and moisture, it tends to return to its original crystalline state, so far as its changed constitution permits. The potash chiefly unites with alumina and silica, and forms potash feldspar (orthoclase chiefly) and potash mica (muscovite). The iron often unites with magnesia, alumina, and silica to form biotite or one of the ferromagnesian minerals, chiefly an amphibole. The lime usually aids in the formation of other silicates of either the feldspar or the ferromagnesian group, while the surplus silica crystallizes into quartz. There is usually a predisposition to form mica in preference to other silicates if the proper constituents are present, and the result is that _mica schists_ and _gneisses_, in which mica abounds, are common products of the metamorphism of shales by contact with bodies of lava. Mica schists and micaceous gneisses are also formed in other ways, and other schists, dependent on the composition of the shales, are formed about intrusions of igneous rock. In all such cases pressure probably attends the heat and is a factor in the development of the schists. When the change induced by the heat is less considerable, the shale is baked, with incipient recrystallization, and often takes the form of _argillite_, a compact, massive sort of shale.

Beds of hydrous iron oxide (limonite) or of iron carbonate (siderite) are usually converted by heat into hematite or magnetite. Beds of peat, lignite, and bituminous coal are converted into anthracite by the driving off of the volatile hydrocarbons. If the process goes to the extreme, graphite is the result.

=Metamorphism by heat and lateral pressure.=—As already indicated, the more common intense pressures experienced by rocks at and near the surface are those that come from lateral thrusts arising from the shrinkage of the earth. These affect one dimension of the rock-mass, while they permit it to expand in one or both of the other dimensions. This produces a strain in all the constituent particles of the rock, and under such strain they pass more readily into solution than when free from strain, and more readily rearrange their molecules internally into positions of less strain. The crystals grow most freely along the planes of least stress, i.e., at right angles to the pressure.[202] As a consequence, where unidimensional pressure and high heat resulting from the compression unite their influence, the metamorphic changes are not only facilitated, but the rearrangement is controlled by the pressure and results in a parallel arrangement of the constituent crystals, giving a foliated or schistose character to the new rock. The changes themselves are much the same as those produced by heat and water without exceptional pressure, though some distinctions may be noted. It is to be observed, however, that two kinds of work are embraced here: the metamorphism of clastic rocks into crystalline schists, which may be regarded as an upbuilding process, anamorphism, and the mashing down of massive crystalline rocks into schists, which may be regarded as a degradational process, katamorphism. In both cases, however, there is solution and rearrangement of the molecules. The katamorphism of basalts and other basic rocks gives basic schists; that of granitic and similar rocks gives gneisses. The anamorphism of basic pyroclastic tuffs and wackes gives basic schists, while that of acid pyroclastics and most shales gives gneisses, mica schists, or similar acidic schists. It is obvious that ordinary shales cannot usually become basic schists, because in producing the original muds, the bases were generally removed; but when shales are highly calcareous and magnesian, as when they grade toward the limestones and dolomites, they may become basic schists by metamorphism, e.g., certain hornblendic schists. It is even more obvious that the limestone and sandstone formations must largely retain their distinct composition. It is thus seen that, in general, a sedimentary series anamorphosed must differ from a crystalline series katamorphosed, though both give rise to foliated or schistose rocks.

=Deep-seated metamorphism.=—When the exceptional pressure arises from the weight of rocks felt at great depth, it is practically equal in all directions and the crystallization probably develops normally and is not forced into the parallel or foliated form. Rocks metamorphosed under these conditions probably tend to take the massive form rather than the schistose form, but this conclusion is theoretical rather than observational, for little or nothing is known of the history of such rocks.

=Completion of the rock cycle.=—The crystallizing processes of metamorphism are fundamentally similar to the processes by which rocks crystallize out of magmas, only in the first case the work is done chiefly by the aid of an aqueous solution, while in the second it is done through a mutual solution of the constituents in themselves, where water was but an incident. If the heat factor in metamorphism be sufficiently increased, aqueous solution may actually grade into magmatic solution through various degrees of softening and melting, and the cycle of changes be closed in upon itself.

VARIOUS CLASSIFICATIONS AND NOMENCLATURES.

From the foregoing sketch of the processes of rock-making it may easily be inferred that the varieties of rocks may be almost unlimited, and that they may be defined, named, and classified on many different bases; for example.

(1) If _the mode of origin_ is chiefly in mind, rocks may be classed as _igneous_ (lavas, tuffs, etc.); _metamorphic_ (schists, gneisses, anthracite, magnetite, etc.); _aqueous_ (water-laid sediments, stalactites, travertine, etc.); _eolian_ (dunes, loess in part); _glacial_ (till, moraines); _clastic_ (mantle-rock, sandstone, conglomerate, etc.); _organic_ (peat, lignite, coal, etc., and indirectly, limestone, chalk, infusorial earth, etc.); and so on.

(2) If the _textural or structural characters_ are in mind, rocks are designated vesicular (pumice, scoria, etc.); rhyolitic (flow-structure rocks); glassy (obsidian, tachylite); porphyritic (distinct crystals in obscure matrix); granitic (well-grained); compact, porous, earthy, arenaceous (sandy), schistose, etc.

(3) If the _chemical composition_ is chiefly regarded, they may be classed as silicious, calcareous, carbonaceous, ferruginous, etc.; or, if the _chemical nature_ is considered, they are grouped as acidic, basic, or neutral.

(4) If the _crystalline character_ is made the basis, they are designated phanerocrystalline (distinctly crystallized), microcrystalline (minutely crystallized), cryptocrystalline (hiddenly crystallized), and amorphous (non-crystalline).

(5) If attention is fastened on _certain ingredients_, rocks are characterized as quartzose, micaceous, chloritic, talcose, pyritiferous, garnetiferous, etc.

(6) When rocks are regarded as _mineral aggregates_, if (_a_) the _aggregates are simple_, they are named from the dominant minerals, as dolomite, hornblendite, garnetite, anorthite, etc.; and if (_b_) the _aggregates are complex_ they take special names, as syenite (orthoclase and hornblende), gabbro (plagioclase feldspar and pyroxene), etc.

(7) When the point of view is _structure of the mass_, they are classed as massive, stratified, shaly, laminated, slaty, foliated, schistose, etc.

(8) When _physical state_ or _genesis_ is considered, they are grouped as clastic, fragmental, or detrital (conglomeratic, brecciated, arenaceous, argillaceous, etc.); or pyroclastic (tufaceous, scoriaceous, agglomeratic); or massive, in a sense slightly different from that above (7).

As sometimes one of these characteristics and sometimes another is most important in a given rock, or in a given study, no one classification is satisfactory in all cases, yet each has its advantages in particular cases.

_New System of Classification and Nomenclature._

The present systems of classifying and naming rocks have grown up gradually out of earlier and cruder methods, many of which were inherited from popular usage. Most of the names and definitions came into use before microscopical and other modern means of study were adopted. These systems, therefore, retain many inherited crudities and inconsistencies, and lack adaptation to present needs. They are too complex and difficult for field use and for general discussions, while not sufficiently exact and systematic for the more rigorous petrological discussions. A more adaptive and consistent practice has been earnestly sought by petrologists, and a new system of classification of igneous rocks has been offered by a group of leading American petrologists, an outline of which is here given.[203] To some extent this may be extended to the metamorphic crystalline rocks with necessary modifications and additions. The classification and nomenclature of the secondary rocks must probably always remain variable and plastic to express the various points of view which it is desirable to take. During the transition to this or some other new system, which seems inevitable, the appended alphabetical reference lists of the most common minerals and rocks, with brief definitions in accordance with current usage, will be found serviceable. The proposed system includes two parts, a _field system_ and a _quantitative system_, the one applicable to rocks on casual inspection, and the other, only after detailed study.

_The proposed field system._

The proposed field names are based largely on _texture_ and _color_. The mineral constituents are used for subdivisions when they can be determined; otherwise they are neglected.

Classifying chiefly on the basis of texture and crystalline state, there are three groups: _Phanerites_, in which all the leading mineral constituents can be seen megascopically; _aphanites_, in which all, or at least an appreciable part, of the constituent minerals cannot be distinguished megascopically; and _glasses_, in which the material is wholly or largely vitreous.

I. The =Phanerites= may be further classified by their chief mineral constituents as follows:

1. _Granites_ (f.n.),[204] consisting largely of _quartz_ and _feldspar_ of any kind, with or without mica, hornblende, pyroxene, or other minerals. This differs from the present common use in not regarding mica as an essential constituent, and in not distinguishing between alkali feldspars and calcic feldspars, thus broadening the class.

2. _Syenites_ (f.n.), consisting predominantly of _feldspar_ of any kind, with subordinate amounts of hornblende, mica, or pyroxene, but with little or no quartz. This differs from the common use in giving hornblende a subordinate place, and in embracing rocks with calcic feldspars, thus broadening the class.

3. _Diorites_ (f.n.), consisting predominantly of _hornblende_ and subordinately of _feldspar_ of any kind, with which there may be mica, pyroxene, or other minerals. This is nearly the present use except that any kind of feldspar may form the subordinate element.

4. _Gabbros_ (f.n.), consisting predominantly of _pyroxene_ and subordinately of _feldspar_ of any kind, with or without other minerals. This nearly coincides with one of the various present uses of the term except that the range of the feldspar is increased.

5. _Dolerites_[205] (f.n.), consisting predominantly of _any ferromagnesian mineral_ not distinguishable as hornblende or pyroxene, with subordinate elements of _feldspar_ of any kind, and with or without other accessory minerals. A name to be used when the dominant mineral is clearly ferromagnesian, but cannot be satisfactorily identified as either hornblende or pyroxene, although it may probably be one of these. In other words, the dolerites (deceptive) embrace the whole diorite-gabbro group when too obscure for separation.

6. _Peridotites_, consisting predominantly of _olivine_ and _ferromagnesian minerals_, _without_ feldspar, or with very little.

7. _Pyroxenite_, consisting essentially of pyroxene without feldspar or olivine.

8. _Hornblendite_, consisting essentially of hornblende without feldspar or olivine.

II. The =Aphanites= may be _non-porphyritic_ or _porphyritic_.

(_a_) Non-porphyritic aphanites when light-colored may be classed as _felsites_; when dark-colored, as _basalts_.

(_b_) The porphyritic aphanites or _porphyries_, when light-colored, are _leucophyres_; when dark-colored, are _melaphyres_ (f.n.). They may be classified further, according to the kind of phenocryst imbedded in the aphanitic ground-mass, as

_Quartz-porphyries_, or quartzophyres;
_Feldspar-porphyries_, or feldspaphyres (not felsophyres);
_Hornblende-porphyries_, or hornblendophyres; and so on.
These may be subclassed by color, as
_Quartz-leucophyres_, light-colored quartz-porphyries;
_Quartz-melaphyres_, dark-colored quartz-porphyries;
_Feldspar-leucophyres_;
_Feldspar-melaphyres_; and so on.

III. The glasses are classified, according to color and luster, into _obsidians_ or _pitchstones_ when dark and lustrous; _perlites_, when a spheroidal fracture gives them a pearly appearance; and _pumice_ when greatly inflated by included gases.

In general discussions, it is regarded as serviceable to use the term _granitoids_ in a broad generic sense, to include all crystalline rocks of the general granitoid type, including the granites, syenites, gneisses, etc. In a similar broad way, the term _gabbroids_ may be used to include the dark crystalline rocks in which the ferromagnesian minerals predominate, as the diorites, gabbros, dolerites, peridotites, etc. In this convenient and comprehensive way, two contrasted groups of igneous rocks may be designated. As the granitoids are usually acidic and the gabbroids usually basic, the grouping represents a broad fact of importance.

THE PROPOSED QUANTITATIVE SYSTEM.

The distinguishing characteristic of the more rigorous system designed to meet the needs of scientific petrology is its quantitative chemical character. All igneous rocks are classified _primarily_ according to their chemical composition and only secondarily according to their mineral constituents, texture, and other characters. The rigorous application of the system requires chemical analyses of the rocks, but as these are not available in many cases, the authors of the system have devised a method of optical mineral analysis by which the nearly exact proportions of all the constituent minerals can be determined, and by knowledge of their chemical nature the results may be converted, by computation, into chemical terms. This can only be done for holocrystalline rocks whose crystals are large enough to be measured under the microscope, but aphanitic rocks may often be approximately classified by comparison with similar rocks already accurately determined. To facilitate this method of chemical analysis by measuring the minerals, the chemical composition of certain common rock-making minerals is expressed in proportional parts and tabulated, and is used somewhat as molecular weight is in ordinary chemical analysis. Certain of these are selected as _standard_ minerals, the selection being such that the standard minerals embrace all the essential elements that enter into the composition of rocks. All other minerals are converted into their chemical equivalents in terms of these standard minerals by the use of the tables. All the mineral constituents being thus reduced to standard minerals, the classification is built up systematically on these standard (or standardized) minerals.

A new system of names is required, and these have been very skillfully formed by selecting significant letters from the names of the leading minerals or from words signifying their preponderance, so that short terms which carry their meaning in their forms, are secured, and this has been done so that these are usually euphonious, however strange they may seem to our preoccupied senses. For example, minerals composed chiefly of _s_ilica and _al_umina are called _salic_; those of _fe_rro_m_agnesian minerals, _femic_; those of _al_uminous _fe_rromagnesian minerals, _alferric_, etc. When in a combination of salic and femic minerals, the salic are extremely abundant, the rock is _per_salic; if notably _do_minant, _do_salic; if the salic and femic minerals are nearly equal, _salfemic_; if the femic are _do_minant, _dofemic_; if extremely abundant, _per_femic, and so on, the system being mnemonic. This method of deriving names is applicable only to a portion of the necessary divisions. For the rest, a series of roots derived from geographic names, with a system of terminations, has been employed.

All standard minerals are divided into two groups of primary importance: one of minerals characterized by alumina, as the feldspars,—orthoclase, albite, anorthite,—leucite, nephelite, sodalite, noselite, and corundum, to which are added the closely associated minerals, quartz and zircon. This is called the _salic_ group. The second group contains minerals characterized by iron and magnesia with no alumina, as hypersthene (enstatite), acmite, olivine, magnetite, hematite, and ilmenite, to which are added the closely associated minerals, titanite, perofskite, rutile, apatite, and all other rock-making minerals except those containing alumina together with iron and magnesia. The second group is called _femic_.

Aluminous ferromagnesian minerals, such as hornblende, augite, mica, etc.; are called _alferric_, and are not classed as standard minerals, because their complexity of composition makes it better to treat them as though made up of the simpler minerals of the standard list.

The composition of all igneous rocks can be expressed in terms of the relative proportions of the two groups of the standard minerals, salic and femic. By subdividing these groups successively on a mineral and chemical basis, a series of classificatory divisions of greater and greater precision has been formed. In each stage of the series, two factors only are compared, and a simple set of ratios has been selected to limit the divisions. Assuming the possibility of a continuous range of variable mixtures of the two factors (_A_ and _B_) from an extreme composed wholly of one (_A_), and an extreme composed wholly of the other (_B_), five ideal cases have been chosen as types or centerpoints about which variation in mixture may take place. These are: