54 TIN AND TOURMALINE. [Feb. I 9°3' 



Boase, Le Neve Foster, and others, was suggestive in offering 

 another possible chemical explanation of the reactions by which 

 tin- dioxide could be separated from solution in magmas containing 

 alkaline borates. Although the deposition of tin is frequently an 

 accompaniment of kaolinization, where fluorine has been active, 

 kaolinization is not essential, as cassiterite occurs in perfectly fresh 

 acidic pegmatites. But the consolidation of a pegmatite results in 

 the exclusion of water to the final stages to such a degree that the 

 last deposition of mineral matter may imitate the ' comby ' structures 

 of ordinary mineral veins ; while the vapours set free begin to 

 attack the crystals of early consolidation, and to produce results 

 akin to secondary alteration. At some stages in this process the 

 Author's supposed chemical reactions may occur, and the granting 

 of one process for the separation of cassiterite does not exclude others 

 that are chemically possible under changed physical conditions. 



