370 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



If this primary origin of mineral substances be admitted, it 

 might at first seem that the elements referred to would be found 

 everywhere, though differing somewhat in the proportions 

 present. In the strictest sense this may be so, but it is not so in 

 any practical sense. There are indeed certain elements, such as 

 silica and alumina, which are scarcely ever absent, even in 

 notable proportions, over any considerable area.* Similar 

 proportions of oxides of iron also are not often absent, although 

 the same cannot be said of the definite compounds of iron with 

 gulphur and arsenic. Gold and silver too seem to be very widely 

 distributed, but these are so valuable that even minute 

 proportions, such as would be overlooked in the case of less 

 valuable substances, are noted. But there are other elements and 

 combinations which, common enough in some localities, are 

 present not at all or only in very minute proportions in others, 

 and this not only in the case of large areas, but even in the 

 different parts of such a small area as our West of England 

 district. 



The elements and mineral substances whose distribution is 

 to be here discussed will be dealt with in groups as follows : 



Group 1.— Fluorine, boron, tin, and tungsten. 



,, 2. — Sulphur, arsenic, copper, zinc, lead, antimony. 

 ,, 3. — Iron, manganese, nickel, cobalt, bismuth, 



uranium, titanium, chromium. 

 ,, 4. — Phosphates. 

 ,, 5. — Carbonates ores and veinstones. 

 ,, 6. — Barium, strontian, cerium. 

 ,, 7. — Gold, silver. 

 ,, 8. — Carbon and hydro-carbons. 



Group 1. — Fluorine^ Boron, Tin, Tungsten. 



These are perhaps the most notable of the irregularly 

 distributed elements in our district, and it will be shown that 

 they are very specially associated with each other. 



Fluorine occurs in tourmaline (schorl) and in fluor spar, 

 also in much smaller quantities in such minerals as gilbertite, 



* Some remarks on the local occurrences of special forms of silica were made 

 in a previous section of ttis chapter. 



