226 



METASOMATIC PROCESSES IN A CASSITEKITE VEIN, 



The chief error in the analyses, arising from impurities in the 

 reagents (the purest obtainable were used), was that in the 

 determination of the alkalies. A blank analysis was made for 

 alkalies, which were found to give rise to an error of 0-31% of 

 K2O, and 0-13% of NagO. The figures given in the table of 

 analyses represent the actual results obtained, less the above 

 corrections for alkalies. 



A series of curves has been drawn, after the method of Pirsson, 

 to illustrate graphically the relationships of the analyses. 



Fig.2. — Graphical representation of the tabulated analyses. 



Abscissas represent distances apart of the rocks in the field. Ordinates^ 

 represent the amounts of constituents present. 

 N.B.— SiOg and AI2O3 plotted to one-tenth the scale of the other 

 constituents. 



From these curves it will be seen that the centre of the vein'is^ 

 highly siliceous, and that there is a decrease in acidity from the 

 centre towards the contact with the'country-rock. It is probable 

 that this is, in some measure, due to differentiation in the 



