ON THE OCCURRENCE OF A NEW SPECIES 

 OF GARNET AT PORT CYGNET. 



By W. A. MACLEOD AND 0. E. White. 



'■■' ' (Plate.) 



^ 



The Igneous jock containing this Garnet as a constant 

 accessory constituent, occurs as an outcrop about 6 feet 

 wide on the beach between Port Cygnet and Lymington. 



It is intruded between other volcanic rocks, and, as far 

 as could be ascertained from a hasty examination, seems 

 to be the only exposure in this locality. From fossils 

 found it is probably contemporaneous with the Permo- 

 Catboniferous sediments. 



The microscopical and chemical examinations of the 

 rock point out its relationship to the Trachyte family, 

 examples of which are so plentiful in this district, and 

 which have been fully described by Messrs. Twelve- 

 trees and Petterd.* Many of the varities of Trachyte 

 mentioned by them have melanite garnet as an accessory 

 constituent. 



The percentage of Si Og in this rock does not run very 

 high for a Trachyte, being as low as 55*87°/o. Trachytes 

 proper often include as much as 62°/q— 64°/^ of Silica. 



The Garnets of a brownish-yellow tirft are scattered 

 abundantly through the rock, and crystals with well 

 developed trapezoidal faces, measuring sometimes over a 

 quarter of an inch diameter, are plentiful. 



Dana (System of Mineralogy, p, 272) says " Garnets con- 

 taining protoxide of iron often become rusty and disinte- 

 grated through the oxidation of-jthe iron, and sometimes 

 are altered more or less completely to limonite, magnetite, 

 or hematite." 



These remarks apply to the mineral in question, which, 

 where accessible to the weather, has lost its lustre, and, in 

 many cases, dissolved away. Pseudomorphs (after garnet) 

 of a yellowish-white metallic mineral (undetermined) 

 occur. These are probably iron pyrites. 



* On Hauyne Trachyte, &c., in districts of Port Cygnet. Proc. Royal 

 Society, Tas., 1899. 



