ORE DEPOSIT THEORIES. 225 
Fluorine, chlorine, boron, and sulphur do generally occur 
in these deposits in small quantities, but never sufficiently 
to account for the presence of such large quantities of the 
metals. Carbonic acid has been suggested as a possible 
explanation, but thereis not much evidence in support of this 
view. It is possiblethat the recent theoretical proposition of 8. 
Arrhenius,* the eminent Swedish physicist, that “water at 
high temperatures acts relatively to silica as a powerful 
acid,” may have an important bearing-on this subject: If 
we may assume that the ores are expelled from the granite 
in the form of. volatile hydrates, this would afford a per- 
fectly satisfactory explanation from a geological point of 
view. 
Tin Veins. 
Although both contact metamorphic deposits and tin 
veins have been deposited from emanations from eruptive 
rocks, their modes of formation are very different. Of the 
whole magmatic water contained originally in the molten 
magmas, one portion is expelled before the eruptive rock 
commences to solidify. This water, as we have seen, pro- 
duces the phenomena of contact metamorphism. A second 
portion remains within the magma until the upper portions 
at least have become consolidated, but finally it also is 
expelled. Yet another portion remains in the magma to 
the end, and may be seen an the multitude of microscopic 
liquid enclosures in the quartz, &c. It is that portion which 
is expelled from the magma last of all which is believed to 
be responsible for the formation of tin veins. The complete 
details of the process by which magmatic water separates 
itself from the rest of the magmas is, of course, largely a 
matter of speculation; but so many significant facts have 
been recorded, and the supposed deposition of tin veins has 
been so faithfully imitated in the laboratory, that it may be 
regarded as scientifically proved that this type of deposit has 
been produced by waters so separated. 
Before this question can be discussed, however, it will 
be necessary to describe the general mode of occurrences 
of the deposits themselves. 
Tin veins are invariably connected with granite, or with 
the dyke-forming or effusive eruptives that correspond to 
*J.H. L. Vogt. Some Problems in the Geology of Ore De- 
posits. Trans. Am. Inst. M. Eng., 1890. 
Q 
