386 TOURMALINE. 



cinth-brown varieties are chiefly brought 

 from Ceylon, and the smoky green and blue 

 from Brazil, on which account they are often 

 called Brazilian Emeralds and Sapphires. 



Though the Tourmaline is occasionally 

 used as a gem, it is chiefly interesting on 

 account of its mineralogical characters, and 

 certain curious physical properties which it 

 possesses. The crystals are generally dif- 

 ferently terminated, which is an exception 

 to the law of crystallography, that all facets 

 of the same kind should be similarly repro- 

 duced on all identically similar elements of 

 a crystal. The electric properties of Tour- 

 maline have relation to this crystallogra- 

 phical anomaly ; and a prism heated in a 

 particular way speedily manifests two kinds 

 of electricity, becoming positively electrical 

 at one end and negatively at the other. It 

 becomes positively electrical when rubbed, 

 and on being heated it becomes electrical 

 while cooling, being positively electrical at 

 the end of the crystal which has the greatest 

 number of facets, and negatively at the 

 opposite end. This state of polarity may 

 be reversed by intense cold ; and if a prism 

 be broken Avhile in an electric state by heat, 

 the fragments present opposite poles, like 

 artificial magnets. The crystals are also 

 frequently particoloured, being of one colour 

 at one end, and of another colour at the end 

 opposite. 



Cut in thin slices, and mounted in an in- 

 strument called a polariscope, the Tourma- 

 line is used to analyse the optical properties 

 of other minerals. 



It is usually found in granite, gneiss, or 

 mica-slate, also in Dolomite or granular 

 limestone, and in sandstone near dykes of 

 igneous rocks. 



Black Tourmaline, or Schorl, is very abun- 

 dant in almost all stanniferous granites, 

 especially in Cornwall, in the neighbour- 

 hood of St. Austell ; and in Devonshire, in 

 the granite of Dartmoor, particularly near 

 Bovey-Tracey. It also occurs in mica- schist 

 at Karosulik, in Greenland ; at Horlberg, 

 near Bodenmais, in Bavaria; at Karing- 

 bricka, in Sweden ; with Emery, at Naxos ; 

 and in the U. S., at Haddam, Connecticut ; 

 Tasmania, imbedded in gravel, overlying 

 granite, on Flinder's Island, in Bass Strait. 

 The rare white variety is found on St. Gott- 

 hard, Siberia, and in Elba, in grey granite ; 

 the green variety (coloured by iron) near 

 Katherinburg, in Siberia; Campo-longo, St. 

 Gotthard, Canton of Tessin, and at Airolo, 

 in Switzerland. Pale brownish crystals at 

 Windisch Kappell, in Carinthia; also in 

 Brazil, and elsewhere. 



TREMOLITE. 



TowANiTE. The name given by Brooke 

 & Miller to Chalcopyrite, after the Cornish 

 locality (Huel Towan), where fine crystals 

 have been found. 



Trapezoidal Kouphone Spar, Mohs. 

 See Leucite. 



Trappisches-eisenerz, Breithaupt See 



TiTANIFEROUS IrON SanD. 



Traubenblei, Hausmann. See Pyro- 



MORPHITE. 



Travertine. The name given to the 

 harder and more compact kinds of Cal- 

 careous Tufa, formed by the evaporation of 

 water holding lime in solution on the sides 

 of rivers, waterfalls, &c. Large deposits of 

 this nature are formed by some of the rivers 

 and springs in Italy. " The calcareous 

 waters of the Anio incrust the reeds which 

 grow on its banks, and the foam of the 

 cataract of Tivoli forms beautiful pendant 

 stalactites. On the sides of the deep chasm 

 into which the cascade throws itself, there 

 is seen an extraordinary accumulation of 

 horizontal beds of Tufa and Travertine, from 

 four to five hundred feet in thickness." — 

 L}eirs Principles of Geology, p. 241. 



The name Travertine is derived from that 

 of the River Tiber, and means Tiber-stone. 



Brit. Mus., Case 46. 



M. P. G. Table-case B, in Recess 6, Nos. 

 206 to 210, and 219 to 223. 



Tremenheerite, Piddington. An impure 

 Indian variety of Graphite. 



Analysis, trom New Jersey : 



Carbon ...'.. 85-7 

 Peroxide of iron . . . 2-5 

 Earthy matter . . .78 

 Water and insoluble matter . 4-0 



1000 



Name. After Major-General Tremen- 

 heere, of the Bengal Engineers. 



Trebiolite, Jameson ; Tremolith, Wer- 

 ner ; Tremolithe, Brochant. A variety of 

 Hornblende, differing from Actinolite prin- 

 cipally in its pale green, grey, or white colour. 

 It occurs both crj'Stallized and in masses 

 which have a columnar composition, and 

 low degrees of transparency. The crystals 

 are often in long slender blades, either dis- 

 tinct and traversing the gangues, or in 

 columnar and radiated aggregations. The 

 clear crystals are called Glassy Actinolite — 

 the fibrous or thin capillary crystals Asbesti- 

 form Tremolite. The fibres are generally 

 slightly elastic. S.G. 2-93. 



Comp. Silicate of Magnesia and Lime. 



Analysis, from Fahlun, by Bonsdoiff: 



Silica 60 iO 



Magnesia . . . . 24-31 



