Vol. 59.] TIN AND TOURMALINE. 53 



6. Tin and Tourmaline. By Donald A. MacAlister, Esq., F.G.S. 

 (Read November 19th, 1902.) 



[Abstract.] 



Cassiterite hardly" ever occurs without tourmaline, although the 

 latter is found without the former ; hence it appears that tourmaline- 

 producing constituents and influences are of wider range than are 

 those of cassiterite. Boron-trioxide is an extremely common accom- 

 paniment of volcanic action, and there can be no doubt that it has 

 acted powerfully in changing such original minerals as the micaceous 

 and felspathic ingredients of crystalline rocks. From a comparison 

 of formulse representing tourmaline and felspar, it is evident that 

 the act of tourmalinization has been accompanied by a loss of soda 

 (which alone is capable of action on tin). The excess of boric acid 

 (which is over and above that required for tourmalinization) will 

 combine with this soda, forming metaborate and pyroborate of 

 soda. The former, acting on disseminated tin-ore, might result in 

 the production of sodium-metastannate and borax. The soluble 

 metastannate is capable of being leached out of the magma, and, 

 by a new reaction, tin-oxide may be precipitated and concentrated, 

 sodium-metaborate being liberated. According to the principle 

 underlying the cooling-curves of solutions, in all probability depo- 

 sition of cassiterite would take place more rapidly at a certain stage 

 in the process of cooling than at others. 



Discussion. 



Mr. J. H. Collins remarked that the Author did not refer to the 

 somewhat extensive literature of the subject, and especially to the 

 writings of Von Buch (1824), Daubree, Foster, and others, which, 

 as well as Daubree's experiments, went to show that fluorine 

 had played a great part in the production of schorl, as also in 

 the origin of cassiterite and of kaolin, in Saxony, Cornwall, and 

 other countries ; so that while there was in these countries schorl 

 without cassiterite and kaolin, there was not cassiterite or kaolin 

 without schorl. A due consideration of these facts might perhaps 

 lend support to the Author's hypothesis, at any rate as to the 

 secondary deposition of cassiterite. Some attention should, however, 

 be given to the fact that in Tuscany and in the Malay Peninsula 

 cassiterite did undoubtedly occur in limestone unaccompanied by 

 schorl, as also in the hornblende-rocks of Pitkaranta in Finland, 

 and in connexion with the andesites of Bolivia. 



Prof. Sollas thought this an interesting speculation and very 

 suggestive for future investigation, which he hoped the Author 

 would pursue both by experiment and observation. 



Mr. T. H. Holland agreed with the previous speaker in welcoming 

 a paper which, not claiming to explain every case, or to be a 

 complete discussion of the well-known work of Elie de Beaumont, 



