XX PRESIDENTIAL ADDRESS, 



( ij.) Ij/ncoKs. — (1.) (iriinkc. — So far from the granite being- 

 one of the oldest rocks of the district, it is in reahty one of the 

 newest, for it can be seen cutting through and altering the 

 Chillagoe Limestone. It occurs in great masses along the 

 northern parts of our area, and in smaller masses to the south, 

 and throughout the district generally. In no part of Queensland 

 have I seen it penetrate the Trias-Jura Beds, and a prolonged 

 study of its behaviour in the field has convinced me that it is not 

 older than Permian times. 



As a rule, it is an ordinary ternary granite, composed of 

 orthoclase, muscovite and quartz. Its more central portions are 

 often coarsely porphyritic, in which case it is generally void of 

 useful minerals. Biotite and plagioclase felspar sometimes form 

 essential ingredients. In some places, as around Calcifer and 

 near Dargalong, it becomes hornblendic, but I have not yet 

 found it as a true quartzless syenite. It sends spurs and dykes 

 into the surrounding rocks, and both the tin and copper deposits^ 

 are almost invariably associated more or less closely with it. 

 There is hardly a mineral deposit of any size more than a mile 

 from its boundary, or from some associated igneous rock. 



It several places it is stanniferous. Thus, at the Hadleigh 

 Castle Mine, near Watsonville, cassiterite is found as a compact 

 ore in irregular masses, and at Mount Borunda, on the Tate, the 

 tin occurs in a more crystalline form in more or less greisenised 

 portions of the rock. 



(2.) P(trj)Ji!/ri/. — The tyi^ical porphyry of the district has a, 

 felsitic groundmass, with sporadic quartz blebs, and isolated 

 crystals of pink orthoclase. 



It sometimes constitutes extensive masses, as around 

 Herberton and Watsonville, and very frequently occurs as elvans. 

 At the North Australian Mine, Watsonville, a curious variety 

 occurs, which is so highly quartzose as to hav'e been mistaken 

 for quartzite. This rock is only known to me elsewhere as 

 occuring around Indooroopilly, Brisbane. 



The porphyries, though of different ages, are newer than the 

 granite, througli Avhich they can be seen to cut. 



Porphyry, whether massive or as ehans, is both here and in 

 South Queensland intimately associated with tin. 



