260 PROCEEDINGS OF SECTIOK C. 



andesite, and plumbago are generally correctly interpreted by the 

 miners, but utter confusion exists in the miners' nomenclature of rocks 

 not belonging to these classes. 



Granite occurs on the western border of the mining field across 

 the Mary River. Diorite and Andesite occurs as dykes and intrusive 

 sheets in many of the mines. Plumbago occurs as beds or "floors" 

 generally overlying, underlying, or interbedded Avith slate or con- 

 glomerates. The reefs are generally very rich where they cut such 

 floors. The plumbago appears to represent either altered coal seams 

 or similar carbonaceous beds. 



By ■' sandstone" is generally understood a tuffy sandstone which 

 has undergone more or less secondary silicification. "' Grey^vacke" is 

 used in the same sense. The constituents of both of these rocks are 

 mostly of volcanic origin, but the volcanic ejacamenta have been redis- 

 tributed under water. Conglomerate is a term given to reddish, 

 purple, and greenish agglomerates, which constitute volcanic tuffs and 

 bieccias subaqueously redistributed. They are more or less crushed 

 and have undergone immense secondary silicification, so that the finer 

 cementing materials have frequently been converted into jasperoid. 



The black slate is a characteristic rock which is seldom confused 

 with other rocks, but frequently the white and bi'ick-coloured phyllites 

 and shales are termed slate. These are also sometimes termed " schist." 

 '' Greenstone" is a term which is used at GjTiipie for a o^reat variety of 

 rocks — namely, greenish tuffy sandstone and conglomerate, chloritic 

 decomposed diorite, chloritic tuffs, chloritised andesite, and green 

 rhyolite. 



Several sections were made of dyke rocks from various parts of 

 the Gympie field and were examined with the following results: — 



(a) " Andesite," Great Northern Mine. — This is a veiy altered 

 diorite porphyry. Secondary calcite and chlorite are im- 

 portant constituents. 

 (6) '■ Inglewood Dyke " rock, Great Scottish Mine. — This has 

 the same composition as (a). 



(c) " Green Diabase," Columbia Extended Mine. — This rock is 



identical with {a) and (6). 



(d) " Diorite," Columbia Extended. — This rock proved to be an 



" amphibolite," consisting chiefly of hornblende, some 

 . I'hombic pyroxene, some plagioclase and subordinate 

 quartz. Several specimens of the stone known as " black- 

 rock" were also sectioned and examined. 



(e) " Black-rock," Beenam Range. — Apparently a reconsti-ucted 



tuffy sandstone, consisting of quartz, felspar, magnetite, 

 and some titanium-rich fibrous red mineral. 



(f) " Speckled Black-rock," Noosa Vale. — This consists mainly 



of felspar and chlorite, and might be called chloritic clay 

 slate. 



(g) " Black-rock," Noosa Vale. — A cherty rock, consisting of 



felspar, hornblende, epidote, magnetite, and kaolin. Prob- 

 ably this too is a silicified tuff. 

 The Gympie rocks are all characterised by intense secondary 

 silicification, but the remarkable schistosity which the I'ocks outside 

 the mining area show is not seen within the mining district. This 



