Centre Valley Tin Ore 



followed the stream towards its source in the Granite 

 Mountains, with a view to further investigations. 



Above the terraces the valley was still in gneiss until 

 the foot of the steep mountain slope was almost reached. 

 About a mile from the foot of the mountain a change 

 came over the appearance of the gneiss, it became much 

 more friable and in places considerable quantities of 

 black tourmaline were to be seen in it. This, so far as 

 we could judge from the weathered specimens which 

 were all that were available, seemed to have taken the 

 place of the felspars. 



A few hundreds of yards farther upstream we came 

 across the junction of the gneiss with a granite similar to 

 that which we had found as pebbles in Centre Valley. 

 Close to its margin the granite was fine-grained and 

 consisted largely of quartz and felspar, there being very 

 little mica, but a little farther up the course of the stream 

 it became coarser and was intersected by numerous veins 

 or dykes of quartz and quartz-porphyry. 



Before describing the complex of mineral veins which 

 we found in the rocks of Granite Mountains, it may be 

 well to give some account of mineral veins in general, 

 and of the various views which are held as to their mode 

 of origin. In the first place, the true mineral vein 

 occupies a more or less vertical rift in either sedimentary, 

 metamorphic, or igneous rocks, but is always definitely 

 associated with the latter. It is generally believed that 

 the veins are the result of the concentration of certain 

 mineral matter, originally disseminated throughout the 

 mass of the molten igneous magma. 



We have already stated in Chapter IV that igneous 

 rocks are composed for the most part of silicates, but 



279 



