110 THE MINERALS OF TASMANIA. 



taining disseminated cassiterite, and in parts extensive 

 bodies of tourmaline and other minerals. At the Mt. 

 Cleveland Tin Mine large quantities occur also containing 

 finely disseminated tin ore. 



Hudlestone has remarked (Pro. Geol. Assn., Vol. XI. 

 1889, p. 104) that iron in rocks is rendered locomotive 

 by means of carbonic acid, a soluble acid carbonate being 

 formed, and is fixed by means of oxygen, the solution of 

 bicarbonate on exposure to air depositing the hydroxide as 

 a kind of bog-ore. 



188. Magnetite (Sesquioxide of Iron). 



Crystallises in the isometric system with a common 

 octahedron habit, but also occurs in dodecahedra; the 

 cubic form is exceedingly rare. The pure mineral contains 

 empirically 28 per cent, of oxygen. It may be recognised 

 by its strong magnetic character, its effect upon the com- 

 pass, and by its black powder. With menaccanite it 

 forms large quantities of black sand, which results from 

 the disintegration of the crystalline rocks. Microscopic- 

 ally it is always present as a constituent in all dense dark- 

 coloured rocks of the basic type. It apparently separated 

 from the molten magma at an early phase of the consolida- 

 tion of the rock, and is oft-en found embedded in other 

 constituents. The irruption of granite or other plutonic 

 rocks is supposed to cause, in many cases, a metamorphism 

 to magnetite of other ores or adjacent rock-masses. It may 

 also be the result of the deoxidation of hematite or of con- 

 tact with effusive rocks. It is supposed that amphibolile 

 often alters this mineral. Ma.rtite is presumed to be a 

 pseudomorph after hematite. A remarkably pure, highly 

 magnetic form occurs in large quantity at the Hampshire 

 Hills. It is somewhat granular in structure, and often 

 presents a striking iridescent tarnish. At Meredith Range 

 to the vicinity of the Pieman River it is in great plenty ; 

 Ilfracombe, with hematite and other ores of iron ; at the 

 Blythe River, near Housetop Mountain ; Mt. Pelion ; Dun- 

 das, near Mt. Black. At the Savage River it shows strong 

 polarity and forms enormous masses associated with pyrites 

 and limonite ; plentiful at Mt. Agnew. In the vicinity of 

 Bell Mount, Middlesex, the massive ore is abundant, occa- 

 sionally in close intermixture with garnet. It also occurs 

 at this locality as beautifully-formed little octahedra, and 

 occasionally dodecahedra, which ai-e really excellent speci- 

 mens of their kind. At the Tenth Legion, near Zeehan, 

 it is found massive, with occasional vughs, in which large 



