BY W. F. PETTERD, CM.Z.S. 115 



•202. Menaccanite (Titanic Iron Oxide). 



This, with pleonaste, constitutes the " black jack " of 

 the East) Coast tin-miners. It is extremely abundant, 

 its principal localities being the Blue Tier, Cascade, Mt. 

 Claude, Denison, Dundas, Blythe River (blue-black to 

 black) ; Greorge's Bay, and other places. The variety 

 nigrine is said to occur abundantly at Rocky Point, West 

 Coast. 



It occurs fairly abundantly in small, well-formed, and 

 highly modified crystals impregnated in the hornblende 

 rock of the Mt. Ramsay Bismuth Mine. 



203. Mesolite (Hydrated Silicate of Aluminium ^Calcium, 



and Soda). 



A zeolite occurring as small globules of a fibrous 

 structure. 



In basalt, near railway-bridge, Hellyer River ; Bell 

 Mount, Middlesex, with other zeolites; Myrtle Bank and 

 other places under like conditions. 



204. Mica Group. 



The various and somewhat numerous species belonging 

 to this important group are all strongly characterised by 

 their basal cleavage, thus yielding easily perfect laminae 

 of extreme tenuity ; or in other words they have a mica- 

 ceous structure. The laminae are remarkably tough, and 

 more or less elastic. All the micas have a splendent 

 pearly and metalloid lustre, although they vary in colour 

 to a remarkable extent. All the members of the group 

 resemble each other, their perfect basal cleavage permit- 

 ting smooth plates to be separated, which are noted for 

 their elasticity. They all belong to the monoclinic system 

 of crystallisation, but they have a pseudo-hexagonal habit, 

 differing from all other monoclinic minerals in the form 

 assumed by the crystals, and by their optical behaviour. 

 The acute bisectrix or first median line is not absolui>ely 

 normal to the basal plane, although it seldom makes an 

 angle of more than five or six degrees with the direction 

 of cleavage, and frequently one of less than half a degree. 

 For this reason cleavage plates of mica show interference 

 figures under convergent polarised light. The micas are 

 essential and important constituents of many igneous 

 rocks, such as granites, gneisses, schists, and others. In 

 pegmatite veins the crystals and plates are often of large 

 dimensions, even up to several feet in diameter in the mica 



