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See also:GREISEN (in See also:French, hyalomicte) , a modification of See also:granite, consisting essentially of See also:quartz 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, and distinguished from granite by the See also:absence of See also:felspar and See also:biotite. In the See also:hand specimen the See also:rock has a silvery glittering See also:appearance from the abundance of lamellar crystals of See also:muscovite, but many greisens have much of the appearance of granite, except that they are paler in See also:colour. The commonest See also:accessory minerals are See also:tourmaline, See also:topaz, See also:apatite, fluorspar and See also:iron oxides; a little felspar more or less altered may also be See also:present and a 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 mica which is biotite or lithionite. The tourmaline in See also:section is brown, See also:green, See also:blue or colourless, and often the same crystal shows many different tints. The white mica forms mostly large plates with imperfect crystalline outlines. The quartz is See also:rich in fluid enclosures. Apatite and topaz are both colourless and of irregular See also:form. Felspar if present may be See also:orthoclase and See also:oligoclase.
Greisen occurs typically in belts or See also:veins intersecting granite. At the centre of each vein there is usually a fissure which may be open or filled with quartz. The greisen bands are from I in. up to 2 ft. or more in thickness. At their See also:outer edges they pars gradually into the granite, for they contain felspar crystals more or less completely altered into aggregates of white mica and quartz. The transition between the two rocks is perfectly See also:gradual, a fact which shows that the greisen has been produced by alteration of the granite. Vapours or fluids rising through the fissure have been the agents which effected the transmutation. They must have contained See also:fluorine, See also:boron and probably also See also:lithium, for topaz, mica and tourmaline, the new minerals of the granite, contain these elements. The See also:change is a See also:post-volcanic
TI
or pneumatolytic one induced by the vapours set See also:free by the granite magma when it cools. Probably the rock was at a relatively high temperature at the 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. A similar type of alteration, the development of white mica, quartz and tourmaline, is found sometimes in sedimentary rocks around granite masses. Greisen is closely connected with See also:schorl rock both in its mineralogical See also:composition and in its mode of origin. The latter is a pneumatolytic product consisting of quartz and tourmaline; it often contains white mica and thus passes by all stages into greisen. Both of these rocks carry frequently small percentages of See also:tin See also:oxide (cassiterite) and may be worked as ores of tin. They are See also:common in See also:Cornwall, See also:Saxony, See also:Tasmania and other districts which are centres of tin-See also:mining. Many other greisens occur in which no tin is found. The analyses show the composition
of Cornish granite and greisen. They make it clear that there has been an introduction of fluorine and boron and a diminution in the alkalies during the transformation of the granitic rock into the greisen. (J. S.
End of Article: GREISEN (in French, hyalomicte)
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