TEA 



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TEA 



merated rocks ; the latter bearing 

 evidence of the action of water in 

 violent motion ; the former show- 

 ing, by their composition and 

 structure, and by the organic re- 

 mains which they frequently con- 

 tain, that they were for the most 

 part deposited in the form of mud 

 and sand, at the bottom of the sea." 



TRANSI'TION LIME' STONE, (calcaire de 

 transition, calcaire intermediare, Pr. 

 ueleryangskalkstein, Germ.) A 

 member of the Grauwacke group, 

 called also Grauwacke Limestone. 



TBANSLT/CENCY. (from trans, through, 

 and luceo, to shine, Lat.) A term 

 used in mineralogy, to express the 

 property which, some minerals pos- 

 sess, of permitting the passage of 

 rays of light, but without sufficient 

 transparency to perceive objects 

 through the mineral. 



TRANSLUCENT. A mineral is said to 

 be translucent when light evidently 

 passes, but objects cannot be dis- 

 tinguished through the mineral. 



TRAP. | (trappa, a stair, Sw. 



TRAP ROCKS. } trapp, a step, Germ, 

 probably from -rpaire^a, Gr. a table.) 

 Bergman gave the name of trap to 

 basalt, which he divided into two 

 families, namely, Common Trap, 

 and Pigurate Trap. The word 

 trap is usually employed to desig- 

 nate certain volcanic rocks, fre- 

 quently occurring in large tabular 

 masses at different heights, forming 

 a succession of terraces or steps. 

 The term is applied to various 

 igneous rocks without any regard 

 to their constituent parts, but 

 merely in reference to their form. 

 Great alterations are produced on 

 stratified rocks by the introduction 

 of trap rocks ; these consist in some 

 cases in the greater degree of in- 

 duration, by which loose grits pass 

 into compact quartz rock, and shale 

 into flinty slate ; coal is converted 

 into coke, or becomes charred, 

 forming an ash-grey porous mass, 

 which breaks into small columnar 



concretions, exactly resembling the 

 coak obtained by baking coal in 

 close iron cylinders. Limestone is 

 often rendered highly crystalline 

 and unfit for lime, when in the 

 vicinity of trap-rock; and slate 

 clay is turned into a substance 

 resembling flinty slate or porcelain 

 jasper. Many of the trap-rocks, 

 like some lavas, afford on decompo- 

 sition one of the most fertile soils 

 with which we are acquainted. A 

 slight acquaintance, says Sir E. 

 Murchison, in his splendid work, 

 " The Silurian System," with vol- 

 canic phenomena teaches us that 

 they are the results of some general 

 and deep-seated cause, which occa- 

 sions eruptions of gaseous and 

 earthy matters, or of lava, both 

 under the atmosphere and beneath 

 the ocean. It is perhaps to sub- 

 marine volcanoes that all our Brit- 

 ish trap-rocks are referable. The 

 most common ingredient in trap as 

 well as in granitic rocks, is felspar. 

 TRAPE'ZIUM. (from rpairegiov, a little 

 table, Gr.) 



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'DAL. In mineralogy, when 

 the surface is composed of twenty- 

 four trapeziums, all equal and 

 similar. 



TRASS. The name given to a tufa- 

 ceous alluvium of the Ehine vol- 

 canoes. Lyell says, * ' this trass is 

 unstratified; and its base consists 

 almost entirely of pumice, in which 

 are included fragments of basalt 

 and other lavas, pieces of burnt 

 shale, slate, and sandstone, and 

 numerous trunks and branches of 

 trees." 



TRAU'MATE. The name given by the 

 French geologists to grauwacke, a 

 term about as euphonious as that 

 they have substituted it for. 



2 L 



