BY W. F. PETTERD, C.M.Z.S. 123 



form, owing to the retardation of development by ofher 

 surrounding rock constituents. 



The cleavage is always perfect, parallel to the basal 

 plane. It is at times sufficiently transparent, coupled with 

 the size of the sheets, to be of great economic importance. 

 It is an essential constituent in most acid rocks, such as 

 granites, gneiss, and similar rocks. The large sheets are 

 usually confined to pegmatite veins, in which case the 

 other essential minerals, with those of an accessory nature, 

 also show an abnormally large development, and thus 

 become of special interest to the mineralogist, as often 

 such species as topaz, tourmaline, beryl, and garnet, among 

 others, reach their maximum growth. In addition, such 

 veins often contain the rarer mineral compounds. 



Muscovite occurs foliated and flexible in the granite 

 district between St. Valentine's Peak and Housetop 

 Mountain (Pro. Roy. Soc. Tas., 1851). At Killikrankie 

 Bay, Flinders Island, this mineral occurs in large sheets in 

 a distinct pegmatite. It is abundant in all the granite 

 districts. 



220. Natrolite (Hydrated Silicate of Aluminium and 



Sodium). 



Although orthorhombic in crystallisation, tliis mineral 

 almost constantly affects a globular, radiating structure. 

 It is usually perfectly white, and may be transparent or 

 translucent. It is an abundant zeolite in the alkaline 

 igneous rocks, but is not absolutely confined to that class. 

 The thin radiating films which occur in the clefts of the 

 dolerite that is so profuse throughout the island have been 

 attributed to this species. It is somewhat abundant as 

 massive crystalline bunches and pockets of agfsflutinated 

 rhombic crystals in the nephelinite of the Shannon Tier. 

 Pseudomorphs of this mineral after sodalite are not 

 uncommon in the elseolite syenite of Port Cygnet, and in 

 the Tertiary basalt of Middlesex, especially in the vicinity 

 of Bell Mount, fine rounded masses are abundant, having 

 the characteristic radiating structure. It also occurs in 

 the trachydolerite of Table Cape and Circular Head. 



221. Nephelite (Orthosilicate of Sodium, Potassium, and 



Aluminium). 



This species occurs crystallised in regular six-sided 

 prisms, with or without terminal planes. The compact 

 massive form is known as elaeolite, which is an essential 

 constituent in the alkaline series of plutonic rocks. 



