ORE DEPOSIT THEORIES. 229 
mineralogy of the characteristic tin-stone deposits. As 
further evidence, they furnished the results of exhaustive 
experiments in the pfoduction of tin oxide from volatile 
compounds. In these experiments stannic fluoride could 
not be used owing to difficulties in manipulation, but they 
were successfully carried out on the analogous compound 
stannic chloride. By introducing the vapours of stannic 
chloride and water into a white-hot porcelain tube, hydro 
chloric acid and small crystals of tin oxide were obtained. 
The reaction takes place according to the formula— 
Sn Cl, + 2H, O=4HCl+Sn0, 
We have no reason to doubt that the entirely analogous 
reaction— 
Sn F, + 2H,O =4HCl+4S8n0, 
would take place under similar circumstances. 
By similar experiments, with suitable vapours acting on 
each other or upon solid substances, many of the accom- 
panying minerals of tin-stone veins were produced, including 
a substance of analogous composition to topaz. 
Recent investigations of the phenomena of magmatic 
segregation and the formation of pegmatite veins have 
thrown much light upon the origin of tin veins. For it is 
now evident from the close connection which exists between 
tin veins and pegmatite veins, and from the fact that they 
have so many characteristic minerals in common, that they 
have been formed from solutions which have a common 
origin. Now, a similar connection to that which has been 
shown to exist between pegmatite veins and tin veins has 
also been proved to exist between pegmatite veins and aplite 
dykes. As bearing on this point, I might mention the 
presence of tin oxide in both the aplite dykes and the peg- 
matite veins at Ben Lomond. It seems, therefore, evident 
that if aplite dykes are of eruptive origin, so also are peg- 
matite veins, and after the exhaustive treatment which this 
question has received at the hands of J. Lehmann, W. C. 
Brogger, Iddings, and many others during the last few years, 
the plutonic origin of pegmatite veins is now generally 
regarded as fully established. The differences between the 
pegmatite veins and aplite dykes may be satisfactorily 
accounted for, on the assumption that the pegmatites were 
deposited from a very much more aqueous magma than the 
aplites. It appears to be probable that at very high tem- 
peratures and pressure magma and magmatic water are 
miscible in all proportions. If this is the case, we may have 
a perfect transition from molten magma to aqueous solu- 
tions on the one hand, and from eruptive dykes to mineral 
