A Manual of Elementary Geology Part 58

DOLERITE. Formerly defined as a synonym of greenstone, which see. But, according to Rose (_ibid._ p. 32.), its composition is black augite and Labrador-felspar; according to Leonhard (_Mineralreich_, &c. p. 77.), augite, Labrador-felspar, and magnetic iron.

DOMITE. An earthy _trachyte_, found in the Puy de Dome, in Auvergne.

EUPHOTIDE. A mixture of grains of Labrador-felspar and diallage. (_Rose_, _ibid._ p. 19.) According to some, this rock is defined to be a mixture of augite or hornblende, and saussurite, a mineral allied to jade. (_Allan's Mineralogy_, p. 158.) _See_ Diallage rock.

FELSPAR-PORPHYRY. _Syn._ Hornstone-porphyry; a base of felspar, with crystals of felspar, and crystals and grains of quartz. _See_ also Hornstone.

GABBRO, _see_ Diallage rock.

GREENSTONE. _Syn._ Dolerite and diorite; components, hornblende and felspar, or augite and felspar in grains. See above, p. 372.

GREYSTONE. (Graustein of Werner.) Lead grey and greenish rock, composed of felspar and augite, the felspar being more than seventy-five per cent.

(_Scrope_, _Journ. of Sci._ No. 42. p. 221.) Greystone lavas are intermediate in composition between basaltic and trachytic lavas.

HORNBLENDE ROCK. A greenstone, composed princ.i.p.ally of granular hornblende, or augite. (_Leonhard_, _Mineralreich_, &c., p. 85.)

HORNSTONE, HORNSTONE-PORPHYRY. A kind of felspar porphyry (_Leonhard_, _ibid._), with a base of hornstone, a mineral approaching near to flint, differing from compact felspar in being infusible.

HYPERSTHENE ROCK, a mixture of grains of Labrador-felspar and hypersthene (_Rose_, _Ann. des Mines_, tom. 8. p. 13.), having the structure of syenite or granite; abundant among the traps of Skye. Some geologists consider it a greenstone, in which hypersthene replaces hornblende.

LATERITE. A red jaspery rock, composed of silicate of alumina and oxide of iron. Abundant in the Deccan, in India; and referred to the trap formation; from Later, a brick or tile.

MELAPHYRE. A variety of black porphyry, the base being black augite with crystals of felspar; from +melas+, _melas_, black.

OBSIDIAN. Vitreous lava like melted gla.s.s, nearly allied to pitchstone.

OPHIOLITE, sometimes same as Diallage rocks (_Leonhard_, p. 77.); sometimes a kind of serpentine.

OPHITE. A green porphyritic rock composed chiefly of hornblende, with crystals of that mineral in a base of the same, mixed with some felspar.

It pa.s.ses into serpentine by a mixture of talc. (_Burat's d'Aubuisson_, tom. ii. p. 63.)

PEARLSTONE. A volcanic rock, having the l.u.s.tre of mother of pearl; usually having a nodular structure; intimately related to obsidian, but less gla.s.sy.

PEPERINO. A form of volcanic tuff, composed of basaltic scoriae.

_See_ p. 374.

PETROSILEX. _See_ Clinkstone and Compact Felspar.

PHONOLITE. _Syn._ of Clinkstone, which see.

PITCHSTONE. Vitreous lava, less gla.s.sy than obsidian; a blackish green rock resembling gla.s.s, having a resinous l.u.s.tre and appearance of pitch; composition various, usually felspar and augite; pa.s.ses into basalt; occurs in veins, and in Arran forms a dike thirty feet wide, cutting through sandstone; forms the outer walls of some basaltic dikes.

PORPHYRY. Any rock in which detached crystals of felspar, or of one or more minerals, are diffused through a base. _See_ p. 372.

POZZOLANA. A kind of tuff. _See_ p. 36.

PUMICE. A light, spongy, fibrous form of trachyte. _See_ p. 373.

PYROXENIC-PORPHYRY, same as augitic-porphyry, pyroxene being Hauy's name for augite.

SCORIae. _Syn._ volcanic cinders; reddish brown or black porous form of lava. _See_ p. 373.

SERPENTINE. A greenish rock, in which there is much magnesia; usually contains diallage, which is nearly allied to the simple mineral called serpentine. Occurs sometimes, though rarely, in dikes, altering the contiguous strata; is indifferently a member of the trappean or hypogene series.

SYENITIC-GREENSTONE; composition, crystals or grains of felspar and hornblende. _See_ p. 372.

TEPHRINE, synonymous with lava. Name proposed by Alex. Brongniart.

TOADSTONE. A local name in Derbys.h.i.+re for a kind of wacke, which see.

TRACHYTE. Chiefly composed of gla.s.sy felspar, with crystals of gla.s.sy felspar. _See_ p. 372.

TRAP TUFF. _See_ p. 374.

TRa.s.s. A kind of tuff or mud poured out by lake craters during eruptions; common in the Eifel, in Germany.

TUFACEOUS CONGLOMERATE. _See_ p. 374.

TUFF. _Syn._ Trap-tuff, volcanic tuff. _See_ p. 374.

VITREOUS LAVA. _See_ Pitchstone and Obsidian.

VOLCANIC TUFF. _See_ p. 374.

WACKe. A soft and earthy variety of trap, having an argillaceous aspect. It resembles indurated clay, and when scratched exhibits a s.h.i.+ning streak.

WHINSTONE. A Scotch provincial term for greenstone and other hard trap rocks.

a.n.a.lYSIS OF MINERALS MOST ABUNDANT IN THE VOLCANIC AND HYPOGENE ROCKS.

Silica. Alu- Mag- Lime. Pot- Soda. Iron. Manga- Remain- mina. nesia. ash. Oxide. nese. der.

Actinolite 64 -- 22 -- -- -- 3 a 4305 C.

(Bergman) trace

Albite (Rose) 6884 2053 -- a trace -- 912 -- -- -- --(mean of 4 6945 1944 013 022 -- 995 a -- -- a.n.a.lyses) trace

Augite (Rose) 5336 -- 499 2219 -- -- 1738 009 -- --(mean of 4 5357 1 1126 209 -- -- 1075 067 -- a.n.a.lyses)

Carbonate of -- -- -- 5633 -- -- -- -- -- Lime (Biot)

Chiastolite 6849 3017 412 -- -- -- 27 -- 027 W.

(Landgrabe)

Chlorite 26 185 8 -- -- 2 43 -- -- (Vauquelin) --(mean of 3 2743 179 1456 050 156 -- 3063 -- 692 W.

a.n.a.lyses)