ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 345 



The economic importance of silicification to the miner is in 

 two directions, — the one favourable, the other unfavourable. By 

 its influence in strengthening the walls of fissures, otherwise 

 much weakened by kaolinization and other changes, it is 

 favourable ; but when, as in many instances, the ore present is, so 

 to speak, buried in large quantities of hard siliceous capel, the 

 cost of excavation and of subsequent treatment is largely 

 increased, and its effect is decidedly unfavourable. 



The cross-courses of the mining regions of the West of 

 England are to a large extent composed of a peculiarly crystall- 

 ized quartz known as cross-course spar, the silica of which has 

 been derived from some unknown source, and when these cross- 

 courses contain oxide of iron this also is often highly siliceous. 

 Mr. Fox has suggested that the different character of the 

 crystallized quartz veinstones in cross-courses as compared with 

 that in lodes has some relation to the different action of local 

 electrical currents in latitudinal and longitudinal fissures. This 

 is certainly so in the West of England, but the same can hardly 

 be said of mining regions generally. 



The thick beds of sandstone which are so frequent and so 

 characteristic of that part of Cornwall which lies immediately to 

 the north and east of Truro, are often little consolidated, and 

 consequently weathered into loose and incoherent sands to a 

 considerable depth. So also the mica traps, which run from 

 Eoscreage beacon to Watergate Bay. But where these sand- 

 stones and traps are crossed by cross-courses, they are mostly 

 found to be indurated and infiltrated with silica, not only in 

 cracks, but throughout their mass. Thus they are able to resist 

 denudation, and the sandstones in such cases stand up above the 

 general level of the country, almost like dykes. This kind of 

 local silicification is common, not only in connexion with rich 

 mineral deposits, but also in situations where valuable minerals 

 are not known or believed to exist. 



The great cairns of quartz found at intervals from the 

 Dodman to Mawgan-in-Meneage seem to be local supersilicifica- 

 tions of fossiliferous sandstones of Lower Silurian Age. 



On the north coast, between Padstow and St. Ives, and also 

 at Wheal Friendship, near Tavistock, in Devon, certain soft and 

 and fine-grained sandstones and mudstones are found to be 



