420 THE GREAT FLAT LODE OF CORNWALL. 



Fluoride of tin at high temperatures is a stable compound, and 

 may have been brought up from great depths in this combination. 

 Oxide of tin is always associated with tourmaline, the existence of 

 which is assumed to be connected with a volatile fluoride of boron. 

 In fact, all the minerals would originally have been fluorides, and 

 have gradually acquired their present combinations. Oxide of tin 

 occurs in the crystalline form of felspar, which is sometimes also 

 replaced by tourmaline. Oxide of tin has been formed experimen- 

 tally by Daubre"e, by heating together perchloride of tin and steam, 

 as well as by dissolving the perchloride of tin in a current of dry 

 carbonic acid. 



Eutile is similarly obtained artificially by heating together steam 

 and the perchloride of titanium, from which it is inferred that titanic 

 acid has resulted from the decomposition of fluoride of titanium, 

 which is associated with the fluoride and chloride of phosphorus and 

 boron. On such a view the ores would be comparable to deposits 

 formed around volcanic vents. 



Apatite is produced by passing a current of perchloride of phos- 

 phorus over caustic lime heated to redness, when the perchloride is 

 absorbed and decomposed. In a like manner, topaz is formed by the 

 action of fluoride of silica on pure alumina ; and corundum has been 

 obtained by the action of volatile metallic fluorides reacting 011 

 oxides. 1 Such an interpretation has the merit of accounting for the 

 minerals associated with tin. 



Tin Lode at Redruth. Around the granite hill of Cam Brae, 

 about a mile south-west of Redruth, are many famous tin mines con- 

 nected with the Great Flat Lode. The lode in the eastern part of 

 the Wheal Uny strikes a little north of west, and being only inclined 

 46 south, is much flatter than most of the tin Veins in Cornwall, 

 which have an average inclination of 70. The leader varies from 

 eighteen inches wide to two inches wide. Its walls have a smooth 

 slickenside character. The lode which lies on the under side of the 

 leader is a compact schorl rock with veins of quartz, cassiterite, 

 chlorite, and iron pyrites. On the upper side of the leader the capel 

 is compact schorl rock. The greyback or black granite is not separ- 

 ated by any line of demarcation from the lode, and the capel is 

 similarly inseparable from the killas. Though this is the section at 

 the 130 fathoms level, it varies a good deal as to the relations of 

 lode to the leader. At a greater depth (175 fathoms) the leader 

 becomes a copper lode two to four feet wide. In some of the other 

 mines the leader becomes a quartz vein. The lode where it contains 

 tin is a mass of schorl rock from four to fifteen feet wide, and one to 

 three per cent, of cassiterite distributed in little grains or strings. 

 The capel is also a schorl rock poor in tin ore, with its constituent 

 minerals arranged in layers, thus indicating, and by its graduation 

 into the adjacent rocks, that the lode itself is only a more altered 

 portion of the granite or killas in which it occurs. Killas is fre- 

 quently altered into schorl rock, sometimes becoming tourmaline 

 1 Daubrde, "Geologie Expcrimentale, " Premiere partie, 1879, 



