BY LEO A. COTTON. 745 



is to be seen in the bed of Murray's Water, about a mile from 

 its junction with Cope's Creek. 



One noteworthy feature of the "Acid Granite" is, that it 

 frequently occurs at the contact of tlie older Tingha Granite and 

 tlie slates. Indeed, if the accompanying geological map be 

 examined, it will be noticed that the large body of the " Acid 

 -Granite," is wedged between the slates on the north and the 

 Tingha Granite on the south. In some places the margin of the 

 slates is separated from the Tingha Granite by a belt of " Acid 

 •Granite," a score or so of yards in width. This marginal distri- 

 bution of the " Acid Granite " is doubtless due to the fact that 

 the contact of the older formations was a surface of structural 

 weakness; and that the **Acid Granite," taking advantage of this, 

 was able to rise, and, by stoping and pressure, to considerably 

 increase its extent. 



Aplitic Dykes. — Closely following on the consolidation of the 

 ^' Acid Granite," a number of aplitic dykes were forced into the 

 granite-systems. These are best observed in the neighbourhood 

 of Oakey Creek and Co[)eton, because here numerous adits have 

 been driven into the granite hills for the purpose of working the 

 diamond-bearing gravels. The dykes are fine-grained quartz- 

 felspar rocks, and are frequently strongly tourmaline-bearing. 

 In several localities where these have been exposed by weathering, 

 the stock and native animals have worn bare and smooth, many 

 square yards by licking the rock, which evidently contains some 

 palatable constituent. The dykes weather to a soft milk-white 

 friable rock of a highly f el spathic nature. 



(b) Felsite- Dykes. — At Copeton, about half-way between the 

 Post Office and the Public School, a dyke of felsite may be seen 

 crossing the race from the "Star of the South" diamond-mine. 

 The rock is greenish in colour, and is felsitic in structure. A 

 similar dyke passes through the "Banker" diamond-mine, and 

 may be traced for at least half a mile on the surface. 



(c) D oler it e- Dykes. — In the same locality a coarse dolerite- 

 dyke has been found. This is situated on Oakey Creek and has 

 been found to be diamond-bearing. It is not quite certain 



