54 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



likely that the rather exceptional fertility of much of the Cornish 

 soil is dependent upon its presence. 



Group 5. Carbonate ores and veinstones. 



Carbonate ores. These are nowhere very common in the 

 district, and in some parts they are entirely absent. Among the 

 most notable examples, the " spathose iron " of the great Perran 

 lode and of the Pawton lode, and the blue and green copper 

 carbonates of the Caradon district may be mentioned. Smaller 

 quantities of these copper carbonates have been found in many 

 of the copper-lodes throughout Cornwall, and very beautiful 

 acicular crystals of carbonate of lead have occurred somewhat 

 sparingly in most of the lead lodes. 



The carbonates of zinc, manganese, and bismuth have also 

 been found in a few instances, but only in very small quantities. 



Carbonate veinstones — Calcite and dolomite have occurred in 

 considerable quantities in several of the Grwinear mines, and 

 especially in the Eosewarne group ; and very beautiful calcite 

 crystals have been found in a great many localities, and especially 

 in the St. Just, Liskeard, Menheniot, and Tamar mines, but 

 seldom in large quantities. The comparative rarity of carbonates 

 in the veins is probably a consequence of the original rarity of 

 limestones in the mining district of the West of England ; and 

 the greater abundance of calcite met with in the veins, in going 

 east, appears to be due to the increasingly larger proportions of 

 calcareous material present in the country-rocks ; and, perhaps, 

 points to a former westward extension of overlying Devonian 

 limestones. There is little evidence of calcareous springs or of 

 carbonic acid gaseous emanations anywhere in the district, and 

 there is good reason to think that all the carbonates of the veins 

 are either direct infiltrations from the country rocks, or else the 

 results of changes affected by these infiltrations on matter 

 already present in the veins. Speaking generally, we may say 

 that carbonate veinstones are, as might be expected, far more 

 abundant where the veins cut stratified rocks of a known fossil- 

 iferous series than elsewhere, while the fossils in the said rocks 

 usually exist near the veins only as cavities or^ more rarely, as 

 pyritous or siliceous casts. 



