See also:

• RHYOLITE (Gr. peiv, to flow, because of the frequency with which they exhibit fluxion structures)

RHYOLITE (Gr. peiv, to flow, because of the frequency with which they exhibit fluxion structures)  , the See also:

• GROUP

group name of a type of volcanic 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, occurring mostly as See also:

• LAVA

lava flows, and characterized by a highly See also:

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

acid See also:

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

composition . They are the most siliceous of all lavas, and, with the exception of the dacites, are the only lavas which contain See also:

• FREE

free See also:

• PRIMARY

primary See also:

• QUARTZ

quartz . In chemical composition they very closely resemble the granites which are the corresponding rocks of plutonic or deep-seated origin; their minerals also See also:

• PRESENT

present many points of similarity to those of See also:

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

granite though they are by no means entirely the same . Quartz, See also:

• ORTHOCLASE

orthoclase and See also:

• PLAGIOCLASE

plagioclase felspars, and See also:

• BIOTITE

biotite are the commonest ingredients of both rocks, but the quartz of rhyolites is full of 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 enclosures and the potash See also:

• FELSPAR, or FELDSPAR

felspar is pellucid sanidine, while the quartz of granite contains dust-like fluid cavities of very See also:

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

minute See also:

• SIZE

size and its potash felspar is of the turbid variety which is properly called orthoclase . The granites also are holocrystalline, while in the rhyolites there are usually porphyritic crystals floating in a See also:

• FINE

fine ground-See also:

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

mass . Rhyolites have also been called liparites because many of the lavas of the Lipari Islands are excellent examples of this group . Above all rocks they have a disposition to assume vitreous forms, as when fused they crystallize with See also:

• GREAT

great difficulty . Hence it has See also:

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

long baffled experimenters to produce See also:

• RHYOLITE (Gr. peiv, to flow, because of the frequency with which they exhibit fluxion structures)

rhyolite synthetically by See also:

• FUSION

fusion; it is stated that these difficulties have now been over-come, but geologists believe that the presence of See also:

• STEAM
• STEAM (0. Eng. steam, vapour, smoke, cf. Du. steam; the origin is unknown)

steam and other gases in the natural See also:

• STATE
• STATE, GREAT OFFICERS OF

state expedites See also:

• CRYSTALLIZATION

crystallization . In crucibles these cannot be retained at the temperatures employed; when the rocks are melted the gases See also:

• ESCAPE (in mid. Eng. eschape or escape, from the O. Fr. eschapper, modern echapper, and escaper, low Lat. escapium, from ex, out of, and cappa, cape, cloak; cf. for the sense development the Gr. iichueoOat, literally to put off one's clothes, hence to sli

escape and on cooling a pure glass is formed . The vitreous forms of rhyolite are known as See also:

• OBSIDIAN

obsidian, See also:

• PERLITE, or PEARLSTONE

perlite and See also:

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

pumice (qq.v.) . The minerals of the first See also:

• GENERATION (from Lat. generare, to beget, procreate; genus, stock, race)

generation, or phenocrysts, of rhyolite are generally orthoclase, See also:

• OLIGOCLASE

oligoclase, quartz, biotite, See also:

• AUGITE

augite or See also:

• HORNBLENDE

hornblende . The felspars are usually glassy clear, small but of well-See also:

• DEVELOPED

developed crystalline See also:

• FORM (Lat. forma)

form: the potash felspar is sanidine, usually See also:

• CARLSBAD

Carlsbad twinned; the soda-See also:

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

lime felspar is almost always oligoclase, with characteristic polysynthetic structure .

Both of these may be corroded and irregular in their outlines; their cleavage and twinning then distinguish them readily from quartz . Glass en-closures, sometimes rectangular with small immobile bubbles, are frequent . The quartz occurs as blebs or sub-rounded grains, which are corroded See also:

• DOUBLE
• DOUBLE (from the Mid. Eng. duble, the form which gives the present pronunciation, through the Old Er. duble, from Lat. duplus, twice as much)

double hexagonal pyramids . Its glass enclosures are many and nearly always rounded or elliptical in See also:

• SECTION
• SECTION (Lat. sectio, cutting, secare, to cut)

section . No proper cleavage is seen in the quartz, though arcuate (conchoidal) fractures may often be noticed; they may have been produced by See also:

• STRAIN (through O. Fr. straindre, estraindre, mod. i treindre, from Lat. stringere, to draw tight, related to stress, stretch, string, &c.)

strain on cooling . Phenocrysts of micropegmatite are known in some rhyolites; they may have the shape of felspar or of quartz crystals; in the former See also:

• CASE
• CASE, JOHN (d. 1600)

case Carlsbad twinning is by no means uncommon, but in other cases See also:

• HOUR

hour-glass structure is very conspicuous . Biotite is always deep 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 or greenish brown, in small hexagonal tablets, generally blackened at their edges by magmatic corrosion . See also:

• MUSCOVITE

Muscovite is not known in rhyolites . Hornblende may be 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 or brown; in the quartz-pantellarites it sometimes takes the form of strongly pleochroic brown cossyrite . Like biotite it is eumorphic but often corroded in a marked degree . Augite, which is equally See also:

• COMMON

common or more common than the other ferro-magnesian minerals, is always green; its crystals are small and perfectly shaped, and corrosion phenomena are very rarely seen in it . See also:

• ZIRCON

Zircon, See also:

• APATITE

apatite and See also:

• MAGNETITE

magnetite are always present in rhyolites, their crystals being often beautifully perfect though never large .

See also:

• OLIVINE

Olivine is never a normal ingredient, but occurs in the hollow See also:

• SPHERULITES (Gr. cchaipa, sphere, Mhos, stone)

spherulites or lithophysae of some rhyolites with See also:

• GARNET
• GARNET, or GARNETT, HENRY (1555—1606)

garnet, See also:

• TRIDYMITE

tridymite, See also:

• TOPAZ

topaz and other minerals which indicate pneumatolytic See also:

• ACTION

action . Among the less common See also:

• ACCESSORY

accessory minerals of the rhyolites are cordierite in crystals which resemble hexagonal prisms but break up under polarized See also:

• LIGHT

light into six radiating sectors owing to complicated twinning: they See also:

• WEATHER (O. Eng. weder; the word is common to Teutonic languages; cf. Du. weder, Dan. veir, Icel. ve8r, and Ger. Wetter and Gewitter, storm; the root is wa- to blow, from which is derived " wind ")

weather to green aggregates of See also:

• CHLORITE

chlorite and muscovite (pinite) ; garnet, See also:

• SPHENE

sphene and orthite may also be met with in rhyolites . The ground-mass of rhyolitic rocks is of three distinct types which are stages in crystalline development, viz. the vitreous, the felsitic or cryptocrystalline, and the microcrystalline . Hence some authorities have proposed to subdivide the group into the vitrophyres, the felsophyres and the granophyres, but this is.not now in use, and the last of these terms has obtained a signification quite different from that originally assigned to it . Mixtures of the different kinds occur; thus a vitreous rhyolite has often felsitic areas in its ground-mass, and in the same lava flow some parts may be vitreous while others are felsitic . The vitreous rhyolites are identical in most respects with. the obsidians, from which they can only be separated in an artificial See also:

• CLASSIFICATION (Lat. classis, a class, probably from the root cal-, cla-, as in Gr. icaMce, clamor)

classification; and in their glassy See also:

• BASE

base the banded or eutaxitic, spherulitic and perlitic structures of pure obsidians are very frequently present (see OBSIDIAN; PERLITE) . The felsoliparites or liparites with stony ground-mass are especially common among the pre-See also:

• TERTIARY

Tertiary igneous rocks (see QUARTZ-See also:

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

PORPHYRY), as liparite glass is unstable and experiences devitrification in course of 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 . Many of these felsites have fluxion banding, spherulites and even perlitic cracks, which are strong See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

evidence that they were originally glassy . In other cases a hyaloliparite, obsidian, or See also:

• PITCHSTONE (German Pechstein, from its resemblance to pitch)

pitchstone becomes felsitic along its See also:

• BORDERS, THE

borders and See also:

• JOINT (through Fr. from Lat. junctum, jungere, to join)

joint planes, or even along perlitic cracks, and we may assume that the once fibrous rock has changed into See also:

• FELSITE

felsite under the action of percolating moisture or even by atmospheric decomposition . In many rhyolites the felsite is See also:

• ORIGINAL

original and represents an incipient crystallization of the vitreous material which took See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place before the rock was yet See also:

• COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

cold . The felsite in turn is liable to 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; it becomes a fine See also:

• MOSAIC (corresponding to Lat. opus musivum, from Gr. µovoeiov, an artificial grotto often decorated with mosaics; the word is only found in the sense of mosaic in late Greek, which generally uses k oXI yrlµa)

mosaic of quartz and See also:

• ALKALI

alkali felspar; and in this way a See also:

• MATRIX

matrix of the third type, the microcrystalline, may develop . This is proved by the occurrence of the remains of spherulitic and perlitic structures in rocks which are no longer felsitic or glassy .

Many micro-crystalline rhyolites have a ground-mass in which much felsitic See also:

• MATTER

matter occurs; but as this tends to recrystallize in course of time, the older rocks of this group show least of it . Whilst no quartz-bearing rhyolites are known to have been erupted in See also:

• RECENT

recent years, See also:

• LACROIX, ANTOINE FRANCOIS ALFRED (1863— )
• LACROIX, PAUL (1806—1884)

Lacroix proved that portions of the " See also:

• DOME (Lat domus, house; Ital. duomo, cathedral)

dome " which See also:

• ROSE
• ROSE (Rosa)
• ROSE, GEORGE (1744-1818)
• ROSE, HUGH JAMES (1795—1838)
• ROSE, WILLIAM STEWART (1775-1843)

rose as a great See also:

• TOWER (Lat. turris; Fr. tour, clocker; Ital. torre; Ger. Thurm)

tower or See also:

• COLUMN (Lat. columna)

column out of the See also:

• CRATER

crater of Mont Pelee after the eruption in 1906 contained small crystals of quartz in the ground-mass . The rock was an acid See also:

• ANDESITE

andesite; and it was ascribed by Lacroix to the action of steam retained in the rock under considerable pressure . The microcrystalline ground-mass of rhyolites is never micrographic as in the porphyries (granophyres); on the other See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

hand it is often micropoikilitic, consisting of small felspars, often sub-rectangular, embedded in little rounded or irregular plates of quartz . The ground-mass of rhyolites is liable to other changes, of which the most important are silicification, kaolinization and sericitization . Among the older rocks of this group it is the exception to find that secondary quartz has not been de-posited in some parts of them . Often indeed the matrix is completely replaced by See also:

• SILICA

silica in the form of finely crystalline quartz or See also:

• CHALCEDONY, or CALCEDONY (sometimes called by old writers cassidoine)

chalcedony; and these rocks on See also:

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

analysis prove to contain over 9o% of silica . In the recent rhyolites of See also:

• HUNGARY
• HUNGARY (Hungarian Magyarorszdg)

Hungary, New See also:

• ZEALAND (also SEALAND Or SEELAND; Danish Sjaelland)

Zealand, &c., the See also:

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

deposit of coarse See also:

• OPAL

opal in portions of the rock is a very common phenomenon . Kaolinization may be due to weathering, and the stony dull See also:

• APPEARANCE (from Lat. apparere, to appear)

appearance of the matrix of many microcrystalline rhyolites is a consequence of the decomposed state of the felspar grains in them; it is even more typically developed by See also:

• FUMAROLE

fumarole action, which replaces the felspars with soft, cloudy 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 products which belong to a See also:

• MINERAL

mineral of the See also:

• KAOLIN

kaolin group . Sericitization, or the development of fine white See also:

• MICA

mica after felspar, is usually associated with shearing, and is commonest in the older rhyolites . Vesicular structure is very common in rhyolites; in fact the pumiceous obsidians have this See also:

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

character in greater perfection than any other rocks (see PUMICE); but even the felsorhyolites are very often vesicular . The cavities are usually lined with opal and tridymite; in the older rocks they may be filled with See also:

• AGATE

agate and chalcedony .

The " See also:

• MILL
• MILL (O. Eng. mylen, later myln, or miln, adapted from the late Lat. molina, cf. Fr. moulin, from Lat. mola, a mill, molere, to grind; from the same root, mol, is derived " meal;" the word appears in other Teutonic languages, cf. Du. molen, Ger. muhle)
• MILL, JAMES (1773-1836)
• MILL, JOHN (c. 1645–1707)
• MILL, JOHN STUART (1806-1873)

mill-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 porphyries," extensively used in See also:

• GERMANY
• GERMANY (Ger. Deutschland)

Germany for grinding See also:

• CORN (a common Teutonic word; cf. Lat. granum, seed, grain)
• CORN (fromm Lat. corms, horn)

corn, are porous rhyolites; the abundance of quartz makes them hard, and their rough surfaces render them peculiarly suitable for this purpose . In some of them the cavities are partlysecondary . These rocks are obtained in the See also:

• ODENWALD

Odenwald, Thuringerwald and See also:

• FICHTELGEBIRGE

Fichtelgebirge . In See also:

• BRITAIN (Gr. Hperavuml vi7vot, Bperravia; Lat. Britannia, rarely Britannia)

Britain a 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 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 Tertiary rhyolite occurs at Tardree, See also:

• ANTRIM
• ANTRIM, RANDAL MACDONNELL, 1ST EARL OF (d. 1636)
• ANTRIM, RANDAL MACDONNELL, 1ST MARQUESS OF (1609-1683)

Antrim (the only See also:

• BRITISH

British rock containing tridymite), and in See also:

• SKYE

Skye . Felsitic rhyolites occur among the Old Red rocks of See also:

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

Scotland (Pent-See also:

• LAND

land Hills, Lorne, &c.), in See also:

• DEVONSHIRE (DEvoN)
• DEVONSHIRE, EARLS AND DUKES OF

Devonshire, and in large See also:

• NUMBERS, BOOK OF
• NUMBERS, PARTITION OF

numbers in See also:

• NORTH
• NORTH, BARONS
• NORTH, MARIANNE (1830—1890)
• NORTH, ROGER (1653-1734)
• NORTH, SIR THOMAS (1535?-16o1?)

North See also:

• WALES (Cymru, Gwalia, Cambria)

Wales . The See also:

• CARNARVONSHIRE (Welsh Caer'narfon, for Caer yn Arfon)

Carnarvonshire rhyolites are often much altered and silicified; many of them have a nodular structure which is very conspicuous on weathered surfaces . The spheroids may be two or three inches in See also:

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

diameter; some of them are built up of See also:

• CON

con-centric shells . RhyoIites are also known from See also:

• FISHGUARD (Abergwaun)

Fishguard, See also:

• MALVERN

Malvern, See also:

• WESTMORLAND
• WESTMORLAND, EARLS OF

Westmorland and Co . See also:

• WATERFORD

Waterford . One of the See also:

• OLDEST

oldest volcanic rocks of Britain (pre-See also:

• CAMBRIAN

Cambrian, Uriconian) is the spherulitic rhyolite of the See also:

• LEA, HENRY CHARLES (1825–1909)

Lea Rock near See also:

• WELLINGTON
• WELLINGTON, ARTHUR WELLESLEY, 1ST DUKE OF (1769-1852)

Wellington, in See also:

• SHROPSHIRE (SALOP)

Shropshire . It shows See also:

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

bright red spherulites in great numbers and is probably an obsidian completely devitrified . Perlitic structure is also visible in it .

In other parts of See also:

• EUROPE

Europe rhyolites have a fairly wide See also:

• DISTRIBUTION (Lat. distribuere, to deal out)

distribution though they are not very numerous . In Hungary (Hlinik, &c.) there are many well-known examples of this class . They extend along the margin of the Carpathians and are found also in Siebenburgen . In See also:

• ITALY
• ITALY (Italia)

Italy they occur in the Euganean Hills and in the Lipari Islands; the latter being the See also:

• PRINCIPAL

principal source of pumice 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 . Rhyolites of Recent See also:

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

age occur in See also:

• ICELAND (Dan. Island)

Iceland (Myvatn, &c.), where they are characterized by the frequent See also:

• ABSENCE (Lat. absentia)

absence of quartz, and the presence of much plagioclase and See also:

• PYROXENE

pyroxene . Some of these rocks have been called See also:

• TRACHYTE (Gr. rpaxus, rough)

trachyte-obsidians, but they seem to be rhyolites which contain an exceptionally large amount of soda . The older rhyolites, which are generally called quartz-porphyries in Germany, are mostly of See also:

• PERMIAN

Permian or Carboniferous age and are numerous in the See also:

• VOSGES
• VOSGES (Lat. Vogesus or Vosagus, Ger. Wasgau or Vogesen)

Vosges, Odenwald, Thuringerwald, &c . They are often accompanied by basic rocks (melaphyres) . Permian rhyolites occur also at See also:

• LUGANO (Ger. Lauis)
• LUGANO, LAKE OF (also called CERESIO)

Lugano in Italy . Rhyolites are known also in See also:

• ASIA

Asia See also:

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

Minor and the See also:

• CAUCASUS

Caucasus, in New Zealand, See also:

• COLORADO

Colorado, See also:

• NEVADA
• NEVADA (a Spanish word meaning " snow-clad " or " snowy land," originally applied to a snow-capped mountain range on the Pacific slope)

Nevada and other parts of western North See also:

• AMERICA

America . In the Yellowstone See also:

• NATIONAL

National See also:

• PARK (Fr. part; Ital. parco; Sp. Marque; O.Eng. pearroc; connected with Ger. pferch, fold, and pfarrei, district, translating med. Lat. parochia, parish)
• PARK, EDWARDS AMASA (1808–1900)
• PARK, MUNGO (1771-1806?)

Park there is a well-known cliff of obsidian which shows remarkably perfect columnar jointing . Some of the rhyolites of Nevada are exceedingly 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 porphyritic minerals, so that they appear at first sight to be holocrystalline rocks, since the ground-mass is scanty and inconspicuous .

To this type the name nevadite has been given, but it is rare and See also:

• LOCAL

local in its distribution . In the See also:

• ISLAND (O.E. ieg =isle, +land')

island.of Pantellaria, which lies to the See also:

• SOUTH
• SOUTH, ROBERT (1634–1716)

south-See also:

• WEST
• WEST, BENJAMIN (1738-1820)
• WEST, NICHOLAS (1461-1533)

west of See also:

• SICILY (Ital. Sicilia)

Sicily, there are rocks of rhyolitic See also:

• AFFINITIES

affinities which present so many unusual features that they have been designated pantellarites . They contain less silica and alumina and more alkalis and See also:

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

iron than do See also:

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

ordinary rhyolites . Their felspars are of the anorthoclase group, being rich in soda together with potash, and are very variable in crystalline development . Aegirine-augite and forms of soda-See also:

• AMPHIBOLE

amphibole are also characteristic of these rocks: dark brown aenigmatite or cossyrite often occur in them . Quartz is not very plentiful; other ingredients are olivine, arfvedsonite and tridymite . The ground-mass varies much, being sometimes quite vitreous, at other times a glass filled with swarms of microliths, while in certain pantellarites it is a microcrystalline aggregate of quartz and alkali felspar . The absence . of plagioclase and biotite are marked distinctions between these rocks and the rhyolites, together with the scarcity of quartz and the prevalence of soda-bearing pyroxenes and amphiboles . Among the Palaeozoic volcanic rocks of Germany there is a group of lavas, the quartz-keratophyres, which are of acid composition and rich in alkali felspar . Their dominant alkali is soda: hence their felspars are See also:

• ALBITE

albite and cryptoperthite, not sanidine as in rhyolites . Quartz occurs sometimes as corroded phenocrysts, but is often scarce even in the ground-mass . Porphyritic biotite or augite are very rare, but occur in the matrix along with felspars and quartz .

Micropegmatite is not infrequent in these rocks, and they may be silicified like the rhyolites . As quartz-keratophyres mostly occur in districts where there has been a See also:

• GOOD, JOHN MASON (1764-1827)

good See also:

• DEAL

deal of folding, they are often crushed and more or less sericitized . They are best known from the Devonian rocks of See also:

• WESTPHALIA (Ger. Westfalen)
• WESTPHALIA, TREATY OF

Westphalia and the Harz, but are also found in See also:

• QUEENSLAND

Queensland, and similar rocks have been described (as soda-felsites) from See also:

• IRELAND
• IRELAND, CHURCH
• IRELAND, CHURCH OF
• IRELAND, JOHN (1761-1842)
• IRELAND, JOHN (1838- )
• IRELAND, WILLIAM HENRY (1777-1835)

Ireland . The rocks which they accompany are usually diabases and spilites . The other group of rhyolitic rocks rich in alkali felspars and soda pyroxenes and amphiboles are the comendites . They are often porphyritic, with crystals of quartz, sanidine, microperthite or albite: the ground-mass is microcrystalline or rarely micrographic, and often filled with spongy growths of aegirine and riebeckite . They are known from the recent eruptive districts of See also:

• EAST
• EAST, ALFRED (1849- )

East See also:

• AFRICA
• AFRICA, ROMAN

Africa, from See also:

• SARDINIA (Gr. 'IXs'o"uva, from a fancied resemblance to a footprint in its shape, Ital. Sardegna)

Sardinia and See also:

• TEXAS

Texas, and very similar rocks occur as intrusive masses which may be grouped with the porphyries . The following analyses show the composition of some of the principal types of rhyolites: SiO, Al20a Fe20a FeO CaO MgO K2O Na2O See also:

• H2O (combined)

H2O I . 76'34 13.22 1.93 1.85 0.21 3.67 2.84 o•61 II . 72.15 13.50 3.12 0.93 0.16 4.54 4'20 o•85 IV . 67.48 9.70 7.42 2.21 1.45 0.77 2.94 7.21 0.96 V . 70.97 13.84 3.21 0.78 1.26 0.20 1.57 6.27 0.74 VI .

74'76 Ii•6o 3.50 0.19 0.07 0.18 4'92 4'35 0'64 I . Rhyolite, Telki Banya, Hungary . II. do . Mafahlid, Iceland . IV . Pantellarite, Pantellaria . V . Quartz-keratophyre, Muhlenthal, Harz . VI . Comendite, Sardinia . We See also:

• NOTE
• NOTE (Lat. nota, mark, sign, from noscere, to know)

note in the rhyolites I.-III. the very high silica, with alkalis and alumina also in considerable amount, while lime, See also:

• MAGNESIA

magnesia and iron are very See also:

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

low . In the pantellarite, keratophyre and comendite the silica tends to be less abundant, while the alkalis, especially soda, increase; they have less alumina but are richer in iron and magnesia.' It is easy to see why the latter types contain less quartz, felspars often very rich in soda, and femic minerals which contain iron and alkalis in notable amounts such as aegirine, riebeckite and arfvedsonite .

(J . S .