T R A 



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T R A 



heights, and forming a succession of ter- 

 races or steps. The term is applied to 

 various igneous rocks, without any re- 

 gard to their constituent parts, but merely 

 in reference to their form. Mr. Bake- 

 well, in his Introduction to Geology, 

 says, " trap-rocks not only reveal the 

 secret of their birth, but, from their 

 close alliance to many of the most an- 

 cient primary rocks, they disclose the 

 operations by which a large portion of 

 the earth's surface was consolidated, in 

 the most remote geological epochs. Many 

 of the trap-rocks are so similar in struc- 

 ture and composition to the products of 

 active volcanoes, and to the beds of lava 

 erupted in our own times, that we may 

 be said to see the very cause in opera- 

 tion by which they were formed. Many 

 of the trap-rocks are also so similar in 

 structure and composition to some of the 

 most ancient primary rocks, that we can 

 scarcely doubt respecting their having 

 had the same origin, though they may 

 have been consolidated under different 

 degrees of heat or pressure, and with 

 different attendant conditions." 



TRA'PPEAN ROCKS. Another name for 

 trap rocks. 



TRAPE'ZIUM. (from rpcnrtZiov, a little 

 table, Gr. trapeze, FT.) 



1. In anatomy, the name given to one of 

 the bones of the carpus, or wrist. 



2. In geometry, a quadrilateral figure, 

 whose four sides are not equal, and none 

 of its sides parallel. 



TRAPEZOI'D. (from rparTrt^ior, a little 

 table, and dSoc , likeness, Gr. trapezo'ide, 

 Fr.) 



1. In anatomy, a small bone of the wrist, 

 placed in the same row with the tra- 

 pezium. 



2. An irregular solid figure, whose four 

 sides are not parallel. 



TRAPEZOI'DAL. In mineralogy, when the 

 surface is composed of twenty-four tra- 

 peziums, all equal and similar. 



TRAU'MATE. (from Qpadff^a, Gr. a frag- 

 ment. ) The name given by the French geo- 

 logists to grauwacke, a term about as eupho- 

 nious as that they have substituted it for. 



TRA'VERTIN. An Italian name for a con- 

 cretionary limestone or calcareous preci- 

 pitate, deposited by water holding in 

 solution a considerable quantity of car- 

 bonate of lime. The water parting with 

 some of its carbonic acid gas, which rises 

 in bubbles to the surface, the lime be- 

 comes deposited. In Italy, immense 

 masses of travertin are being constantly 

 formed from waters copiously charged 

 with carbonate of lime. At the baths of 

 San Filippo a manufactory of medallions 

 in basso-relievo is carried on. The water 

 is first conducted into pits, where it frees 



itself from its grosser parts ; it is then 

 conducted by a tube to the top of a small 

 chamber, and made to fall through a 

 space of ten or twelve feet in height. 

 The stream is broken in its descent by 

 means of numerous sticks placed across, 

 by which the spray is dispersed upon 

 moulds, which are slightly soaped, and 

 the result is a cast of the figures formed 

 in the mould, of a solid, marble-like sub- 

 stance. The waters of the lake of the 

 Solfatara, between Rome and Tivoli, are 

 so supersaturated with carbonic acid gas, 

 that they appear in a constant state of 

 ebullition, from the extrication of the 

 gas. The Coliseum, and the majority of 

 the public buildings of Rome, are com- 

 posed of travertin. 



TRE'MOLITE. (from Tremola, a valley of 

 St. Gothard, where it was first found.) 

 A nearly white, grey, or bluish, variety of 

 horneblende or hemiprismatic augite. It 

 occurs in crystals, but most commonly in 

 fibrous or radiated masses, composed of 

 minute, imperfect prisms or fibres. There 

 are several subvarieties of tremolite, 

 namely, common tremolite, glassy tre- 

 molite, fibrous tremolite, and Baikalite. 

 Tremolite is found almost exclusively in 

 primary rocks, but it sometimes occurs 

 in secondary. Specific gravity from 2 '9 

 to 3-2. Hardness from 5-0 to 6'0. Be- 

 fore the blow-pipe, tremolite fuses into a 

 white glass, full of pores. Its consti- 

 tuents are silex, lime, magnesia, water 

 and carbonic acid, and oxide of iron. 

 Very differing analyses are, however, 

 given by different chemists and mineralo- 

 gists ; some describe it as containing fluo- 

 rid acid and alumina. 



TRIA'NDRIA. (from rptiq, three, and 

 avijp, a male, Gr.) The name of the 

 third class in Linnaeus' sexual system, 

 consisting of plants with hermaphrodite 

 flowers, having three stamens or male 

 organs. This class is divided into three 

 orders : 1. Monogynia. 2. Digynia. 

 3. Trigynia. 



TRICA'PSULAR. A plant that has three 

 capsules to each flower. 



TRICU'SPIDATE. Three-pointed. 



TRIDA'CNA. A genus of sub transverse 

 inequilateral, equivalve, marine, bivalves, 

 belonging to Lamarck's family of Tri- 

 dacnacea, and found both recent and 

 fossil. Recent tridacnse are found at 

 depths varying to seven fathoms, moored 

 by a byssus to rocks, and on coral reefs. 

 The shells of this genus are exceedingly 

 beautiful, being radiately ribbed, the ribs 

 adorned with vaulted foliations, and waved 

 at the margins. The hinge is formed of 

 two compressed and entering teeth. One 

 species, the tridacna gigas, is met with of 

 immense size. Fossil shells of this genus 



