226 PROCEEDINGS OF SECTION C. 
granite (quartz porphyry, rhyolite, &c.). Corresponding 
tin-stone deposits never occur with basic eruptives, so that 
we may at once conclude that tin veins are in some way 
* genetically connected with the acid eruptives. Although 
tin-stone veins occur in the solid granite, they are sometimes 
crossed by granitic dykes (aplites), which proves indispu- 
tably that they were formed after the consolidation of the 
upper portions of the granite, but before the last phase of 
the eruption had been completed. The most characteristic 
minerals of tin lodes are such as contain the elements 
fluorine and boron, such as fluorspar with 48-7 per cent. 
fluorine, topaz with 14 to 18 per cent. fluorine, potassium 
and lithium micas with 4 per cent. to 9 per cent. fluorine, 
fluoric apatite with up to 3-8 per cent. fluorine, and the 
‘boric silicates, tourmaline with from 8 to 10 per cent. 
B,O, and 1 to 3 per cent. F, axinite with from 4°6 to 5°6 
per cent. B:O;, and datolite with from 19 to 22 per cent. 
B:Os, &c. Other minerals frequently met with in tin 
veins, such as cassiterite, wolframite, columbite, beryl, and 
tourmaline, also occur in pegmatite veins, and in some 
localities the pegmatite veins are closely connected with the 
tin-stone deposits. Of sulphides, there are present iron, 
copper, and arsenical pyrites, bismuth sulphide, and often 
small quantities of galena and zinc-blende. One of the 
most common phenomena connected with tin ore deposits 
in granite is a remarkable metamorphism of the wall-rock 
known as greissenisation, and this may sometimes also be 
observed in connection with pegmatite veins, or at least in 
veins which are intermediate between tin veins and peg- 
matite veins. Greissenisation consists of the more or less 
complete replacement of the feldspar and biotite of the 
granite with such minerals as potash-mica, topaz, tour- 
maline, cassiterite, quartz, &c. This greissen often contains 
so much cassiterite that it becomes the object of mining. 
Tasmania is well-known as a tin-producing State, and, as 
might be supposed, it furnishes examples which have a direct 
bearing on the origin of tin deposits. Of the several impor- 
tant tin-mining districts, I regret that I am _ personally 
acquainted with only one in which typical tin veins occur, 
namely,. the Ben Lomond district, but fortunately this 
district has been exceptionally favoured as regards the 
occurence of phenomena which have a direct bearing on the 
origin of tin veins. I may mention, however, that von 
Firks has published an exhaustive monograph on the Mt. 
Bischoff deposits, in which he draws some very interesting 
