G R A 



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G R E 



sandstone is a graywacke, coloured red 

 by the accidental admixture of oxide of 

 iron ; it possesses all the mineral charac- 

 ters, and occupies the geological position 

 of, graywacke. The rock, though com- 

 posed of substances of various colours, 

 usually exhibits some shade of gray or 

 brown ; it is sometimes of considerable 

 hai-dness, and susceptible of a high polish. 

 Graywacke is often distinctly stratified, 

 but the strata are not usually parallel to 

 those of the subjacent rocks. The com- 

 mon and slaty varieties often alternate 

 with each other, and both are traversed 

 by veins of quartz. This rock is remark- 

 ably metalliferous ; and its ores occur 

 both in beds and veins. Most of the 

 mines of the Hartz are contained in gray- 

 wacke. 



Graywacke abounds in Germany and 

 in Scotland ; indeed, nearly all the moun- 

 tains of Scotland north of the Frith of 

 Forth are chiefly composed of it. In the 

 neighbourhood of Mont Blanc, and in 

 other parts of the Alps, it occurs at a 

 great elevation, forming large masses in 

 vertical beds. 



GRA'YWACKE SLATE. A variety of gray- 

 wacke, in which the grains are so minute 

 as to be scarcely perceptible by the naked 

 eye. 



GREEN-EARTH. The Griin Erde of Werner ; 

 the Talc Zographique of Haiiy ; the 

 Chlorite Baldogee of Brongniart. A va- 

 riety of talc, occurring in vesicular cavi- 

 ties in amygdaloid. Its colour is a plea- 

 sant green, more or less deep, sometimes 

 bluish or grayish-green, and passing to 

 olive and blackish -green. Its fracture is 

 dull, and fine-grained earthy, or slightly 

 conchoidal. It is somewhat unctuous to 

 the touch, and adheres to the tongue. 

 Easily reducible to powder. Specific 

 gravity 2*63. Kirwan. 



According to Vauquelin it consists of 

 silex 52'0, magnesia 6'0, oxide of iron 

 23-4, alumina 7'0, potash 7'4, water 4'0. 

 It is met with in the mountainous districts 

 of England and Scotland. It is the moun- 

 tain-green of artists ; and, when ground 

 with oil, is employed as a paint. 



GREEN-SAND. A member of the chalk 

 formation, called also Shanklin sand. The 

 beds of sand, sandstone, and limestone, 

 which form the lowermost strata of the 

 chalk formation, have obtained the name 

 of green-sand, from the circumstance of 

 their containing a considerable quantity 

 of chlorite, or green earth, scattered 

 throughout their substance. In describ- 

 ing the group of deposits to which the 

 name of green-sand, or Shanklin-sand, is 

 appropriated, geologists state that they 

 admit of a triple division ; the first, or 

 uppermost, consists of sand, with irregular 



concretions of limestone and chert, some- 

 times disposed in courses oblique to the 

 general direction of the strata. The se- 

 cond consists chiefly of sand, but in some 

 places is so mixed with clay, or with oxide 

 of iron, as to retain water. The third, 

 and lowest group, abounds much more in 

 stone ; the concretional beds being closer 

 together and more nearly continuous. 

 The total thickness of the green-sand, 

 where it is fully developed, is more than 

 400 feet. The animal remains of the 

 green-sand are exclusively marine. The 

 French have denominated this formation 

 glauconie crayeuse, and craie chloritee. 

 It is very common to divide the green- 

 sand into the upper green-sand and the 

 lower green-sand, the two being separated 

 by the gault. 



GREE'NSTONE. The Griinstein of Werner ; 

 Roche Amphibolique of Haiiy ; the dia- 

 base of the French geologists. A granular 

 rock composed of hornblende and felspar, 

 in the state of grains, or sometimes of 

 small crystals. Greenstone contains a 

 larger quantity of felspar than basalt, and 

 the grains both of hornblende and felspar 

 are less amalgamated. It is a variety of 

 trap rock. The hornblende usually pre- 

 dominates, and frequently gives to this 

 aggregate a greenish hue, from which cir- 

 cumstance it obtains its name. Green- 

 stone occurs in beds of considerable mag- 

 nitude, and sometimes forms whole moun- 

 tains. It often appears in conical hills, 

 or presents high, mural precipices, whose 

 fronts are frequently composed of nume- 

 rous columns of various sizes, resembling 

 basalt. Sometimes it forms only the 

 summits of mountains. Small veins of 

 actinolite, epidote, felspar, prehuite, 

 quartz, &c., &c., frequently are found 

 traversing greenstone. 



GREGA'RIOUS. (gregarfas, Lat.) Living 

 in flocks or herds ; animals whose habits 

 lead them to assemble in numbers. 



GRE'NATITE. Prismatoidal garnet. See 

 Granatite. 



GRE'YSTONE. A rock of greyish or green- 

 ish colour, composed of felspar and au- 

 gite, the former being more than 75 per 

 cent, of the whole. 



GRE'YWACKE. For a full description of 

 this, see Graywacke. 



GREY-WEATHERS. The name given to 

 large boulders of siliceous sandstone. 

 There is a singular assemblage of these 

 erratic blocks in a field on the borders of 

 Wiltshire, not far from Marlboro ugh. 

 The immense blocks forming, as is sup- 

 posed, the druidical temple at Stonehenge, 

 are composed of this siliceous sandstone. 

 tf Boulders of druid sandstone," observes 

 Dr. Mantell, " also occur in the shingle 

 bed, and calcareous deposit, at Brighton, 



