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.
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.
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.
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 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
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 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 silica and alumina are called salic; those of ferromagnesian minerals, femic; those of aluminous ferromagnesian minerals, alferric, etc. When in a combination of salic and femic minerals, the salic are extremely abundant, the rock is persalic; if notably dominant, dosalic; if the salic and femic minerals are nearly equal, salfemic; if the femic are dominant, dofemic; if extremely abundant, perfemic, 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:
Division lines half-way between these points occur as the following ratios: