746 THE TIN-DEPOSITS OF NEW ENGLAND, N.S.W., I., 



whether the occurrence is a pipe or a dyke, but there can be no- 

 doubt that this interesting rock is a true diamond-matrix. 

 Several other rather similar occurrences have been observed, but 

 need further investigation. 



The Tertiary Rocks. 



Siliceous Springs — After the final intrusions of the " Acid 

 Granites," a long period of quiescence prevailed, daring which 

 denudation and stream-action profoundly modified the topography. 

 Many tin-bearing reefs were disintegrated, and their metallic 

 contents concentrated in the valleys and watercourses. At length 

 the time was ripe for the initiation of another cycle of igneous 

 activity. This period of Tertiary vulcanism was ushered ia 

 by the development of siliceous springs. The effect of these is 

 evident on every hand. At Newstead, perhaps, the most striking 

 effects are to be seen. Here, on the eastern slope of the granite 

 hill, there occurs a very hard and highly siliceous rock covering 

 an area of some acres. This is locally known as " The Glassy 

 Bir." It is of no great thickness, usually from 3 to 10 feet, and 

 invariably overlies the normal granite. It occurs casing the 

 present slope of the hill to a height of about 100 feet above 

 Newstead Creek. It is a very compact rock, consisting entirely 

 of fine-grained quartz-granules firmly cemented by silica. At a 

 little distance from, and higher than the "Glassy Bar," a shaft 

 was sunk on an outcrop of quartz. The shaft was continued for 

 some distance on the same type of quartz, which did not present 

 any resemblance to a lode. Fine crystals of cassiterite were 

 found included in the quartz, which was rather glassy in appear- 

 ance. A few yards to the west another shaft was sunk on a dark, 

 almost black-looking quartz. I am of the opinion- that the 

 cassiterite contained in the quartz was not derived from an. 

 extraneous source, but that it crystallised from the siliceous 

 solution. This massive quartz-structure rises through the solid 

 granite massif. 



About half a mile to the west of these shafts, a most 

 interesting occurrence of cemented conglomerate is to be seen. 



