228 METASOMATIC PROCESSES IN A CASSITERITE VEIN, 



decrease; and this would indicate that the width of the fissure^ 

 along which the solution ascended, could not have extended 

 much beyond C. This would indicate a width of about 3 inches 

 for the original fissure. Between C and E the curves indicate 

 an increasing amount of alkalies and lime from the vein to the 

 country-rock. Alumina seems to be fairlj^ uniformly distributed 

 between C and G, being somewhat more abundant towards C. 

 This would indicate that the ferromagnesian product of the 

 differentiation of the vein-material contained more alumina than 

 the country-rock. 



From the foregoing it would appear that the primary consti- 

 tuents of the intruding solutionwere SiOo, FeO, F, Sn02, MoSg, 

 FegO.. and possibly AloOg and MgO. The other constituents 

 of the vein, as analysed, were probably almost wholly derived 

 from the country-rock. The chief of these areCaO, NagO, K2O, 

 and possibly TiOo. 



A comparison of E^^ and G^ with E and G shows that these 

 deductions are in harmony with those made by Dr. Dalmer on 

 Analyses E^and G^. With regard to the alteration in the country- 

 rock Gi caused by the metasomatic action yielding the product 

 El ; Lindgren states that "Dr.Dalmer concludes that the principal 

 changes consisted in the addition of FeO, F, SnOo and possibly 

 AloOo, while KoO, Na„0, and SiOo have been subtracted. In 

 the absence of further knowledge of the relations of volume 

 during the alteration, it is scarcely possible to conclude, from 

 the comparison of these analyses alone, what the actual changes 

 have been," 



In attempting to trace the actual metasomatic changes, I was 

 soon convinced that though, with my present data, the problem 

 was an indeterminate one, yet the following generalisations 

 seemed to be justified. 



The resulting product of metasomatism has given rise to a 

 non-ferruginous mica, to fluorite and possibly topaz. The nature 

 of the mica, which is the characteristic replacing constituent, is 

 an indeterminate problem. It depends on the amount of secon- 

 dary haematite present; and also on the composition of the biotite^ 



