PETROLOGY , the See also:

• SCIENCE (Lat. scientia, from scire; to learn, know)

The volcanic rocks, found typically as See also:

• LAVA

A granite arises by the consolidation of a molten magma (a fused rock mass; Gr. µfiy,ua, from µaaaety, to knead) at high temperatures and great pressures and its component minerals are such as are formed in such circumstances. Exposed to moisture, carbonic See also:

• ACID (from the Lat. root ac-, sharp; acere, to be sour)

Crystalline rock masses have See also:

• CON

The igneous (See also:

• LAT

These are sometimes placed into a See also:

• SPECIAL

geologist in the field depends principally on them and on a few rough chemical and physical tests; and to the See also:

• PRACTICAL

His researches are See also:

• MODELS

All other crystalline bodies, being doubly refracting, will appear See also:

• BRIGHT, JOHN (1811-188g)
• BRIGHT, SIR CHARLES TILSTON

By taking the See also:

• AVERAGE

This is obtained by a wide angled achromatic See also:

• CONDENSER

The preparation is allowed to cool and then the rock chip is again ground down as before, first with carborundum and, when it becomes transparent, with fine emery till the desired thickness is obtained. It is then cleaned, again heated with a little more balsam, and covered with a cover glass. The labour of grinding the first surface may be avoided by cutting off a smooth slice with an iron disk armed with crushed See also:

• DIAMOND

III.). The refractive index is also clearly shown by the See also:

• APPEARANCE (from Lat. apparere, to appear)

These operations are precisely similar to those employed by the mineralogist in the examination of plates cut from crystals. It is sufficient to point out that the petrological microscope in its See also:

• MODERN

Fluids are used which do not attack the majority of the rock-making minerals and at the same time have a high specific gravity. Solutions of See also:

• POTASSIUM [symbol K (from kalium), atomic weight 39.114 0=16)]

2.64) and See also:

• ORTHOCLASE

Many silicates are insoluble in acids and cannot be tested in this way, but others are partly dissolved, leaving a film of gelatinous silica which can be stained with colouring matters such as the See also:

• ANILINE, PHENYLADIINE, or AMINOBENZENE, (C6H5NH2)

In many cases no experiment is necessary. Every stage in the origin of See also:

• CLAYS, PAUL JEAN (1819-1900)

See also:

• ADAMS
• ADAMS, ANDREW LEITH (1827-1882)
• ADAMS, CHARLES FRANCIS (1807-1886)
• ADAMS, HENRY (1838— )
• ADAMS, HENRY CARTER (1852— )
• ADAMS, HERBERT (i858— )
• ADAMS, HERBERT BAXTER (1850—1901)
• ADAMS, JOHN (1735–1826)
• ADAMS, JOHN QUINCY (1767-1848)
• ADAMS, SAMUEL (1722-1803)
• ADAMS, THOMAS (d. c. 1655)
• ADAMS, WILLIAM (d. 162o)

As crystallization was going on while the mass was still creeping forward over the surface of the earth, the latest formed minerals (in the ground-mass) are commonly arranged in subparallel winding lines following the direction of movement (fluxion or fluidal structure) (see Pl. I. See also:

• FIGS

I. figs. I, 4. 5). A common feature of glassy rocks is the presence of rounded bodies (spherulites: Gr. ocPai pa, See also:

• BALL (in Mid. Eng. bal; the word is probably cognate with " bale," Teutonic in origin, cf. also Lat. falls, and Gr. 7raXXa)
• BALL, JOHN (1585-1640)
• BALL, JOHN (1818-1889)
• BALL, JOHN (d. 1381)
• BALL, SIR ALEXANDER JOHN, BART
• BALL, THOMAS (1819- )

II. fig. r). Microscopic examination of the phenocrysts often reveals that they have had a complex history. Very frequently they show successive layers of different composition, indicated by See also:

• VARIATIONS
• VARIATIONS, CALCULUS
• VARIATIONS, CALCULUS OF

4, 5, 9). That these are igneous is proved by the manner in which they have burst through the superincumbent strata, filling the Plutonic or cracks with ramifying veins; that they were at a very Abyssal high temperature is equally clear from the changes which Types. they have induced in the rocks in contact with them. But as their heat could dissipate only very slowly, because of the masses which covered them, complete crystallization has taken place and no vitreous rapidly chilled matter is present. As they have had time to come to See also:

• REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

The same distinction holds between elaeolite and nepheline. Leucite is common in lavas, very rare in plutonic rocks. Muscovite is confined to the intrusives. These differences show the influence of the physical conditions under which consolidation takes place. There is a certain class of intrusive rocks which have risen upwards towards the surface, but have failed to reach it, and have solidified in fissures as dikes and intrusive sills at no great See also:

• DEPTH

But there are many kinds of rock which are not found to occur normally in any other manner. As examples we may cite the See also:

• LAMPROPHYRES (from Gr. Xaurp6s, bright, and the terminal part of the word porphyry, meaning rocks containing bright porphyritic crystals)

It occurs principally in See also:

• COMBINATION (Lat. combinare, to combine)

These must be regarded only as general tendencies, which are modified by physical conditions in a manner not as yet understood. It is possible by inspection of a rock analysis to say approximately what minerals the rock will contain, but there are numerous exceptions to any rule which can be laid down. Hence we may say that except in acid or siliceous rocks containing 66 % of silica and over, quartz will not be abundant. In basic Mineral rocks (containing 6o% silica or less) it is rare and Mineral accidental. If magnesia and iron be above the average Flom while silica is See also:

• LOW, SETH (1850- )
• LOW, WILL HICOK (1853- )

There are certain minerals which are practically confined to deep-seated intrusive rocks, e.g. See also:

• MICROCLINE

It is often separated from the others as the " See also:

• ALKALI

Commonest Alkali Felspar, Soda Lime Felspar Nepheline or Minerals. Nepheline or Leu- Nepheline or Leu- Leucite, Augite, cite,Augite,See also:

• HORN
• HORN (a common Teutonic word, cognate with Lat. cornu; cf. Gr. Kipas)
• HORN (Lat. cornu; corresponding terms being Fr. cor, trompe; Ger. Horn; Ital. corno)
• HORN, ARVID BERNHARD, COUNT (1664-1742)
• HORN, PHILIP DE MONTMORENCY, COUNT

Any chemical distinctions between the different groups, though implied, are relegated to a subordinate position. It is admittedly artificial but it has grown up with the growth of the science and is still adopted as the basis on which more minute subdivisions are erected. The subdivisions are by no means of equal value. The syenites, for example, and the peridotites, are far less important than the granites, diorites and gabbros. Moreover, the effusive andesites do not always correspond to the plutonic diorites but partly also to the gabbros. . As the different kinds of rock, regarded as aggregates of minerals, pass gradually into one another, transitional types are very common and are often so important as to receive special names. The quartz-syenites and nordmarkites may be interposed between granite and syenite, the tonalites and adamellites between granite and diorite, the monzonites between syenite and diorite, norites and hyperites between diorite and gabbro, and so on. There is of course a large number of recognized rock species not included in the tables given. These are of two kinds, either belonging to groups which are subdivisions of those enumerated (bearing the same relation to them that species do to genera) or rare and exceptional rocks that do not fall within any of the main subdivisions proposed. The question may be asked—When is a rock entitled to be recognized as belonging to a distinct species or variety and deserving a name for itself? It must, first of all, be proved to occur in considerable quantity at some locality, or better still at a series of localities or to have been produced from different magmas at more than one period of the earth's history. In other words, it must not be a mere See also:

• ANOMALY (from Gr. lwvw)

Moreover, it should have a distinctive mineral constitution, differing from other rocks, or some-thing individual in the characters of its minerals or of its structures. It is often surprising how See also:

• PECULIAR

Plutonic or Granite. Syenite. Diorite. Gabbro. Peridotite. A b y s s a l } Quartz - Orthoclase- Porphyrite. Dolerite. Picrite. type. porphyry. porphyry. Andesite. Basalt. See also:

• LIMBURGITE

Intrusive or Obsidian. . Trachyte. H y p a b y s- sal type. Lavas or} Eff usiv e type. to be unique, turn up with identical features in widely scattered According to modern views two explanations of these facts are regions, alnoite, for example, occurs in Norway, See also:

• SCOTLAND
• SCOTLAND, CHURCH OF
• SCOTLAND, EPISCOPAL CHURCH OF

Geological occurrence, structure, mineralogical constitution, the hitherto accepted criteria for the discrimination of rock species were relegated to the background. The completed rock analysis is first to be interpreted in terms of the rock-forming minerals which might be expected to be formed when the magma crystallizes, e.g. quartz felspars of various kinds, olivine, al:ermannite, felspathoids, magnetite, corundum and so on, and the rocks are divided into groups strictly according to the relative proportion of these minerals to one another. There is no need here to describe the minutia of the process adopted as the authors have stated them very clearly in their See also:

• TREATISE

Thus the lavas of the Hawaiian Islands are mostly basaltic, as are those of See also:

• OREGON

AOKKOS, See also:

• PIT (O. E. pytt, cognate with Du. put, Ger. Pfutze, &c., all ultimately adaptations of Lat. puteus, well, formed from root pu-, to cleanse, whence gurus, clean, pure)

As a rock solidifies the minerals which crystallize follow one another in a more or less well-defined order, the most basic (according to Rosenbusch's See also:

• LAW
• LAW (0. Eng. lagu, M. Eng. lawe; from an old Teutonic root lag, " lie," what lies fixed or evenly; cf. Lat. lex, Fr. loi)
• LAW, JOHN (1671—1729)
• LAW, WILLIAM (1686-1761)

Hence as crystallization goes on the residual liquor must contain an ever-increasing proportion of volatile constituents. It is conceivable that in the final stages the still uncrystallized part of the magma has more resemblance to a solution of mineral matter in superheated steam than to a dry igneous fusion. Quartz, for example, is the last mineral to form in a granite. It bears much of the See also:

• STAMP (from " to stamp," to strike or tread heavily, hence to impress, O. Eng. stempen, Du. stampen, Ger. stampfen, whence, O. Fr. estamper, mod. etamper)

The Sequen of first minerals to separate belong to a group known seque ..e as the See also:

• MINOR
• MINOR (Lat. for smaller, lesser)
• MINOR, ROBERT CRANNELL (1839-1904)

A variety of this, known as ophitic (Pl. III, fig. 6), is very characteristic of many dolerites and diabases, in which large plates of augite enclose many small laths of plagioclase felspar. Biotite and hornblende frequently enclose felspar ophitically; less commonly iron oxides and sphene do so. In peridotites the " lustre-mottled " structure arises from pyroxene or hornblende enveloping olivine in the same manner (Pl. III, fig. 8). In these cases no crystallographic relation exists between the two minerals (enclosing and enclosed). But often the surrounding mineral has been laid down on the surface of the other in such a way that they have certain crystalline Parallel faces or axes parallel to one another. This is known Growths. as " parallel growth." It is best seen in zoned crystals of plagioclase felspar, which may range in composition from anorthite to oligoclase, the more acid layers being deposited regularly on the surfaces of the more basic. Biotite and muscovite, hornblende and augite, enstatite and diallage, epidote and orthite, very frequently are associated in this way. When two minerals crystallize simultaneously they may be intergrown in " graphic " See also:

• FASHION (adapted from Fr. facon, agon, Lat. factio, making, facere, to do or make)

The best example is quartz Graphic and orthoclase occurring together as micropegmatite Inter- (Pl. II, figs. 6 and 8). The quartz forms angular growths. patches in the felspar, which though separated have the same crystalline See also:

• ORIENTATION

At the same time the mathematical theory of the physical processes involved has received much attention, and serves both to See also:

• DIRECT

As crystallization goes on, these gases are set free and their pressure must increase to some extent. Moreover, the felspar is not one mineral but two or perhaps three, there being always soda felspar and potash felspar and usually also a small amount of lime felspar in these porphyries. In a typical basic rock the conditions are even more complex. A deterite, for example, usually contains, as its last products of crystallization, pyroxene and felspar. Of these the latter consists of three distinct species, the former of an unknown number; and in each case they can form mixed crystals, to a greater or less extent with one another. From these considerations it will be clear that the properties of solutions of two or three independent components, do not necessarily explain the process of crystallization in any igneous rock. Very frequently in porphyries not only quartz but felspar also is present in large well-formed early crystals. Similarly in basalts, augite and felspar may appear both as phenocrysts and as components of the ground-mass. As an explanation of this it has been suggested that supersaturation has taken place. We may suppose that the augite which was in excess of the proportion necessary to form the felspar-augite, eutectic mixture, first separated out. When the remaining solution reached the eutectic composition the felspar did not at once start crystallizing, perhaps because nuclei are necessary to initiate crystal-growth and these were not at hand; augite went on crystallizing while felspar lagged behind. Then felspar began and as the mixture was now supersaturated with that mineral a considerable amount of it was rapidly thrown out of the solution.

At the same time there would be a tendency for part of the augite, already crystallized, to be dissolved and its crystals would be corroded, losing their sharp and perfect edges, as is often observed in rocks of this group. When the necessary adjustments had been made the eutectic mixture would be established and thereafter the two minerals would consolidate simultaneously (or nearly so) till crystallization was complete. There is a good See also:

• DEAL

It has been found, for example, that in the case of three minerals—one of which is independent, while the two others can form mixed crystals—there is a large number of possible sequences; and, what is very important, one mineral may separate out entirely at an early stage, or its crystallization may be interrupted and not continuous. The ternary eutectic, which is produced by a mixture of three independent minerals, may not in such a case be the last substance to crystallize, and may not be present at all. This is very much in accordance with the observed facts of petrology; for usually in a rock there is one mineral which indubitably was the last of all to finish crystallizing and contained no appreciable quantity of the others. As yet we know little about such important questions as the composition of the eutectic mixtures of rock-minerals, their latent heat of fusion, specific heats, mutual solubilities, See also:

• INVERSION (Lat. invertere, to turn about)

Epigenitic change (secondary processes) may be arranged under a number of headings, each of which is typical of a group of rocks Secondary or rock-forming minerals, though usually more than Second. one of these alterations will be found in progress in the same rock. Silicification, the replacement of the minerals by crystalline or crypto-crystalline silica, is most common in acid rocks, such as rhyolite, but is also found in serpentine, &c. Kaolinization is the decomposition of the felspars, which are the commonest minerals of igneous rocks, into kaolin (along with quartz, muscovite, &c.); it is best shown by granites and syenites. Serpentinization is the alteration of olivine to serpentine (with magnetite) ; it is typical of peridotites, but occurs in most of the basic rocks. In uralitization secondary hornblende replaces augite; this occurs very generally in diabases; chloritization is the alteration of augite (biotite or hornblende) to chlorite, and is seen in many diabases, diorites and greenstones. Epidotization occurs also in rocks of this group, and consists in the development of epidote from biotite, hornblende, augite or plagioclase felspar. The sedimentary rocks, which constitute the second great group, have many points in common that distinguish them from the igneous and the metamorphic. They have all originated on the surface of the earth, and at the period of their formation were exposed only to the temperature of the air and to atmospheric pressure (or the pressures which exist at the bottoms of seas and lakes). Their minerals are in most cases not susceptible to change when exposed to moist air or sea, and many of them are hydrated (chlorite, micas, &c.), or oxidized (iron ores), or contain carbonic acid (calcite, dolomite). The extent, however, to which this is the case depends largely on the rapidity with which they have accumulated; coarse rocks quickly piled up often consist of materials only partly weathered. When crystalline, the sedimentary rocks are usually soluble at low temperatures. The members of this group occur in beds or strata, hence they are often known as the stratified rocks; the upper beds are always of later formation than those which underlie them, except (as may happen when great disturbance has taken place) the whole series is inverted or overturned.

Many of the stratified rocks have been formed by the agency of moving water (See also:

• RIVERS
• RIVERS, ANTHONY WOODVILLE, or WYDEVILLE, 2ND EARL (c. 1442—1483)
• RIVERS, EARL
• RIVERS, RICHARD SAVAGE, 4TH EARL (c. 1660—1712)

The best example of elastic. these are the soils, but in elevated regions angular broken rock often covers large areas. More usually they are transported by wind or water, and become sorted out according to their size and See also:

• DENSITY (Lat. densus, thick)

These quartz grains have been rolled along and are usually rounded and worn (Pl. IV., fig. I). More or less of garnet felspar, tourmaline, zircon, rutile, &c., are mixed with the quartz, because these are hard minerals not readily decomposed. The mechanical sorting by the transporting agencies is usually somewhat incomplete, and mixed types of sediment result, such as gravels containing sand, or clays with coarser arenaceous particles. Moreover, successive layers of deposit may not always be entirely similar, and alternations of varying composition may follow one another in thin laminae: e.g. laminae of arenaceous material in beds of clay and shale. Organic matter is frequently mingled with the finer-grained sediments. These three types have been named the psephitic (or pebbly; Gr. +b See also:

• COS
• COS, or STANKO (Ital. Stanchio, Turk. Islan-keui, by corruption from Etc rav KW)

IV. fig. 2), and only accidentally contain other rocks or fossils. They are stratified, and may be coarse or fine, but are usually much less perfectly sorted out, according to their fineness, than ordinary aqueous or aeolian deposits. The glacial clays (See also:

• BOULDER
• BOULDER (short for " boulder-stone," of uncertain origin; cf. Swed. bullersten, a large stone which causes a noise of rippling water in a stream, from bullra, to make a loud noise)

Thus limestones are dolomitized or converted into ironstones, flints and cherts, by percolating waters which remove the lime salts and substitute for them compounds of iron, magnesia, See also:

• SILICON [symbol Si, atomic weight 28.3 (0= 16)]

Some kinds of siliceous See also:

• SINTER

Limestones, which were originally a loose See also:

• ACCUMULATION (from Lat. accumulare, to heap up)

They include representatives of nearly all kinds of the other two classes, their common characteristic being that they have all undergone considerable alterations in structure or in mineral composition. The agencies of metamorphism (q.v.) are of two kinds—thermal and regional. In the former case contact with intrusive igneous masses, such as granite, laccolites or dikes, have indurated and recrystallized the original rock. In the second case the actions are morecomplex and less clearly understood; it is evident that pressure and interstitial movement have had a powerful influence, possibly assisted by rise of temperature. In thermal or contact alteration the rocks are baked, indurated, and often in large measure recrystallized. In regional metamorphism recrystallization also goes on, but the final products are usually schists and gneisses. It is as a rule not difficult to distinguish the two classes of metamorphic rocks at a glance, and they may conveniently be considered separately. When a rock is contact altered by an igneous intrusion it very frequently becomes harder, more crystalline and more lustrous, owing to the development of many small crystals in its The mass. Many altered rocks of this type were formerly T hermo called hornstones, and the term hornfelses (Ger. phitarmo-Hornfels) is often used by geologists to signify those fine grained, compact, crystalline products of thermal metamorphism. A shale becomes a dark argillaceous hornfels, full of tiny plates of brownish biotite; a marl or impure limestone changes to a grey, yellow or greenish lime-silicate-hornfels, tough and splintery, with abundance of augite, garnet, See also:

• WOLLASTONITE

If the rock was originally banded or foliated (as, for example, a laminated sandstone or a foliated calc-schist) this character may not be obliterated, and a banded hornfels is the product; fossils even may have their shapes preserved, though entirely recrystallized, and in many contact altered lavas the steam cavities are still visible, though their contents have usually entered into new combinations to form minerals which were not originally present. The minute structures, however, disappear, often completely, if the thermal alteration is very profound; thus small grains of quartz in a shale are lost or blend with the surrounding particles of clay, and the fine ground-mass of lavas is entirely reconstructed. By recrystallization in this manner peculiar rocks of very distinct types are often produced. Thus shales may pass into cordierite rocks, or may show large crystals of See also:

• ANDALUSITE

In a few cases rocks are fused and in the dark glassy product minute crystals of See also:

• SPINEL

consists in a definite arrangement of the minerals, so that such as are platy or prismatic (e.g. mica and hornblende, which are very common in these rocks) have their longest axes arranged parallel to one another. For that reason many of these rocks split readily in one direction (schists). The minerals also tend to aggregate in bands; thus there are seams of quartz and of mica in a mica schist, very thin, but consisting essentially of one mineral. These seams are called folia (leaflets), and though never very pure or very persistent they give the rock a streaked or banded character when they are seen edgewise (Pl. IV. figs. 6, 7, 8). Along the folia composed of the soft or fissile minerals the rocks will sever most readily, and the freshly split specimen will appear to be faced or coated with this mineral; for example, a piece of mica schist looked at face See also:

• WISE, HENRY ALEXANDER (1806-1876)
• WISE, ISAAC MAYER (1819-1900)

6) of the foliation is by no means uncommon, and then the splitting faces are undulose or puckered. The origin of schistosity or foliation is not perfectly understood, but it is clear that in many cases it is due to pressure, acting in a direction perpendicular to the banding, and to interstitial movement or See also:

• INTERNAL

Another group is rich in quartz (quartzites, quartz schists and quartzose gneisses), with variable amounts of white and black mica, garnet, felspar, See also:

• ZOISITE

Most of these augen gneisses are metamorphic granites, but sometimes a conglomerate bed simulates a gneiss of this kind rather closely. There are other gneisses, which were derived from felspathic sandstones, grits, arkoses and sediments of that order; they mostly contain biotite and muscovite, but the hornblende and pyroxene gneisses are usually igneous rocks allied in composition to the hornblende-granites and quartz-diorites. The metamorphic forms of dolerite, basalt and the basic igneous rocks generally have a distinctive facies as their pyroxene and olivine are replaced by dark green hornblende, with often epidote, garnet and biotite. These rocks have a well See also:

• DEVELOPED