T R A 



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T R A 



riety, of schorl. It is the tourmalin of 

 Werner ; the tourmaline verte of Haiiy ; 

 the schorl electrique of Brochant. 

 It is of various colours, the shades of 

 some of which are so dark as to approach 

 nearly to black. Its hardness is about 7'5. 

 Specific gravity from 3'0 to 3'2. By 

 friction it yields vitreous electricity; by 

 heating, vitreous electricity at one extre- 

 mity and resinous electricity at the other. 

 It occurs imbedded in granite, gneiss, 

 mica slate, &c. in Scotland, Sweden, 

 America, Spain, and other parts. Its 

 property of polarizing light is thus de- 

 scribed by Mrs. Somerville. " If a brown 

 tourmaline, which is a mineral generally 

 crystallized in the form of a long prism", 

 be cut longitudinally, that is, parallel to 

 the axis of the prism, into plates about 

 the thirtieth of an inch in thickness, and 

 the surfaces polished, luminous objects 

 may be seen through them, as through 

 plates of coloured glass. If one of these 

 plates be held perpendicularly between 

 the eye and a candle, and turned slowly 

 round in its own plane, no change will 

 take place in the. image of the candle. 

 But if the plate be held in a fixed posi- 

 tion, with its axis or longitudinal section 

 vertical, when a second plate of tourma- 

 line is interposed between it and the eye, 

 parallel to the first, and turned slowly 

 round in its own plane, a remarkable 

 change will be found to have taken place 

 in the nature of the light. For the 

 image of the candle will vanish and ap- 

 pear alternately at every quarter revolu- 

 tion of the plate, varying through all de- 

 grees of brightness, down to total or 

 almost total evanescence, and then in- 

 creasing again by the same degrees as it 

 had before decreased." 

 TRA'CHEA. (trachea, Lat. from rpaxvz, 

 Gr. rough.) 



1. The windpipe, or that canal which 

 leads from the throat to the lungs. 



2. In botany, the vessels of plants in 

 which the internal fibres run in a spiral 

 direction ; they are also called air-tubes. 



TRA'CHEAL. Belonging to the trachea, as 

 tracheal-vessels. 



TRACHE'LIPODS. (from rpa%7;Xoc, the 

 neck, and TTOVQ, a foot, Gr.) In La- 

 marck's arrangement, the third order of 

 Molluscans : they have the greatest part 

 of the body spirally convolved, always 

 inhabiting a spiiivalve shell ; the foot 

 free, attached to the neck, formed for 

 creeping. Trachelipods may be divided 

 into .herbivorous and carnivorous, the 

 latter possessing a respiratory siphon, 

 which the herbivorous have not. This 

 order contains fourteen families, and up- 

 wards of seventy genera. 



TRA'CHYTE. (from rpaxvff, Gr. rough.) 



A kind of volcanic porphyry, usually 

 containing crystals of glassy felspar, and 

 excessively rough to the touch. It is not 

 found in this country, but is very abun- 

 dant in the neighbourhood of all volcanic 

 craters. Trachyte sometimes possesses a 

 columnar structure ; it is generally of a 

 coarse grain, and with a degree of po- 

 rosity. From this latter circumstance it 

 easily breaks down, and forms frequently 

 a conglomerate with other substances. 

 It sometimes is found to contain augite 

 and horneblende. In some districts tra- 

 chyte seems little else than granite which 

 has been again fused, and, having been 

 exposed to different conditions, no longer 

 presents the appearance of granite. 

 TRACHY'TIC. Composed of trachyte ; re- 

 sembling trachyte ; rough. 

 TRANSITION ROCKS. ~*j The name 



TRANSITION SERIES. ) transition 



TRANSITION FORMATIONS. ) has been 

 applied to certain rocks from an opinion 

 that they had been formed at a period 

 when the globe was undergoing a great 

 change, fitting it for the reception of 

 organized beings. The term, though no 

 longer applicable in its original significa- 

 tion, is still retained. The rocks usually 

 included in the transition series are the 

 Dudley limestone, the Caradoc sand- 

 stones, and the British and Llandilo 

 rocks ; the whole possessing a thickness 

 of upwards of two thousand four hundred 

 yards, and containing, throughout, or- 

 ganic remains. The transition rocks rest 

 upon the rocks called primary, and are 

 themselves covered by the old red sand- 

 stone formation. Professor Buckland 

 observes, "it is most convenient to in- 

 clude within the transition series, all 

 kinds of stratified rocks, from the earliest 

 slates, in which we find the first traces of 

 animal or vegetable remains, to the ter- 

 mination of the great coal formation. 

 TRANSLU'CENCY. (from trans, through, 

 and luceo, to shine, Lat.) A term used 

 in mineralogy to express the property, 

 which some minerals possess, of permit- 

 ting the passage of rays of light, but with- 

 out sufficient transparency to perceive 

 objects through the mineral. 

 TRANSLU'CENT. A mineral is said to be 

 translucent when light evidently passes, 

 but objects cannot be distinguished through 

 the mineral. 



TRAP. > (trappa, a stair, Sw. trapp, 



TRAP ROCKS. > a step, Germ, probably 

 from rpaTTf^a, Gr. a table.) Kirwan gave 

 the name of trap to basalt, which he 

 divided into two families, namely, Com- 

 mon Trap, and Figurate Trap. The word 

 trap is usually employed to designate 

 certain volcanic rocks, frequently occur- 

 ring in large tabular masses at different 



