228 PROCEEDINGS OF SECTION C. 
tites sometimes contain tin oxide, with or without tourma- 
line, and there is one instance of an extraordinarily rich 
patch of tin oxide occurring in a pegmatite vein (the old 
Lomond Mine). Another stanniferous pegmatite vein con- 
tains giant crystals of beryl, which Mr. W. H. Twelvetrees 
has shown by microscopic examination to contain numerous 
fluid enclosures of carbonic acid, with moving bubbles. 
The pegmatite veins are also often accompanied by a 
certain amount of greissenisation, meaning by the latter 
term not the formation of typical greissen, but the modified 
form of that rock, which is common throughout the district. 
It may be mentioned that true stanniferous greissen occurs 
at the Roy’s Hill Tin Mine, a granite contact deposit some 
15 miles from the Ben Lomond district proper. 
The age of the tin deposits in the Ben Lomond district, 
though ‘it has not yet been so accurately determined as in 
some other districts, is evidently nearly as great as that of 
the granite itself. The latter is younger than the Silurian 
strata in which it forms intrusions and has produced meta- 
morphism, and older than the Permo-Carboniferous strata 
which rest horizontally upon its eroded surface. It is there 
fore probably of Devonian-age. The stanniferous quartz 
veins penetrate the Silurian strata, but are cut off sharply 
at the contact of the granite and the Permo-Carboniferous 
strata. The bottom layers of the latter are also often com- 
posed of an old stanniferous wash. This is believed to be 
the oldest known occurrence of stanniferous gravels in the 
world. . 
It is therefore evident that the tin veins were formed be- 
fore the Permo-Carboniferous, and presumably during the 
Devonian Period. They belong, therefore, at least to the 
same geological period as the granite. The granite is 
traversed by numerous aplitic dykes, but up to the present 
no case has been recorded in which a tin vein has been 
intersected by one of these. The aplitic (quartz feldspar) 
dykes, however, sometimes contain considerable quantities 
of tin oxide, and even a little galena, both of which have 
the appearance of being original constituents of the rock. 
As early as 1840-50 Elie de Beaumont and A. Daubrée 
sought to explain the formation of tin veins by pneumato- 
lysis. They proved the stability of stannic fluoride at high 
temperatures, and assumed that the tin had ascended in 
this state of combination from the deep-seated “ granitic 
hearth,” together with fluoride of boron, silicon, and gaseous 
chlorine, and phosphorous compounds. They supported this 
hypothesis by pointing to the prominent part which com- 
pounds of fluorine, boron, and phosphorous take in the 
