See also:

• CLAY (from O. Eng. claeg, a word common in various forms to Teutonic languages, cf. Ger. Klei)
• CLAY, CASSIUS MARCELLUS (1810-1903)
• CLAY, CHARLES (1801–1893)
• CLAY, FREDERIC (1838–1889)
• CLAY, HENRY (1777–1852)

CLAY (from O. Eng. claeg, a word See also:

• COMMON

common in various forms to See also:

• TEUTONIC
• TEUTONIC (GERMANIC) LANGUAGES

Teutonic See also:

• LANGUAGES

languages, cf. Ger. Klei)  , commonly defined as a See also:

• FINE

fine-grained, almost impalpable substance, very soft, more or less coherent when dry, plastic and retentive of See also:

• WATER

water when wet; it has an " earthy " odour when breathed upon or moistened, and consists essentially of hydrous See also:

• ALUMINIUM (symbol Al; atomic weight 27.0)

aluminium silicate with various impurities . Of See also:

• CLAY (from O. Eng. claeg, a word common in various forms to Teutonic languages, cf. Ger. Klei)
• CLAY, CASSIUS MARCELLUS (1810-1903)
• CLAY, CHARLES (1801–1893)
• CLAY, FREDERIC (1838–1889)
• CLAY, HENRY (1777–1852)

clay are formed a See also:

• GREAT

great number of rocks, which collectively are known as " clay-rocks " or " pelitic rocks "(from Gr. rrrlXos, clay), e.g. mudstone, shale, See also:

• SLATE (properly CLAY SLATE; in M. Eng. slat or sclat, from O. Fr. esclat, a small piece of wood used as a tile; esclater, to break into pieces, whence modern Fr. eclat, the root being seen also in Ger. schleissen, to split)

slate: these exhibit in greater or less perfection the properties above described according to their freedom from impurities . In nature, See also:

• CLAYS, PAUL JEAN (1819-1900)

clays are rarely See also:

• FREE

free from See also:

• FOREIGN

foreign ingredients, many of which can be detected with the unaided See also:

• EYE
• EYE (O. Eng. edge, Ger. Auge; derived from an Indo-European root also seen in Lat. oc-ulus, the organ of vision (q.v.)

eye, while others may be observed by means of the See also:

• MICROSCOPE (Gr. µucp6s, small, asenreiv, to %view)

microscope . The commonest impurities are: (1) organic See also:

• MATTER

matter, humus, &c . (exemplified by clay-soils with an admixture of See also:

• PEAT (possibly connected with Med. Lat. petia, pecia, piece, ultimately of Celtic origin; cf. O. Celt. pet, O. Ir. pit, Welsh peth, portion)

peat, oil shales, carbonaceous shales); (2) fossils (such as See also:

• PLANTS

plants in the shales of the See also:

• LIAS

Lias and See also:

• COAL

Coal See also:

• MEASURES

Measures, shells in clays of all See also:

• GEOLOGICAL

geological periods and in fresh water marls); (3) carbonate of See also:

• LIME
• LIME (O. Eng. lim, Lat. limes, mud, from linere, to smear)

lime (rarely altogether absent, but abundant in marls, See also:

• CEMENT (from Lat. caementum, rough pieces of stone, a shortened form of caedimentum, from caedere, to cut)

cement-stones and argillaceous limestones); (4) sulphide of See also:

• IRON [symbol Fe, atomic weight 55.85 (0=16)]

iron, as pyrite or See also:

• MARCASITE

marcasite (when finely diffused, giving the clay a dark See also:

• GREY, CHARLES GREY, 2ND EARL (1764-1845)
• GREY, HENRY GREY, 3RD EARL (1802-1894)
• GREY, LADY JANE (1537-1554)
• GREY, SIR EDWARD
• GREY, SIR GEORGE (1812–1898)

grey-See also:

• BLUE (common in different forms to most European languages)

blue See also:

• COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour, which weathers to See also:

• BROWN
• BROWN, CHARLES BROCKDEN (1771-181o)
• BROWN, FORD MADOX (1821-1893)
• BROWN, FRANCIS (1849- )
• BROWN, GEORGE (1818-188o)
• BROWN, HENRY KIRKE (1814-1886)
• BROWN, JACOB (1775–1828)
• BROWN, JOHN (1715–1766)
• BROWN, JOHN (1722-1787)
• BROWN, JOHN (1735–1788)
• BROWN, JOHN (1784–1858)
• BROWN, JOHN (1800-1859)
• BROWN, JOHN (1810—1882)
• BROWN, JOHN GEORGE (1831— )
• BROWN, ROBERT (1773-1858)
• BROWN, SAMUEL MORISON (1817—1856)
• BROWN, SIR GEORGE (1790-1865)
• BROWN, SIR JOHN (1816-1896)
• BROWN, SIR WILLIAM, BART
• BROWN, THOMAS (1663-1704)
• BROWN, THOMAS (1778-1820)
• BROWN, THOMAS EDWARD (1830-1897)
• BROWN, WILLIAM LAURENCE (1755–1830)

brown--e.g . See also:

• LONDON

London Clay; also as nodules and concretions, e.g . See also:

• GAULT

Gault); (5) oxides of iron (staining the clay See also:

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

bright red when ferric See also:

• OXIDE

oxide, red ochre; yellow when hydrous, e.g. yellow ochre); (6) See also:

• SAND
• SAND, GEORGE (1804-1876)

sand or detrital See also:

• SILICA

silica (forming loams, arenaceous clays, argillaceous sandstones, &c.) . Less frequently See also:

• PRESENT

present are the following:—See also:

• ROCK
• ROCK (O.Fr. rake, Sp. rota, Ital. rocca; possibly from a Lat. form rupica, from rupes, rock)
• ROCK, DANIEL (1799–187t)

rock See also:

• SALT (a common Teutonic word, cf. Dutch zout, Ger. Salz, Scand. salt; cognate with Gr. &Xs, Lat. sal)
• SALT, SIR TITUS, BART

salt (Triassic clays, and marls of See also:

• CHESHIRE

Cheshire, &c.); See also:

• GYPSUM

gypsum (London Clay, Triassic clays); See also:

• DOLOMITE

dolomite, phosphate of lime, See also:

• VIVIANITE

vivianite (phosphate of iron), oxides of See also:

• MANGANESE (symbol Mn; atomic weight, 54.93 (0=16)], a metallic chemical element. Its dioxide (pyrolusite)

manganese, See also:

• COPPER (symbol Cu, atomic weight 63.1, H=1, or 63.6, O = 16)

copper ores (e.g . Kupferschiefer), See also:

• WAVELLITE

wavellite and See also:

• AMBER

amber . As the impurities increase in amount the clay rocks pass gradually into argillaceous sands and sandstones, argillaceous limestones and See also:

• DOLOMITES, THE

dolomites, shaly coals and clay ironstones . Natural clays, even when most ,pure, show a considerable range of See also:

• COMPOSITION
• COMPOSITION (Lat. compositio, from componere, to put together)

composition, and hence cannot be regarded as consisting of a single See also:

• MINERAL

mineral; clay is a rock, and has that variability which characterizes all rocks . Of the essential properties of clay some are merely See also:

• PHYSICAL

physical, and depend on the See also:

• MINUTE
• MINUTE (Lat. minutes, small; minuere, to make less)

minute See also:

• SIZE

size of the particles .

If any rock be taken (even a piece of pure See also:

• QUARTZ

quartz) and crushed to a very fine See also:

• POWDER (through O. Fr. puldre, modern poudre, from Lat. pulvis, pulveris, dust)

powder, it will show some of the peculiarities of clays; for example, it will be plastic, retentive of moisture, impermeable to water, and will shrink to some extent if the moist See also:

• MASS (O.E. maesse; Fr. messe; Ger. Messe; Ital. messa; from eccl. Lat. missa)
• MASS, IN

mass be kneaded, and then allowed to dry . It happens, however, that many rocks are not disintegrated to this extreme degree by natural processes, and weathering invariably accompanies disintegration . Quartz, for example, has little or no cleavage, and is not attacked by the See also:

• ATMOSPHERE (Gr. fir µ6r, vapour; orkaipa, a sphere)

atmosphere . It breaks up into fragments, which become rounded by See also:

• ATTRITION (Lat. attritio, formed from atterere, to rub away)

attrition, but after they reach a certain minuteness are See also:

• BORNE, KARL LUDWIG (1786–1837)

borne along by currents of water or See also:

• AIR (from an Indo-European root meaning " breathe," " blow ")
• AIR, or ASBEN

air in a See also:

• STATE
• STATE, GREAT OFFICERS OF

state of suspension, and are not further reduced in size . Hence sands are more coarse grained than clays . A great number of rock-forming minerals, however, possess a See also:

• GOOD, JOHN MASON (1764-1827)

good cleavage, so that when bruised they split into thin fragments; many of these minerals decompose somewhat readily, yielding secondary minerals, which are comparatively soft and have a scaly See also:

• CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character, with eminently perfect cleavages, which facilitate splitting into exceedingly thin plates . The See also:

• PRINCIPAL

principal substances of this description are See also:

• KAOLIN

kaolin, See also:

• MUSCOVITE

muscovite and See also:

• CHLORITE

chlorite . Kaolin and muscovite are formed principally after See also:

• FELSPAR, or FELDSPAR

felspar (and the felspars are the commonest minerals of all crystalline rocks); also from See also:

• NEPHELINE

nepheline, See also:

• LEUCITE

leucite, See also:

• SCAPOLITE (Gr. O'Kairos, rod, )tLOoc, stone)

scapolite and a variety of other rock-forming minerals . Chlorite arises from See also:

• BIOTITE

biotite, See also:

• AUGITE

augite and See also:

• HORNBLENDE

hornblende . See also:

• SERPENTINE

Serpentine, which may be fibrous or scaly, is a secondary product of See also:

• OLIVINE

olivine and certain pyroxenes . Clays consist essentially of the above ingredients (although serpentine is not known to take See also:

• PART

part in them to any extent, it is closely allied to chlorite) . At the same See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time other substances are produced as decomposition goes on .

They are principally finely divided quartz, See also:

• EPIDOTE

epidote, See also:

• ZOISITE

zoisite, See also:

• RUTILE

rutile, See also:

• LIMONITE, or BROWN IRON ORE

limonite, See also:

• CALCITE

calcite, See also:

• PYRITES

pyrites, and very small particles of these are rarely absent from natural clays . These fine-grained materials are at first mixed with broken and more or less weathered rock fragments and coarser mineral particles in the See also:

• SOIL

soil and subsoil, but by the See also:

• ACTION

action of See also:

• WIND
• WIND (a common Teut. word, cognate with Skt. vats, Lat. ventus, cf. " weather," to be of course distinguished from to " wind," to coil or twist, O.Eng. windan, cf. "wander," "wend," &c.)

wind and See also:

• RAIN (O.E. regn; the word is common to Teutonic languages, cf. Ger. Regen, Swed. and Dan. regn; it has been connected with Lat. rigare, to wet, Gr. 13pixeav)

rain they are swept away and deposited in distant situations . " See also:

• LOESS (Ger. Loss)

Loess " is a fine calcareous clay, which has been wind-borne, and subsequently laid down on the margins of dry See also:

• STEPPES

steppes and deserts . Most clays are water-borne, having been carried from the See also:

• SURFACE

surface of the See also:

• LAND

land by rain and transported by the See also:

• BROOKS, CHARLES WILLIAM SHIRLEY (1816-1874)
• BROOKS, PHILLIPS (1835-1893)

brooks and See also:

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

rivers into lakes or the See also:

• SEA (in O. Eng. sae, a common Teutonic word; cf. Ger. See, Dutch Zee, &c.; the ultimate source is uncertain)
• SEA, COMMAND OF THE

sea . In this state the fine particles are known as " mud." They are deposited where the currents are checked and the water becomes very still . If temporarily laid down in other situations they are ultimately lifted again and removed . A little clay, stirred up with water in a See also:

• GLASS
• GLASS (O.E. glees, cf. Ger. Glas, perhaps derived from an old Teutonic root gla-, a variant of glo-, having the general sense of shining, cf. " glare," " glow ")
• GLASS, STAINED

glass See also:

• VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

vessel, takes See also:

• HOURS, CANONICAL

hours to See also:

• SETTLE
• SETTLE, ELKANAH (1648–1724)

settle, and even after two or three days some remains in suspension; in fact, it has been suggested that in such cases the clay forms a sort of "colloidal See also:

• SOLUTION (from Lat. solvere, to loosen, dissolve)

solution" in the water . Traces of dissolved salts, such as See also:

• COMMON

common salt, gypsum or See also:

• ALUM

alum, greatly accelerate deposition . For these reasons the principal gathering places of fine pure clays are deep, still lakes, and the sea bottom at considerable distances from the See also:

• SHORE

shore . The coarser materials settle nearer the land, and the shallower portions of the sea See also:

• FLOOR (from O. Eng. flor, a word common to many Teutonic languages, cf. Dutch vloer, and Ger. Flur, a field, in the feminine, and a floor, masculine)

floor are strewn with See also:

• GRAVEL, or PEBBLE BEDS

gravel and sand, except in occasional depressions and near the mouths of rivers where mud may gather . Farther out the great mud deposits begin, extending from 50 to 200 M. from the land, according to the amount of sediment brought in, and the See also:

• RATE

rate at which the water deepens . A See also:

• GIRDLE (O. Eng. gyrdel, from gyrdan, to gird; cf. Ger. Gurtel, Dutch gordel, from giirten and gorden; "gird" and its doublet " girth " together with the other Teutonic cognates have been referred by some to the root ghar—to seize, enclose, seen in Gr. )(s

girdle of mud accumulations encircles all the continents .

These sediments are fine and tenacious; their principal components, in addition to clay, being small grains of quartz, See also:

• ZIRCON

zircon, See also:

• TOURMALINE

tourmaline, hornblende, felspar and iron compounds . Their typical colour is blackish-blue, owing to the abundance of sulphuretted See also:

• HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

hydrogen; when fresh they have a sulphurous odour, when weathered they are brown, as their iron is present as hydrous oxides (limonite, &c.) . These deposits are tenanted by numerous forms of marine See also:

• LIFE

life, and the See also:

• SULPHUR

sulphur they contain is derived from decomposing organic matter . Oecasionally water-logged plant debris is mingled with the mud . In a few places a red colour prevails, the iron being mostly oxidized; elsewhere the muds are See also:

• GREEN, A
• GREEN, ALEXANDER HENRY (1832—1896)
• GREEN, DUFF (1791—1875)
• GREEN, JOHN RICHARD (1837—1883)
• GREEN, MATTHEW (1696-1737)
• GREEN, THOMAS HILL (1836-1882)
• GREEN, VALENTINE (1739–1813)
• GREEN, WILLIAM HENRY (.1825–1900)

green owing to abundant See also:

• GLAUCONITE

glauconite . Traced landwards the muds become more sandy, while on their See also:

• OUTER

outer margins they grade into the abysmal deposits, such as the See also:

• GLOBIGERINA, A

globigerina See also:

• OOZE (O. Eng. wdse, cognate with an obsolete waise, mud; cf. O. Nor. veisa, muddy pool)

ooze (see OCEAN AND OCEANOGRAPHY) . Near volcanoes they contain many volcanic minerals, and around See also:

• CORAL

coral islands they are often in large part calcareous . Microscopic sections of some of the more coherent clays and shales may be prepared by saturating them with See also:

• CANADA

Canada See also:

• BALSAM (from Gr. f &X raµov, through Lat. balsamum, contracted by popular use to O. Fr. basme, mod. Fr. bdme; Eng. balm)

balsam by See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

long boiling, and slicing the resultant mass in the same manner as one of the harder rocks . They show that clay rocks contain abundant very small grains of quartz (about o•or to 0.05 mm. in See also:

• DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter), with often felspar, tourmaline, zircon, epidote, rutile and more or less calcite . These may See also:

• FORM (Lat. forma)

form more than one-third of an See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary shale; the greater part, however, consists of still smaller scales of other minerals (o•or mm. in diameter and less than this) . Some of these are recognizable as See also:

• PALE (through Fr. pal, from Lat. palms, a stake, for paglus, from the stem pag- of pangere, to fix; " pole " is from the same original source)

pale yellowish and See also:

• WHITE
• WHITE, ANDREW DICKSON (1832– )
• WHITE, GILBERT (1720–1793)
• WHITE, HENRY KIRKE (1785-1806)
• WHITE, HUGH LAWSON (1773-1840)
• WHITE, JOSEPH BLANCO (1775-1841)
• WHITE, RICHARD GRANT (1822-1885)
• WHITE, ROBERT (1645-1704)
• WHITE, SIR GEORGE STUART (1835– )
• WHITE, SIR THOMAS (1492-1567)
• WHITE, SIR WILLIAM ARTHUR (1824--1891)
• WHITE, SIR WILLIAM HENRY (1845– )
• WHITE, THOMAS (1628-1698)
• WHITE, THOMAS (c. 1550-1624)

white See also:

• MICA

mica; others seem to be chlorite, the See also:

• REMAINDER, REVERSION

remainder is perhaps kaolin, but, owing to the minute size of the flakes, they yield very indistinct reactions to polarized See also:

• LIGHT

light . They are also often stained with iron oxide and organic substances, and in consequence their true nature is almost impossible to determine .

It is certain, however, that the finer-grained rocks are richest in alumina, and in combined water; hence the inference is clear that kaolin or some other hydrous aluminium silicate is the dominating constituent . These results are confirmed by the See also:

• MECHANICAL

mechanical See also:

• ANALYSIS (Gr. avci and Meta, to break up into parts)

analysis of clays . This See also:

• PROCESS

process consists in finely pulverizing the soil or rock, and levigating it in vessels of water . A See also:

• SERIES (a Latin word from serere, to join)

series of powders is obtained progressively finer according to the time required to settle to the bottom of the vessel . The clay is held to include those particles which have less than 0•005 mm. diameter, and contains a higher percentage of alumina than any of the other ingredients . As might be inferred from the See also:

• DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

differences they exhibit in other respects, clay rocks vary greatly in their chemical composition . Some of them contain much iron (yellow, blue and red clays); others contain abundant See also:

• CALCIUM [symbol Ca, atomic weight 40.0 (o= x6)]

calcium carbonate (calcareous clays and marls) . Pure clays, however, may be found almost quite free from these substances . Their silica ranges from about 6o to 45%, varying in accordance with the amount of quartz and See also:

• ALKALI

alkali-felspar present . It is almost always more than would be the See also:

• CASE
• CASE, JOHN (d. 1600)

case if the rock consisted of kaolin mixed with muscovite . Alumina is high in the finer clays (18 to 3o%), and they are the most aluminous of all sediments, except See also:

• BAUXITE

bauxite . See also:

• MAGNESIA

Magnesia is never absent, though its amount may be less than r %; it is usually contained in minerals of the chlorite See also:

• GROUP

group, but partly also in dolomite .

The alkalis are very interesting; often they form 5 or to% of the whole rock; they indicate abundance of white micas or of undecomposed particles of felspar . Some clays, however, such as fireclays, contain very little potash or soda, while they are See also:

• RICH, BARNABE (c. 1540-1617)
• RICH, CLAUDIUS JAMES (1787-1821)
• RICH, JOHN (1692-1761)
• RICH, PENELOPE, LADY (c. 1562–1607)
• RICH, RICHARD, 1ST BARON RICH (1490?-1567)

rich in alumina; and it is a See also:

• FAIR

fair inference that hydrated aluminous silicates, such as kaolin, are well represented in these rocks . There are, in fact, a few clays which contain about 45 % of alumina, that is to say, more than in pure kaolin . It is probable that these are related to bauxite and certain kinds of See also:

• LATERITE (Lat. later, a brick)

laterite . A few of the most important clay rocks, such as See also:

• CHINA

china-clay, See also:

• BRICK (derived according to some etymologists from the Teutonic bricke, a disk or plate; but more authoritatively, through the French brique, originally a " broken piece," applied especially to bread, and so to clay, from the Teutonic brikan, to break)

brick-clay, red-clay and shale, may he briefly described here . China-clay is white, friable and earthy . It occurs in regions of See also:

• GRANITE (adapted from the Ital. granite, grained; Lat. granum, grain)

granite, See also:

• PORPHYRY (IlopcPisptos) (A.D. 233-c. 304)

porphyry and See also:

• SYENITE

syenite, and usually occupies See also:

• FUNNEL (through an O. Fr. founil, found in Breton, from Lat. infundibulum, that through which anything is poured, from fundere, to pour)

funnel-shaped cavities of no great superficial See also:

• AREA

area, but of considerable See also:

• DEPTH

depth . It consists of very fine scaly kaolin, larger, shining plates of white mica, grains of quartz and particles of semi-decomposed felspar, tourmaline, zircon and other minerals, which originally formed part of the granite . These clays are produced by the decomposition of the granite by See also:

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

acid vapours, which are discharged after the igneous rock has solidified (" See also:

• FUMAROLE

fumarole or pneumatolytic action") . See also:

• FLUORINE (symbol F, atomic weight 19)

Fluorine and its compounds are often supposed to have been among the agencies which produce this See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change, but more probably carbonic acid played the principal role . The felspar decomposes into kaolin and quartz; its alkalis are for the most part set free and removed in solution, but are partly retained in the white mica which is constantly found in crude china-clays . Semi-decomposed varieties of the granite are known as china-See also:

• STONE
• STONE (0. Eng. shin; the word is common to Teutonic languages, cf. Ger. Stein, Du. steen, Dan. and Swed. sten; the root is also seen in Gr. aria, pebble)
• STONE, CHARLES POMEROY (1824-1887)
• STONE, EDWARD JAMES (1831-1897)
• STONE, FRANK (1800-1859)
• STONE, GEORGE (1708—1764)
• STONE, LUCY [BLACKWELL] (1818-1893)
• STONE, MARCUS (184o— )
• STONE, NICHOLAS (1586-1647)

stone .

The kaolin may be washed away from its See also:

• ORIGINAL

original site, and deposited in hollows or lakes to form beds of white clay, such as See also:

• PIPE

pipe-clay; in this case it is always more or less impure . Yellow and pinkish varieties of china-clay and pipe-clay contain a small quantity of oxide of iron . The best known localities for china-clay are See also:

• CORNWALL

Cornwall, See also:

• LIMOGES

Limoges (See also:

• FRANCE
• FRANCE, ANATOLE (1844– )

France), See also:

• SAXONY
• SAXONY (Ger. Provinz Sachsen)

Saxony, Bohemia and China; it. is found also in See also:

• PENNSYLVANIA
• PENNSYLVANIA, UNIVERSITY OF

Pennsylvania, N . Carolina and elsewhere in the See also:

• UNITED

United States . See also:

• FIRE (in O. Eng. f j'r; the word is common to West German languages, cf. Dutch vuur, Ger. Feuer; the pre-Teutonic form is seen in Sanskrit pu, pavaka, and Gr. irup; the ultimate origin is usually taken to be a root meaning to purify, cf. Lat. purus)

Fire-clays include all those varieties of clay which are very refractory to See also:

• HEAT (0. E. haktu, which like " hot," Old Eng. hat, is from the Teutonic type haita, hit, to be hot; cf. Ger. hitze, heiss; Dutch, hitte, heet, &c.)

heat . They must contain little alkalis, lime, magnesia and iron, but some of them are comparatively rich in silica . Many of the clays which pass under this designation belong to the Carboniferous See also:

• PERIOD
• PERIOD (Gr. irepioSos, a going or way round, circuit, aepi, round, and &Sis, way, road)

period, and are found underlying seams of coal . Either by rapid growth of vegetation, or by subsequent percolation of organic solutions, most of the alkalis and the lime have been carried away . Any argillaceous material, which can be used for the manufacture of bricks, may be called a brick-clay . In See also:

• ENGLAND
• ENGLAND, THE CHURCH OF

England, Kimmeridge Clay, Lias clays, London Clay and pulverized shale and slate are all employed for this purpose . Each variety needs See also:

• SPECIAL

special treatment according to its properties . The true brick-clays, however, are superficial deposits of See also:

• PLEISTOCENE

Pleistocene or See also:

• QUATERNARY

Quaternary See also:

• AGE (Fr. age, through late Lat. aetaticum, from aetas)

age, and occur in hollows, filled-up lakes and deserted stream channels .

Many of them are derived from the glacial 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)

boulder-clays, or from the washing away of the finer materials contained in older clay formations . They are always very impure . The red-clay is an abysmal formation, occurring in the sea bottom in the deepest part of the oceans . It is estimated to See also:

• COVER (from the Fr. convert, from couvrir, to cover, Lat. cooperire)

cover over fifty millions of square See also:

• MILES, NELSON APPLETON (1839— )

miles, and is probably the most extensive See also:

• DEPOSIT (Lat. depositurn, from deponere, to lay down, to put in the care of)

deposit which is in course of See also:

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

accumulation at the present See also:

• DAY (O. Eng. dreg, Ger. Tag; according to the New English Dictionary, " in no way related to the Lat. dies")
• DAY, JOHN (1574-1640?)
• DAY, THOMAS (1748-1789)

day . In addition to the reddish or brownish argillaceous See also:

• MATRIX

matrix it contains fresh or decomposed crystals of volcanic minerals, such as felspar, augite, hornblende, olivine and pumiceous or palagonitic rocks . These must either have been ejected by submarine volcanoes or drifted by the wind from active vents, as the fine ash discharged by See also:

• KRAKATOA (KRAKATAO, KRAKATAU)

Krakatoa was wafted over the whole globe . Larger rounded lumps of See also:

• PUMICE (Lat. purnex, spumex, spuma, froth)

pumice, found in the clay, have probably floated to their present situations, and sank when decomposed, all their cavities becoming filled with sea water . Crystals of See also:

• ZEOLITES

zeolites (See also:

• PHILLIPSITE

phillipsite) form in the red-clay as radiate, nodular See also:

• GROUPS
• GROUPS, THEORY

groups . Lumps of manganese oxide, with a See also:

• BLACK
• BLACK, ADAM (1784-1874)
• BLACK, JEREMIAH SULLIVAN (1810–1883)
• BLACK, WILLIAM (1841-1898)

black, shining outer surface, are also characteristic of this deposit, and frequently encrust pieces of pumice or See also:

• ANIMAL
• ANIMAL (Lat. animalis, from anima, breath, soul)

animal remains . The only fossils of the clay are See also:

• RADIOLARIA

radiolaria, sharks' See also:

• TEETH (O.E. Eel); plural of tooth, O.E. top)

teeth and the See also:

• EAR (common Teut.; O.E. are, Ger. Ohr, Du. oor, akin to Lat. auris, Gr. ovs)

ear-bones of whales, precisely those parts of the See also:

• SKELETON

skeleton of marine creatures which are hardest and can longest survive exposure to sea-water . Their See also:

• COMPARATIVE

comparative abundance shows how slowly the clay gathers . Small rounded spherules of iron, believed by some to be meteoric dust, have also been obtained in some See also:

• NUMBERS, BOOK OF
• NUMBERS, PARTITION OF

numbers .

Among the rocks of the continents nothing exactly the same as this remarkable deposit is known to occur, though fine dark clays, with manganese nodules, are found in many localities, accompanied by other rocks which indicate deep-water conditions of deposit . Another type of red-clay is found in caves, and is known as See also:

• CAVE (Lat. cavea, from caves, hollow)
• CAVE, EDWARD (1691–1754)
• CAVE, WILLIAM (1637–1713)

cave-See also:

• EARTH
• EARTH (a word common to Teutonic languages, cf. Ger. Erde, Dutch aarde, Swed. and Dan. jord; outside Teutonic it appears only in the Gr. 'pq'e, on the ground; it has been connected by some etymologists with the Aryan root ar-, to plough, which is seen in
• EARTH, FIGURE OF
• EARTH, FIGURE OF THE

earth or red-earth (terra rossa) . It is fine, tenacious and bright red, and represents the insoluble and thoroughly weathered impurities which are See also:

• LEFT

left behind when the calcareous matter is removed in solution by carbonated See also:

• WATERS, TERRITORIAL

waters . Similar residual clays sometimes occur on the surface of areas of See also:

• LIMESTONE

limestone in hollows and fissures formed by weathering . Boulder-clay is a coarse unstratified deposit of fine clay, with more or less sand, and boulders of various sizes, the latter usually marked with glacial striations . Some clay rocks which have been laid down by water are very See also:

• UNIFORM

uniform through their whole thickness, and are called mud-stones . Others split readily into fine leaflets or laminae parallel to their bedding, and this structure is accentuated by the presence of films of other materials, such as sand or See also:

• VEGETABLE

vegetable debris . Laminated clays of this sort are generally known as shales; they occur in many formations but are very common in the Carboniferous . Some of them contain much organic debris, and when distilled yield See also:

• PARAFFIN

paraffin oil, See also:

• WAX

wax, compounds of See also:

• AMMONIA (NH3)

ammonia, &c . In these oil-shales there are clear, globular, yellow bodies which seem to be resinous . It has been suggested that the admixture of large quantities of decomposed fresh-water See also:

• ALGAE

algae among the original mud is the origin of the paraffins . In New See also:

• SOUTH
• SOUTH, ROBERT (1634–1716)

South See also:

• WALES (Cymru, Gwalia, Cambria)

Wales, See also:

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

Scotland and several parts of See also:

• AMERICA

America such oil-shales are worked on a commercial See also:

• SCALE
• SCALE (1) A

scale .

Many shales contain great numbers of ovoid or rounded septarian nodules of clay ironstone . Others are rich in pyrites, which, on oxidation, produces sulphuric acid; this attacks the aluminous silicates of the clay and forms aluminium sulphate (alum shales) . The lias shales of See also:

• WHITBY

Whitby contain blocks of semi-mineralized See also:

• WOOD, ANTHONY A2 (1632-1695)
• WOOD, JOHN GEORGE (1827—1889)
• WOOD, MRS HENRY [ELLEN] (1814—1887)
• WOOD, SEARLES VALENTINE (1798—188o)

wood, or See also:

• JET (Fr. jais, Ger. Gagat)

jet, which is black with a resinous lustre, and a fibrous structure . The laminated structure of shales, though partly due to successive very thin sheets of deposit, is certainly de-pendent also on the See also:

• VERTICAL (from Lat. vertex, highest point)

vertical pressure exerted by masses of super-See also:

• INCUMBENT (from Lat. incumbere, to lean, lie upon)

incumbent rock; it indicates a transition to the fissile character of clay slates . (J . S .