BY LEO A. COTTON. 747 



It is known as the " Kaioola Cemenced Run." This was a 

 prebasaltic stream running in a nortlierly direction, and has 

 been traced from the granite-hill at Newstead, for about half a. 

 mile to the north, where it ends abruptly. This channel was, in 

 places, exceptionally rich in cassiterite, but the very hard 

 nature of the cementing silica rendered it difficult to work 

 economically. It appears probable that the silica which i& 

 responsible for the formation of the "Glassy Bar " and the 

 " Karoola Cemented Run," had its origin in thermal springs 

 issuing from rather large channels in the granite. 



In several other localities, namely Elsmore, Topper's Mountain, 

 Stannifer, on the Inverell to Armidale Road east of Stannifer, 

 and at the Lion Lode, I have seen similar cemented drifts. The 

 rock found in these places is a hard grit consisting of quartz- 

 grains, from 2 to 5 mm. in diameter, cemented by silica. Fre- 

 quently a considerable amount of iron oxide is present in the 

 cementing material. In the proximity of reefs, this rock is often 

 rich enough in cassiterite to pay for extraction, and at Elsmore a 

 quantity of the rock has been treated. Here I have studied the 

 deposit in some detail, and have found tliat the rock is normally 

 an even-grained compact rock, the grains of which are of the 

 same size as those in the underlying and adjacent granite; the 

 grains are not at all waterworn. In places the rock is rendered 

 porphyritic by the inclusion of large angular fragments of reef- 

 material. Several tin-bearing reefs were noted in this rock. 

 Thus, ^vhile the nature of the cemented rock, with its cassiterite- 

 content, indicates its age as younger than the " Acid Granite. "^ 

 the presence in it of the tin-bearing reefs points to the opposite 

 conclusion. The explanation of this is, that the cemented rock 

 has been formed from the residual quartz resulting from the 

 decomposition of granite iji situ. The reefs in the granite, being 

 composed chiefly of quartz, are very resistant, and have remained 

 practically intact in some cases; while the felspar in the sur- 

 rounding granite decomposed, leaving only a residue of quarlz- 

 grains for a depth of several feet. 



In many cases these deposits are overlaid by basalts, and the 

 cementing may have been accomplished by the squeezing out of 



