TEA 



[ 440 



TEA 



TRACHE'LIPODS. (from r/ja^Xov, the 

 neck, and TTOVS, a foot, Gr.) In 

 Lamarck's arrangement, the third 

 order of Molluscans : they have the 

 greatest part of the body spirally 

 convolved, always inhabiting a 

 spirivalve shell ; the foot free, at- 

 tached to the neck, formed for 

 creeping. Trachelipods may be 

 divided into herbivorous and car- 

 nivorous, the latter possessing a 

 respiratory siphon, which the her- 

 bivorous have not. This order 

 contains fourteen families, and up- 

 wards of seventy genera. 



TBA'CHYTE. (from rpaxvs, Gr. rough.) 

 A kind of volcanic porphyry, usu- 

 ally containing crystals of glassy 

 felspar, and excessively rough to 

 the touch. It is found in the 

 neighbourhood of all volcanic cra- 

 ters. Trachyte sometimes possesses 

 a columnar structure ; it is gener- 

 ally of a coarse grain, and with a 

 degree of porosity. From this 

 latter circumstance it easily breaks 

 down, and forms frequently a con- 

 glomerate with other substances. 

 It sometimes is found to contain 

 augite and hornblende. In some 

 districts trachyte seems little else 

 than granite which has been again 

 fused, and, having been exposed to 

 different conditions, no longer pre- 

 sents the appearance of granite. 

 Mr. Poulett Scrope proposes to 

 divide trachyte as follows: 1. 

 Compound trachyte, with mica, 

 hornblende, or augite, sometimes 

 both, and grains of titaniferous 

 iron. 2. Simple trachyte, without 

 any visible ingredients but felspar. 

 3. Quartziferous trachyte, when 

 containing numerous crystals of 

 quartz. 4. Silicious trachyte, when 

 apparently much silex enters into 

 its composition. 



TRACHYDO'LERITE. A sort of inter- 

 mediate lava. The name trachy- 

 dolerite is proposed by Abich to be 

 given to certain lavas where the 

 minerals pass into one another, or 



are so intimately blended that they 

 cannot be distinguished. 

 TRANSITION ROCKS. \ The name 



TRANSITION SERIES. > transition 



TRANSITION TORMA'TIONS. ) given by 

 Werner, 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 : or it was 

 adopted to express the theory that, 

 at this period, the causes which 

 had given rise to crystalline forma- 

 tions were still in action ; while 

 those which produced stratified 

 sedimentary rocks, including or- 

 ganic remains, were only beginning 

 to operate. The term, though no 

 longer applicable in its original 

 signification, is still retained. The 

 rocks usually included in the tran- 

 sition* series are the Dudley lime- 

 stone, the Caradoc sandstones, and 

 the British and Llandilo rocks ; the 

 whole possessing a thickness of up- 

 wards of two thousand four hundred 

 yards, and containing, throughout, 

 organic remains. The transition 

 rocks rest upon the rocks called 

 primary, and are themselves cov- 

 ered by the old red sandstone for- 

 mation. Buckland observes, "it 

 is most convenient to include 

 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 termi- 

 nation of the great coal formation. 

 Upwards of six hundred species of 

 fossils have been discovered in the 

 transition rocks. They are mostly 

 peculiar to these rocks, though 

 some occur also in the carbonifer- 

 ous system, and are nearly all 

 marine. About 66 per cent, belong 

 to genera supposed to be extinct. 

 The mineral character of the tran- 

 sition formations present alterna- 

 tions of slate and shale, with slaty 

 sandstone, limestone, and conglo- 



