BY W. F. PETTERD, C.M.Z.S. 129 



Heazlewood District," by W. H. Twelvetrees and the 

 writer, Pro. Roy. Soc. Tas., 1897). 



229. Orthite (Orthosilicate of Cerium, Yttrium, Calcium, 



and Aluminium). 



This mineral is also known as allanite. It is found as 

 occasional phenocrysts and as scattered brownish crystals 

 in the haiiyne-syenite-porphyry at Mt. Livingstone, and in 

 the solvsbergite porphyry south of Regatta Point, Port 

 Cygnet. 



230. Orthoclase (FolysUicote of Aluminium and Potas- 



sium). 



The common potash felspar that is characteristic of the 

 acid rocks comprised in the granite, elvan, and rhyolite 

 families. 



Exceptionally well-developed monoclinic crystals of this 

 felspar are abundant in the immediate neighbourhood of 

 Killikrankie Bay, Flinders Island (Gould, Pro. Roy. Soc. 

 Tas., 1871). The individual crystals are often 3 to 4 

 inches in length and proportionately wide. They are of a 

 milky-w^hite to yellow-brown colour, and are remarkable 

 e-amples of the species. The occurrence is in pegmatite 

 veins, which traverse the granite rock, and the associated 

 quartz and muscovite are also unusually enlarged. Druses 

 of crystals are not uncommon, with which well-developed 

 topaz is not unusual. Near the Great Republic Mine, 

 Ben Lomond, many very fine crystals have also been col- 

 lected. 



Orthoclase occurs massive and highly crystalline, of a 

 flesh-red colour, on the east bank of the Mersey River, 

 above Gad's Hill ; and of a white to pale-green colour, 

 on the west bank of the Mersey, distant about 2 miles 

 above the crossing. 



An angulated variety occurs in a pegmatite band pene- 

 trating the granite at Harmon's Rivulet, near the Parson's 

 Hood. Fairly good crystals occur, with microcline (a tri- 

 clinic potash of felspar) as a vein, in the granite near 

 George's Bay. It is abundant in syenite, near Liena, Upper 

 Mersey River. Somewhat fine crystals can be readily 

 broken out of the granite at Mt. Stronach, near Scotts- 

 dale. It occurs as a constituent in the porphyroid of the 

 Lynchf ord-R osebery districts. 



The glassy variety known as sanidine is abundant, often 

 in well-developed crystals, in alkaline rocks of Port 

 Cygnet. 



