Finlayson. — Scheelite of Otago. 119 



1. Unaltered rock ; specific gravity = 2-695. 



2. Altered rock ; specific gravity = 2-693. 



3. Altered rock, recalculated on a basis of constant alumina. 



4. Gains and losses of altered rock. 



In this case the loss of half the total mass of the rock, in- 

 cluding 40 per cent, of the original rock, is in accordance 

 with the fact that the Macrae's veins are segregated veins. In 

 other words, wall-rock + lode (gangue) = original rock. This 

 approximate equation will hold good for volumes as well as for 

 masses — that is to say, the wall-rock has suffered a correspond- 

 ing diminution of 50 per cent, in volume. This explains why 

 the specific gravity of the altered rock (2-69) is equal to that 

 of the unaltered rock. 



Further, there is to be observed in the wall-rock a depletion 

 of lime and addition of alkalies, as at Grlenorchy, and likewise 

 due to the processes by which the scheelite was deposited. The 

 notable loss of carbon-dioxide is no doubt due to the destruc- 

 tion of calcite during the metasomatic action. 



The bunchy tendency of the ore, particularly at Macrae's, is 

 evidence of the segregation of the mineral during the forma- 

 tion of the lodes. 



(5.) Genesis op New Zealand Tungsten-ores. 



J. D. Irving, in a description of the tungsten-deposits of the 

 Black Hills of South Dakota,* deposits which occur in associa- 

 tion with crystalline limestone, has divided tungsten-ore de- 

 posits into two classes : — 



(1.) " Primary deposits," associated with granitic rocks, in 

 veins with cassiterite, and minerals such as tourmaline, beryl, 

 and fluor-spar. Such were concentrated by the pneumatolytic 

 phase of activity of the granitic magma. 



(2.) " Secondary deposits," formed by solution of bodies of 

 the first type and metasomatic redeposition in higher levels. 



The scheelite of Otago is thus a typical secondary deposit. 

 As regards the other type, it is probable that the wolfram of 

 Stewart Island, which has been described by Mr. Alex. McKay 

 as occurring in the neighbourhood of granitic rocks, and in 

 association with cassiterite, gahnite, and topaz, f is a primary 

 deposit as defined above. 



.Further, it is evident that the tungstic acid of the scheelite 

 has ascended through the schists by way of the lode-fissures, 



* J. D. Irving, " Wolframite in the Black Hills of South Dakota," 

 Trans. Amer. Inst. Min. Eng., vol. xxxi (1902), p. 683. 



t A. McKay, " Reports of Geological Explorations, 1888-89," p. 74. 



