ORIGIN AND DEVELOPMENT OP ORB-DEPOSITS. 55 



The entire absence of carbonates from the veins cutting the 

 granite, and their extreme rarity in those found close to the 

 granite is noteworthy, and would appear to negative, for this 

 district, the now almost abandoned idea that the kaolinization 

 of granite is due to the action of carbonic acid on f elspathic rocks* 



Group 6. Barium, Strontium, Calcium, Sfc. 



Barium, so far as is yet known in the West of England, can 

 hardly be regarded as a rock-constituent at all, though it is 

 likely that its silicate compounds would be found, if sought for, 

 in many of the eruptive rocks. As a vein-constituent it is known 

 only as a sulphate. It is abundi.mt in the Teign valley, 

 where it occurs associated with lead-ores ; it has also occurred 

 with the lead ores of Wheal Mary Ann and Wheal Trelawney 

 near Liskeard, with oxidized iron ores at Eestormel, and 

 with sulphuretted copper ores at Ale-and-Cakes in Grwennap. 

 In all these localities west of the Teign, the barytes has 

 been rather rare, and this rarity is in striking contrast 

 with its abundance in many other well-known mining regions, 

 such as the Eammelsberg in the Hartz, and at Aspen in 

 Colorado. 



Strontium is very much rarer than barium, in our district as 

 elsewhere ; in fact its compounds have only been found, and 

 that sparingly, at Sidmouth in South Devon (which, indeed, is 

 scarcely within our district at all), and at Binner Downs Mine, 

 near Hayle. 



Calcium, as a rock-constituent, not only exists in the great 

 rock-masses of South Devon, but also in many dark lenticular 

 patches in various parts of Cornwall. On the whole, it may be 

 said that it becomes more and more rare in proceeding from west 

 to east, but even in the extreme west there are few rocks which 

 do not contain as much as one-half per cent. In the veins it 

 occurs as calcite, dolomite, and fluor spar as already noted. 



Magnesium is an important constituent of serpentine, and of 

 all the basic eruptive rocks, as silicate ; but it is very rare in 

 acidic eruptives. It is generally present, but only in small pro- 

 portions, in the stratified rocks. 



* gee =' Nature and Origin of Clays," Min. Mag., 1888, p. 211, 



