148 TOURMALliS'E-HKAKINU KUCKS, .Ml', li K l.MSKI Iv'K . 



(3.) Some of the quartz has preceded the felspar in 

 order of crystallisation. The structure may be 

 described as partly panidiomorphic. 



Normal Aplites. 



Many of the aplites do not contain quartz tourmaline 

 nodules. They occur in the form of dykes in the normal 

 granite and in the tourmaline granite, and as masses and 

 nodules in the normal granite. One case was observed in 

 which a small dyke of aplite contained a central seam of 

 large crystals of quartz and felspar (pegmatite) with small 

 cavities or druses, into which the crystal faces project. In 

 many other cases miarolitic cavities were observed irregu- 

 larly distributed through the dykes. Microscopically, the 

 normal aplites closely resemble the groundmass of the tour- 

 maline aplites. They are somewhat even-grained rocks, 

 composed essentially of orthoclase, plagioclase f el-spar, with 

 a little biotite and accessory tourmaline and apatite. The 

 felspa,rs are much decomposed ; the plagioclase felspar, 

 which is relatively less abundant than in the normal granite, 

 appears to be oligoclase^andesine, or albite, and has preceded 

 orthoclase in order of crystallisation. The qiiattz is more 

 abundant than in the normal granites, and in some cases 

 the grains carry rods of tourmaline quite similar in appear- 

 ance to those described in connection with the normal 

 granite. Their disposition is very capricious, as they are 

 numerous in some grains and quite absent from others. 

 The biotite shows bleaching and resorption phenomena ; 

 apatite occurs as an inclusion in biotite. Some of the 

 quartz has defined crystal edges ; it also appears as eyes in 

 the orthoclase. Vvhen present in the latter form, tour- 

 maline rods are absent. 



Quartz Tourmaline Reefs. 



The rocks which have now been, described form in part 

 the wall rocks of numerous quartz tourmaline reefs v/hich 

 occur throughout the district. We propose to use the 

 term " reef " in the sense in which it is used locally, and by 

 most mining men in Australia, viz., to denote any non- 

 clastic tabular deposit composed largely of silicious material 

 irrespective of its supposed mode of formation. Until com- 

 paratively recent years the filling-matter of fissures was 

 regarded as being divisible into two well-defined classes — 

 one formed by cooling from a molten condition, the other 

 formed by precipitation from aqueous solution ; and these 

 classes are still known as dykes and veins respectively. It 



