BY W. F. PETTERD, C.M.Z.S. 193 



extreme alteration and decomposition by substitution and 

 hydration, and by this means, as before stated, pass into 

 such substances as the hydro-micas. 



331. Tourmaline (Silicate of Aluininium^ Boron, and other 

 Elemeiitsj. 



This species belongs to the hexagonal system of crystallisa- 

 tion, but commonly assumes the form of three-sided prisms, 

 which are strongly striated longitudinally, and have rhom- 

 bohedral faces at the crystal extremities. The opposite 

 extremes of the crystals are dissimilarly terminated, hence 

 they are hemimorphic ; they are also pyro-electric to a 

 marked degree. This mineral affects commonly a columnar 

 habit, which is often strongly radiating or divergent. It 

 varies in colour to an infinite degree. In this island it is 

 only known to occur black, brown, and green. Many of the 

 highly-coloured kinds are valued as gems, such as the 

 rubellite or red variety, the indicolite or blue variety ; 

 but the green, puce, and other colours are also highly valued 

 when clear and transparent. Tourmaline of the black 

 or schorl variety is a most persistent companion of tin 

 ore, and with the exception of quartz the most abundant 

 associate. It is often minutely disseminated throughout 

 tin -bearing rocks. Rocks are almost always tourmalin- 

 iferous in the tin-mining districts here as elsewhere. It 

 is generally supposed to have been formed by the action 

 of boron and fluoric vapours acting on the felspathic and 

 micaceous constituents of the acid stanniferous rocks. 

 Rosenbusch says (" Microscopical Physiography," p. 184) 

 that, '' Tourmaline is not directly secreted out of the erup- 

 tive magma in eruptive rocks, but resulted from the action 

 of fumaroles, carrying fluorine and boron, on the eruptive 

 rock, especially on its felspar and mica." The common 

 black variety occurs abundantly in drift at Moorina, and 

 in the vicinity of Mt. Cameron, the undivided prisms some- 

 times measuring over 2\ inches in diameter; at Killi- 

 krankie Bay and the south end of Flinders Island it occurs 

 of large size, penetrating the altered granite as well as in 

 free lumps ; in the tin-wash it occurs with topaz and large 

 quartz crystals; at Heemskirk it is remarkably plentiful, 

 often in radiating bunches of considerable size — the crys- 

 tals have been found penetrating and enclosed in quartz. 

 At Ben Lomond it is equally abundant, and generally on 

 the eastern tinfields ; at Schouten Main ; Granite Tor ; Mt. 

 Cameron ; Housetop ; Cape Barren Island ; Pieman 

 River ; Meredith Range ; at Mt. Ramsay ; in the stan- 



