164 ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 



Manganese is often found in connexion with very dark- 

 coloured and somewhat hard slates in East Cornwall and West 

 and North Devon. In such cases the slates are commonly found 

 to be bleached where they are in contact with the deposits, as at 

 Lew Trenchard and many other places. 



Iron deposits in the West of England when non-siliceous 

 are commonly associated with killas of very moderate hardness, 

 or with greatly softened and kaolinized granite. When the iron 

 ore is siliceous, the country rock is also hardened by infiltration 

 of silica, and the deposit becomes consequently worthless. 



From what has here been said, it is sufficiently obvious that 

 the Cornish miner's dislike of firm highly fissile slate, such as is 

 suitable for roofing, of highly crystalline and sound granite or 

 elvan such as would be valued for building, of clean and solid 

 looking limestone or sandstone — and generally of fresh looking 

 rocks containing pure transparent quartz, little altered felspar 

 or other unchanged minerals ; or free from cracks, spots, or stains 

 is well-founded ; while the opposite signs are rightly looked 

 upon with favour. Thus we may say in regard to ore-deposits 

 generally that "contact metamorphism" is universal; or at least 

 the exceptions, if any, are extremely few. 



7. Ore-shoots occurring in lodes which traverse stratified 

 rocks have usually a pretty definite dip, the direction of which 

 is determined by the relations existing between the bearing 

 and underlie of the lodes, and the strike and dip of the beds, as 

 shewn by M. Leon Moissenet.* In cases where the strata are 

 horizontal, or where the bearing of the lode coincides with the 

 strike of the beds, no such definite dip of the ore-shoots is of 

 course to be expected. 



A deep valley, the lower slope of a hill, or at any rate a 

 moderately diversified surface contour, is regarded as more 

 favourable than a table land or the crest of a hill. It would 

 lead us too far to discuss here the reasons for these very obvious 

 facts, they however may be readily inferred from what has been 

 stated above. 



* See Moissenet's " Observations on the rich parts of the lodes of Cornwall. 

 Translated by the present author, 1877. 



