R. H. Rastall — The Genesis of Tungsten Ores. 245 



than tin, or, what amounts to the same thing, has a lower freezing- 

 point in certain compounds. If this be so, it would naturally be 

 expected that tungsten minerals would tend to travel further from 

 their original source than tin minerals, and in some instances it 

 seems to be established that this is actually the case. 



An unusual type of tungsten lode containing a considerable amount 

 of titanium minerals is found in the Eastern Alps.^ The other 

 associates are molybdenite, beryl, quartz, felspar, and apatite. This 

 is an unusual combination, since titanium minerals seem to be 

 decidedly rare in tin-tungsten lodes. 



Wolfram sometimes occurs in lead-silver lodes, as, for example, at 

 l^eudorf in the Harz, where it is accompanied by fluorite, but this 

 association seems to be decidedly uncommon. 



In concluding the descriptive portion of this section attention may 

 be drawn to the remarkable fluorite veins of San Eoque in' Brazil 

 described by Valentin.^ Although not, containing any tungsten ores, 

 nevertheless there are affinities to the tungsten type, and these may 

 be considered as the extreme case of this kind of differentiation. 

 The veins consist almost exclusively of quartz and fluorite, the 

 latter showing a great variety of colours. There is occasionally 

 a little pyrite, but no other metallic minerals. These veins appear 

 to be in close connexion with the intrusion of the granite of Achala, 

 and represent the consolidation product of the last residue of the 

 magma after the metallic constituents, if present, had been strained 

 off and crystallized at higher temperatures. 



Summary of Part II. 



To sum up this part of the subject, it appears that the wolframite 

 deposits without tinstone include a considerable number of types of 

 very varying character. Some of them are clearly of direct 

 magmatic origin, and formed in a manner exactly similar to the 

 wolframite- cassiterite lodes of granite areas. That is to say, they 

 are produced from the granitic magma by differentiation carried 

 a stage further than in the case of the tin-bearing lodes, leading to 

 a complete separation of tin and tungsten. This process may, in 

 fact, be regarded as an example of fractional distillation on a large 

 scale. The extreme case of this kind of differentiation is afforded 

 by the quartz -fluorite veins of San Roque, which are analogous to 

 the topaz veins of some tin-bearing areas. 



Eut in addition to the foregoing comparatively simple case it will 

 be seen that this class also includes several varieties of widely 

 different origin ; some of these may be due to peculiar forms of 

 differentiation, whereas others are certainly metasomatic ; for 

 example, the replaced dolomite in the Black Hills. It is doubtful 

 whether any true contact deposits occur ; those hitherto assigned to 

 this origin may be capable of explanation in some other way. 

 Finally, in some areas our knowledge of the geological conditions is 



' Weinschenk, "Die Minerallagerstatten des Gross-Venediger Stockes " : 

 Zeits. fiir Krist.,.vol. xxvi, 1896. 



- Valentin, " TJber das Flussspathvorkommen von San Eoque " : Zeits. fiir 

 prakt. Geol., 1896, p. 104. 



