G E A 



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G R A 



GEANULA'TION. {granulation, Fr.) 



1. The operation by which metals 

 are reduced into small grains. 



2. The act of forming into bodies 

 resembling aggregates of grains. 



GBA'NTJXITE. A crystalline aggregate 

 of quartz and felspar, in which the 

 quartz occurs in thin flakes. 



GEA'PHIC GEA'NITE. Called also Peg- 

 matite. A variety of granite, compo- 

 sed of felspar and quartz, so arran- 

 ged as to produce an imperfect 

 laminar structure. When a section 

 of graphic granite is made at right 

 angles to the alternations of the 

 constituent minerals, broken lines, 

 resembling Hebrew characters, pre- 

 sentthemselves; hence its derivation . 



GEA'PHITE. Another name for black- 

 lead, or plumbago ; carburet of 

 iron. Graphite is of a dark steel- 

 gray, or nearly iron-black. It 

 leaves on paper a well denned, 

 shining trace, which has very near- 

 ly the colour of the mass, and con- 

 sists of minute grains. It is per- 

 fectly opaque, easily scraped by a 

 knife, and soils the fingers. It is 

 a conductor of electricity, and when 

 rubbed on sealing-wax till a metal- 

 lic trace appears, communicates no 

 electricity to the wax. Specific 

 gravity from 1-98 to 2'26. Consti- 

 tuent parts, carbon 92, iron 8. 



GEA'PTOLITE. (graptolithus, Linnasus.) 

 Graptolites form a genus of the 

 family of sea-pens. " These pen- 

 like, serrated, fossils have," says 

 Sir E. Murchison, " a great vertical 

 range in the older or Protozoic 

 rocks, being found from the lower 

 part of the Ludlow formation, 

 down to very ancient beds in the 

 Cambrian system." " Very differ- 

 ent opinions," says the Danish 

 naturalist, Dr. Beck, "have been 

 entertained as to the place grapto- 

 lites hold in the series of living 

 beings, but that of Prof. Nilsson 

 may come nearest to the truth, 

 who conceives the graptolite to be 

 a polyparium of the ceratophydian 



family. Yet I am more inclined 

 to regard them as belonging to the 

 group Pennatulinae, the Linnsean 

 Yirgularia being the nearest form 

 in the present state of nature to 

 which they may be compared. I 

 am now acquainted with six or 

 seven species of graptolites, all 

 occurring in the oldest fossiliferoua 

 strata, where they are associated 

 with Trilobites, Orthoceratites, &c. 

 This genus was established by 

 Linnaeus. The graptolite of the 

 most ancient fossiliferous rocks 

 occurs in greatest abundance in a 

 finely levigated mudstone, for it 

 too was a dweller in the mud." 

 Hugh Miller. A fossil zoophyte, 

 found in the Silurian shales. 

 GEAVITA'TION. (gravitation, Fr. gram- 

 tazione, It.) The difference be- 

 tween gravity and the centrifugal 

 force induced by the velocity of 

 rotation or revolution : the force 

 which causes substances to fall to 

 the surface of the earth, and which 

 retains the celestial bodies in their 

 orbits; its intensity increases as 

 the squares of the distance decrease. 

 ' 'Gravitation , ' ' says Mrs Somerville, 

 " not only binds satellites to their 

 planets, and planets to the sun, but 

 it connects sun with sun throughout 

 the wide extent of creation, and is 

 the cause of the disturbances, as 

 well as of the order, of nature; 

 since every tremor it excites in any 

 one planet is immediately trans- 

 mitted to the farthest limits of our 

 system, in oscillations, which cor- 

 respond in their periods with the 

 cause producing them." 

 GBA'VITY. (gravitt, Fr. gravitti, It.) 

 The reciprocal attraction of matter 

 on matter. The force of gravity is 

 everywhere perpendicular to the 

 surface, and in direct proportion to 

 the quantity of matter. 

 GEA'YWACKE. \ (from grauwacTce, 

 GEAT/WACKE. f Germ, a com- 

 GEAFWACKE'. k pound of grau, 

 GBE'YWACKE. / grey, and ivacke, 



