98 Transactions. 



interest in the Globe-Progress Mine. Since that date the yield of quartz 

 has rapidly increased, and reached its height about 1904. 



The mines at present in operation are the Wealth, Progress, Big River, 

 Blackwater (Consolidated Company's mine), and Keep It Dark (dating 

 from 1873). Their statistics, operations, and plants are fully described in 

 the New Zealand Mining Handbook for 1906. 



The section accompanying this paper shows the position and depth 

 of the principal shafts and workings in the different mines. 



Future Prospects. 



Any future developments will occur, as indicated, along the auriferous 

 zone, and prospecting is rendered difficult by the fact that reefs are not 

 readily located except when blocks of quartz happen to outcrop. There 

 can be no doubt that a great deal of payable ore still exists in many parts 

 of the field, but the difficulty of locating it is an obstacle which demands 

 considerable initial outlay in prospecting. 



In regard to the prospects of deep mining, we have seen that at about 

 sea-level the blocks of quartz are notably smaller in all the mines, although 

 the gold-content is generally still payable. As the blocks get smaller the 

 expenses of working increase, and it seems very probable that the limit of 

 profitable mining will be reached not many hundred feet below the present 

 deepest workings. In other words, the blocks will become too few and 

 too scarce to justify further development in depth. 



Lastly, the very high returns secured in the early days (2 oz. to 4 oz. 

 per ton) from the upper levels indicate secondary enrichment, and it is 

 therefore obvious that in any future work the upper levels will be the most 

 remunerative. 



Art. XVII. — Geology of Rarotonga and Aitutahi. 



By Dr. P. Marshall, M.A., D.Sc, F.G.S., University of Otago. 



\^Read before the Otago Imtitute, \Oth November, 1908.] 



Rarotonga. 



But few statements have hitherto been made as to the geological nature of 

 this island, and they are very general. Mr. Percy Brown has lately been 

 good enough'to forward me specimens of rock from several localities on the 

 island, and from the study of these, as well as the statements of Mr. Brown, 

 Mr. James Allen, M.P.. and those in the Government Year-book, the following 

 description has been compiled. 



Relatively few soundings have been made near Rarotonga, but there 

 is at present no reason to doubt that it is surrounded on all sides by water 

 between 2,000 and 3,000 fathoms in depth. It is situated to the east of 

 the deep trench which extends from New Zealand almost to Samoa. 



The volcanic rock appears to rise directly from the ocean-shore without 

 any intervening fringe of raised coral rock, though the island is surrounded 

 with a fringing reef of coral. This appears to prove that no change in eleva- 

 tion has taken place since the volcanic action ceased. 



In the neighbouring island of Tonga Mr. Lister has proved an elevation 

 of 1,000 ft., while the atoll of Palmerston and others seem to prove consider- 



